Serial Communication Data Communications Data communications refers to the ability of one computer to exchange data with another computer or a peripheral

Serial Communication

Data Communications• Data communications refers to the ability of one computer to exchange data

with another computer or a peripheral• Physically, the data comm. path may be a short, 5 to 10 feet ribbon cable

connecting a microcomputer and parallel printer; or it might be a high speed telecommunications port connecting two computers thousands of miles apart.

• Standard data communication interfaces and standards are needed• Centronic’s parallel printer interface• RS-232 defines a serial communications standard• We focus on serial I/O this week• 8251 USART (Universal Synchronous/Asynchronous Receiver/Transmitter) is

the key component for converting parallel data to serial form and vice versa• Two types of serial data communications are widely used

– Asynchronous communications– Synchronous communications

Types of Transmission

8251A programmable Communication Interface

Introduction 8251A is a USART (Universal Synchronous Asynchronous Receiver

Transmitter) for serial data communication.

Programmable peripheral designed for synchronous /asynchronous serial data communication, packaged in a 28-pin DIP.

Receives parallel data from the CPU & transmits serial data after conversion.

Also receives serial data from the outside & transmits parallel data to the CPU after conversion.

Pin diagram

Block diagram of the 8251 USART

Sections of 8251A Data Bus buffer Read/Write Control Logic Modem Control Transmitter Receiver

1. Data Bus Buffer D0-D7 : 8-bit data bus used to read or write status, command word or

data from or to the 8251A

2. Read/Write Control logic

• Includes a control logic, six input signals & three buffer registers: Data register, control register & status register.

• Control logic : Interfaces the chip with MPU, determines the functions of the chip according to the control word in the control register & monitors the data flow.

Input signals CS – Chip Select : When signal goes low, the 8251A is selected by the

MPU for communication. C/D – Control/Data : When signal is high, the control or status register is

addressed; when it is low, data buffer is addressed. (Control register & status register are differentiated by WR and RD signals)

WR : When signal is low, the MPU either writes in the control register or sends output to the data buffer.

RD : When signal goes low, the MPU either reads a status from the status register or accepts data from data buffer.

RESET : A high on this signal reset 8252A & forces it into the idle mode. CLK : Clock input, usually connected to the system clock for

communication with the microprocessor.

Control Register

16-bit register for a control word consist of two independent bytes namely mode word & command word.

Mode word : Specifies the general characteristics of operation such as baud, parity, number of bits etc.

Command word : Enables the data transmission and reception. Register can be accessed as an output port when the Control/Data

pin is high.

Status register• Checks the ready status of the peripheral.

• Status word in the status register provides the information concerning register status and transmission errors.

Data register Used as an input and output port when the C/D is low

CS

C/D WR

RD

Operation

00001

0011×

1001×

011 0 ×

MPU reads data from data bufferMPU writes data from data bufferMPU writes a word to control registerMPU reads a word from status registerChip is not selected for any operation

3. Modem Control DSR - Data Set Ready : Checks if the Data Set is ready when

communicating with a modem.

DTR - Data Terminal Ready : Indicates that the device is ready to accept data when the 8251 is communicating with a modem.

CTS - Clear to Send : If its low, the 8251A is enabled to transmit the serial data provided the enable bit in the command byte is set to ‘1’.

RTS - Request to Send Data : Low signal indicates the modem that the receiver is ready to receive a data byte from the modem.

4. Transmitter section Accepts parallel data from MPU & converts them into serial data.

Has two registers: Buffer register : To hold eight bits Output register : To convert eight bits into a stream of serial bits.

The MPU writes a byte in the buffer register. Whenever the output register is empty; the contents of

buffer register are transferred to output register. Transmitter section consists of three output & one

input signals TxD - Transmitted Data Output : Output signal to transmit

the data to peripherals TxC - Transmitter Clock Input : Input signal, controls the

rate of transmission. TxRDY - Transmitter Ready : Output signal, indicates the

buffer register is empty and the USART is ready to accept the next data byte.

TxE - Transmitter Empty : Output signal to indicate the output register is empty and the USART is ready to accept the next data byte.

5. Receiver Section Accepts serial data on the RxD pin and converts them to

parallel data. Has two registers :

Receiver input register Buffer register

Receive Buffer

Receive control

Input Register

RxRDY

RxC

RxD

When RxD goes low, the control logic assumes it is a start bit, waits for half bit time, and samples the line again. If the line is still low, the input register accepts the following data, and loads it into buffer register at the rate determined by the receiver clock.

RxRDY - Receiver Ready Output: Output signal, goes high when the USART has a character in the buffer register & is ready to transfer it to the MPU.

RxD - Receive Data Input : Bits are received serially on this line & converted into a parallel byte in the receiver input register.

RxC - Receiver Clock Input : Clock signal that controls the rate at which bits are received by the USART.

DMA Controller 8237

Introduction • The DMA I/O technique provides direct access to the memory

while the microprocessor is temporarily disabled.• This chapter also explains the operation of disk memory systems

and video systems that are often DMA-processed. • Disk memory includes floppy, fixed, and optical disk storage.

Video systems include digital and analog monitors.

13–1 BASIC DMA OPERATION

• Two control signals are used to request and acknowledge a direct memory access (DMA) transfer in the microprocessor-based system.– the HOLD pin is an input used to request a

DMA action– the HLDA pin is an output that acknowledges

the DMA action• Figure 13–1 shows the timing that is typically found on these two

DMA control pins.

Figure 13–1 HOLD and HLDA timing for the microprocessor.

– HOLD is sampled in any clocking cycle – when the processor recognizes the hold, it stops executing software

and enters hold cycles– HOLD input has higher priority than INTR or NMI– the only microprocessor pin that has a higher priority than a HOLD is

the RESET pin

• HLDA becomes active to indicate the processor has placed its buses at high-impedance state.– as can be seen in the timing diagram, there are

a few clock cycles between the time that HOLD changes and until HLDA changes

• HLDA output is a signal to the requesting device that the processor has relinquished control of its memory and I/O space. – one could call HOLD input a DMA request

input and HLDA output a DMA grant signal

Basic DMA Definitions • Direct memory accesses normally occur between an I/O device and

memory without the use of the microprocessor. – a DMA read transfers data from the memory

to the I/O device – A DMA write transfers data from an I/O device

to memory• Memory & I/O are controlled simultaneously.

– which is why the system contains separate memory and I/O control signals

• A DMA read causes the MRDC and IOWC signals to activate simultaneously.– transferring data from memory to the I/O device

• A DMA write causes the MWTC and IORC signals to both activate. • 8086/8088 require a controller or circuit such as shown in Fig 13–2 for control

bus signal generation.• The DMA controller provides memory with its address, and controller signal

(DACK) selects the I/O device during the transfer.

Figure 13–2 A circuit that generates system control signals in a DMA environment.

• Data transfer speed is determined by speed of the memory device or a DMA controller. – if memory speed is 50 ns, DMA transfers occur

at rates up to 1/50 ns or 20 M bytes per second – if the DMA controller functions at a maximum

rate of 15 MHz with 50 ns memory, maximum transfer rate is 15 MHz because the DMA controller is slower than the memory

• In many cases, the DMA controller slows the speed of the system when transfers occur.

• The switch to serial data transfers in modern systems has made DMA is less important.

• The serial PCI Express bus transfers data at rates exceeding DMA transfers.

• The SATA (serial ATA) interface for disk drives uses serial transfers at the rate of 300 Mbps– and has replaced DMA transfers for hard disks

• Serial transfers on main-boards between components using can approach

20 Gbps for the PCI Express connection.

13–2 THE 8237 DMA CONTROLLER

• The 8237 supplies memory & I/O with control signals and memory address information during the DMA transfer.– actually a special-purpose microprocessor

whose job is high-speed data transfer between memory and I/O• Figure 13–3 shows the pin-out and block diagram of the 8237

programmable DMA controller.

Figure 13–3 The 8237A-5 programmable DMA controller. (a) Block diagram and (b) pin-out. (Courtesy of Intel Corporation.)

• 8237 is not a discrete component in modern microprocessor-based systems.– it appears within many system controller chip sets

• 8237 is a four-channel device compatiblewith 8086/8088, adequate for small systems. – expandable to any number of DMA channel inputs

• 8237 is capable of DMA transfers at rates up to 1.6M bytes per second. – each channel is capable of addressing a full

64K-byte section of memory and transfer up to 64K bytes with a single programming

8237 Pin Definitions

• Clock input is connected to the system clock signal as long as that signal is 5 MHz or less.– in the 8086/8088 system, the clock must be inverted for

the proper operation of the 8237

CLK


• Chip select enables 8237 for programming. • The CS pin is normally connected to the output of a decoder. • The decoder does not use the 8086/8088 control signal

IO/M(M/IO) because itcontains the new memory and I/O control signals (MEMR, MEMW, IOR and IOW).

CS


• The reset pin clears the command, status, request, and temporary registers.

• It also clears the first/last flip-flop and setsthe mask register.– this input primes the 8237 so it is disabled

until programmed otherwise

RESET


• A logic 0 on the ready input causes the8237 to enter wait states for slowermemory components.

READY

HLDA • A hold acknowledge signals 8237 that the

microprocessor has relinquished control ofthe address, data, and control buses.


• DMA request inputs are used to request a transfer for each of the four DMA channels.– the polarity of these inputs is programmable, so

they are either active-high or active-low inputs

DREQ0–DREQ3

DB0–DB7 • Data bus pins are connected to the processor data

bus connections and used during the programming of the DMA controller.


• I/O read is a bidirectional pin used during programming and during a DMA write cycle.

IOR

IOW • I/O write is a bidirectional pin used during

programming and during a DMA read cycle.


• End-of-process is a bidirectional signalused as an input to terminate a DMA process or as an output to signal theend of the DMA transfer.– often used to interrupt a DMA transfer at

the end of a DMA cycle

EOP


• These address pins select an internalregister during programming and providepart of the DMA transfer address during a DMA action.– address pins are outputs that provide part of

the DMA transfer address during a DMA action

A0–A3


• Hold request is an output that connects tothe HOLD input of the microprocessor inorder to request a DMA transfer.

HRQ


• DMA channel acknowledge outputs acknowledge a channel DMA request.

• These outputs are programmable as either active-high or active-low signals. – DACK outputs are often used to select the

DMA- controlled I/O device during the DMA transfer.

DACK0–DACK3


• Address enable signal enables the DMA address latch connected to the DB7–DB0 pins on the 8237.– also used to disable any buffers in the

system connected to the microprocessor

AEN


• Address strobe functions as ALE, exceptit is used by the DMA controller to latchaddress bits A15–A8 during the DMA transfer.

ADSTB

MEMR • Memory read is an output that causes memory to

read data during a DMA read cycle.


• Memory write is an output that causes memory to write data during a DMA write cycle.

MEMW

8237 Internal Registers

• The current address register holds a 16-bit memory address used for the DMA transfer. – each channel has its own current address

register for this purpose• When a byte of data is transferred during a DMA operation, CAR is either incremented

or decremented.– depending on how it is programmed

CAR


• The current word count register programsa channel for the number of bytes (up to 64K) transferred during a DMA action.

• The number loaded into this register is one less than the number of bytes transferred. – for example, if a 10 is loaded to CWCR, then

11 bytes are transferred during the DMA action

CWCR


• The base address (BA) and base word count (BWC) registers are used whenauto-initialization is selected for a channel.

• In auto-initialization mode, these registersare used to reload the CAR and CWCRafter the DMA action is completed. – allows the same count and address to be used

to transfer data from the same memory area

BA and BWC


• The command register programs the operation of the 8237 DMA controller. • The register uses bit position 0 to select the memory-to-memory DMA transfer

mode.– memory-to-memory DMA transfers use DMA channel 0 to hold the source address– DMA channel 1 holds the destination address

• Similar to operation of a MOVSB instruction.

CR

Figure 13–4 8237A-5 command register. (Courtesy of Intel Corporation.)


• The mode register programs the mode of operation for a channel. • Each channel has its own mode register as selected by bit positions 1 and

0. – remaining bits of the mode register

select operation, auto-initialization, increment/decrement, and mode for the channel

MR

Figure 13–5 8237A-5 mode register. (Courtesy of Intel Corporation.)


• The bus request register is used to requesta DMA transfer via software.– very useful in memory-to-memory transfers,

where an external signal is not available tobegin the DMA transfer

BR

Figure 13–6 8237A-5 request register. (Courtesy of Intel Corporation.)


• The mask register set/reset sets or clears the channel mask.– if the mask is set, the channel is disabled– the RESET signal sets all channel masks

to disable them

MRSR

Figure 13–7 8237A-5 mask register set/reset mode. (Courtesy of Intel Corporation.)


• The mask register clears or sets all ofthe masks with one command instead of individual channels, as with the MRSR.

MSR

Figure 13–8 8237A-5 mask register. (Courtesy of Intel Corporation.)


• The status register shows status of each DMA channel. The TC bits indicate if the channel has reached its terminal count (transferred all its bytes).

• When the terminal count is reached, theDMA transfer is terminated for most modesof operation. – the request bits indicate whether the DREQ

input for a given channel is active

SR

Figure 13–9 8237A-5 status register. (Courtesy of Intel Corporation.)

Software Commands • Three software commands are used to

control the operation of the 8237. • These commands do not have a binary bit pattern,

as do various control registerswithin the 8237. – a simple output to the correct port number

enables the software command

• Fig 13–10 shows I/O port assignments that access all registers and the software commands.

Figure 13–10 8237A-5 command and control port assignments. (Courtesy of Intel Corporation.)

8237 Software Commands

• Acts exactly the same as the RESET signalto the 8237.– as with the RESET signal, this command

disables all channels

Master clear

Clear mask register • Enables all four DMA channels.

8237 Software Commands

• Clears the first/last (F/L) flip-flop within 8237.• The F/L flip-flop selects which byte (low or high order) is read/written in the

current address and current count registers. – if F/L = 0, the low-order byte is selected– if F/L = 1, the high-order byte is selected

• Any read or write to the address or count register automatically toggles the F/L flip-flop.

Clear the first/last flip-flop

Programming the Address and Count Registers

• Figure 13–11 shows I/O port locations for programming the count and addressregisters for each channel.

• The state of the F/L flip-flop determines whether the LSB or MSB is programmed. – if the state is unknown, count and address

could be programmed incorrectly

• It is important to disable the DMA channel before address and count are programmed.

Figure 13–11 8237A-5 DMA channel I/O port addresses. (Courtesy of Intel Corporation.)

• Four steps are required to program the 8237:– (1) The F/L flip-flop is cleared using a clear F/L

command– (2) the channel is disabled– (3) LSB & MSB of the address are programmed– (4) LSB & MSB of the count are programmed

• Once these four operations are performed, the channel is programmed and ready to use.– additional programming is required to select

the mode of operation before the channel is enabled and started

The 8237 Connected to the 80X86• The address enable (AEN) output of 8237 controls

the output pins of the latches and outputs of the 74LS257 (E). – during normal operation (AEN=0), latches

A & C and the multiplexer (E) provide addressbus bits A19–A16 and A7–A0

• See Figure 13-12.

Figure 13–12 Complete 8088 minimum mode DMA system.

• The multiplexer provides the system control signals as long as the 80X86 is in control of the system.– during a DMA action (AEN=1), latches A & C

are disabled along with the multiplexer (E)– latches D and B now provide address bits

A19–A16 and A15–A8

• Address bus bits A7–A0 are provided directly by the 8237 and contain part of the DMA transfer address.

• The DMA controller provides conntrol signals.

Memory-to-Memory Transfer with the 8237

• Memory-to-memory transfer is much more powerful than the automatically repeated MOVSB instruction. – most modern chip sets do not support the memory-to-

memory feature

• 8237 requires only 2.0 µs per byte, which is over twice as fast as a software data transfer.

• This is not true if an 80386, 80846, or Pentium is in use in the system.

Sample Memory-to-Memory DMA Transfer

• Suppose contents of memory locations 10000H–13FFFH are to be transferred to locations 14000H–17FFFH. – accomplished with a repeated string move instruction

or with the DMA controller

• Example 13–1 shows the software requiredto initialize the 8237 and program latch B in Figure 13–12 for this DMA transfer.

Sample Memory Fill Using the 8237

• To fill an area of memory with the same data, the channel 0 source register is programmed to point to the same address throughout the transfer. – accomplished with the channel 0 hold mode

• The controller copies the contents of this single memory location to an entire blockof memory addressed by channel 1.

• This has many useful applications.

DMA-Processed Printer Interface

• Fig 13–13 illustrates the hardware addedto Fig13–12 for a DMA-controlled printer interface. – software to control this interface is simple as

only the address of the data and number of characters to be printed are programmed

• Once programmed, the channel is enabled, and the DMA action transfers a byte at atime to the printer interface.– each time a printer ACK signal is received

Figure 13–13 DMA-processed printer interface.

8279 KEYBOARD AND DISPLAY INTERFACING

Features: It is designed by Intel It is support 64 contact key matrix with two more keys

“CONTROL” and “SHIFT” It provides 3 operating modes 1.Scanned keyboard mode 2.Scanned sensor matrix mode

3.Strobed Input mode. It has inbuilt debounce key .

It provides 16 byte display RAM to display 16 digits and interfacing 16 digits.

It provides two output modes: 1.Left entry (Typewriter type). 2.Right entry (Calculator type). Simultaneous keyboard and display operation facility allows

to interleave keyboard and display software. The interrupt output of 8279 can be used to tell CPU that the

key press is detected, this eliminates the need of software polling.

PIN DIAGRAM OF 8279

LOGIC SYMBOL

Cpu interface pins:DB0-DB7 : These are bidirectional data bus lines. The data

and command words to and from the CPU are transferred on these lines.

RD, WR ( Input / Output ) READ/WRITE :These input pins enable the data buffers to receive or send data over the data bus.

A0(Address lines) : A high on this line indicates the transfer of a command or status information. A low on this line indicates the transfer of data. This is used to select one of the internal registers of 8279.

IRQ : This interrupt output lines goes high when there is a data in the FIFO sensor RAM. The interrupt lines goes low with each FIFO RAM read operation but if the FIFO RAM further contains any key-code entry to be read by the CPU, this pin again goes high to generate an interrupt to the CPU.

Vss, Vcc : These are the ground and power supply lines for the circuit.

SL0-SL3-Scan Lines : These lines are used to scan the key board matrix and display digits. These lines can be programmed as encoded or decoded, using the mode control register.

Key board Data:RL0 - RL7 - Return Lines : These are the input lines which are

connected to one terminal of keys, while the other terminal of the keys are connected to the decoded scan lines. These are normally high, but pulled low when a key is pressed.

SHIFT : The status of the shift input lines is stored along with each key code in FIFO, in scanned keyboard mode. It is pulled up internally to keep it high, till it is pulled low with a key closure.

CNTL/STB- CONTROL/STROBED I/P Mode : In keyboard mode, this lines is used as a control input and stored in FIFO on a key closure. The line is a strobed lines that enters the data into FIFO RAM, in strobed input mode. It has an interrupt pull up. The lines is pulled down with a key closer.

Display data:OUT A0 – OUT A3 and OUT B0 – OUT B3 : These are the output

ports for two 16*4 or 16*8 internal display refresh registers. The data from these lines is synchronized with the scan lines to scan the display and keyboard. The two 4-bit ports may also as one 8-bit port.

BD – Blank Display : This output pin is used to blank the display during digit switching or by a blanking closure.

Block Diagram

It consists 4 main section.1. CPU interface and control section.2. Scan section3. Keyboard Section4. Display section.CPU INTERFACE AND CONTROL SECTION:It consists of 1. Data buffers2. I/O control3. Control and timing registers.4. Timing and control logic.

Data Buffers: 8-bit bidirectional buffer. Used to connect the internal data bus and external data bus.

I/O control: I/O control section uses the A0,CS,RD and WR signals to

controls the data flow. The data flow is enabled by CS=0otherwise it is the high

impedance state. A0=0 means the data is transferred. A0=1 means status or command word is transferred.

I/O control signals listed below

A0 RD WR Interpretation

0 1 0 Data from CPU to 8279

0 0 1 Data from 8279 to CPU

1 1 0 Command word from CPU to 8279

1 0 1 Status word from 8279 to CPU

TIMING AND CONTROL REGISTERS:Store the keyboard and display modes and others

operating condition programmed by the CPU.The modes are programmed by sending proper command

A0=1.

TIMING AND CONTROL: It consist timing counter chain.First counter is divided by N prescalar that can be

programmed to give an internal frequency of 100 KHz.

The other counter is divide by basic internal frequency .Listed below

Parameter Timings

Keyboard and time 5.1 msec

Keyboard and debounce time 10.3 msec

Key scan time 80 µ sec

Display scan time 10.3 msec

Digit ON time 480 µ sec

Blanking time 160 µ sec

Internal clock time 10 µ sec

Scan Section It has two modes,1. Encoded mode2. Decoded mode.

ENCODED MODE: It provide binary count from 0000 to 1111 by four scan

lines(SC3-SC0)by active high inputs. It is externally decoded to provide 16 scan lines

Display use all 16 lines to interface 16 digit 7 segment display.

But keyboard use only 8 scan lines out of 16 lines.

DECODED MODE: In this mode ,the internal decoder decodes the least 2

significant bits. It is provide four possible combination from (SC0-SC3) such

as 1110 ,1101 ,1011 and 0111. This four active low outputs line is used to directly to

interface 4 –digit 7-segment display ,8*4 matrix keyboard

Keyboard sectionThis is consist of,Return buffers.Keyboard debounce control.FIFO / sensor RAM.FIFO / sensor RAM status.

RETURN BUFFERS:8 return lines(RL7-RL0) are buffered and latched

by when each row scan in scanned keyboard or sensor matrix mode.

In strobed mode ,the contents of return lines are transferred to FIFO Ram.

KEYBOARD DEBOUNCE AND CONTROL: It is enabled only when keyboard mode is selected. In this mode , return lines are scanned whether any

keys are closed in the row. If debounce circuit is detect any closed switch it

wait about 10 msec. It is continued , the status of SHIFT and CONTROL

keys are transferred into RAM.

FIFO/SENSOR RAM:This is a dual function of 8*8 RAM.

In scanned key board mode and Strobed input mode , It is FIFO.

Each new entry is written into successive RAM position and read in the order of entry.

In sensor matrix mode it is a sensor RAM. Each sensor RAM is loaded with corresponding sensor RAM

status.

FIFO/SENSOR RAM status: This is used to tell the status of FIFO/SENSOR RAM. The status of logic also makes IRQ signal is High , When FIFO is

empty.

Display section:It consists of,1. Display RAM.2. Display Address registers.3. Display registers.DISPLAY RAM: It is a 16*8 RAM. Which stores 16 digits display codes. It can be accessed by CPU directly. In Decoded mode,8279 uses only first four location of Display

RAM. In Encoded mode,8279 uses only first eight location of Display

RAM. And all 16 location for 16 digits display.

DISPLAY ADDRESS REGISTERS: Used to hold address of the byte currently write or read by

the CPU and scan count value. In auto increment mode, address in the register is

automatically incremented for each write or read.DISPLAY REGISTERS: It is a Two 4-bit registers such as , A and B. They hold the bit patterns of character to be displayed. The content of display registers A and B can B blanked and

inhibited individually.

Operating modesIt is two types,1. Input modes.2. Display modes.INPUT MODES: It is basically 3 types,1. Scanned keyboard.2. Scanned sensor matrix.3. Strobed mode.SCANNED KEYBOARD:Key board can be scanned in two ways.1.Encoded Scan 2.Decoded Scan.

ENCODED SCAN: In this scan, scan lines (SL2-SL0) are decoded externally to

provide 8 scan lines. Additionally it provides 8 return lines. So the size of matrix keyboard is 8*8 (i.e Scan * Return)=64. When the key is pressed , it is stored the status of return lines

, Scan lines ,SHIFT and CNTL/STB keys into FIFO RAM. The Scanned keyboard structure is,

B7 B6 B5 B4 B3 B2 B1 B0

CNTL SHIFT SCAN RETURN

Example: Find the key code for given condition below:CNTL/STB SHIFT keys are open.The pressed keys are to scan lines 2 and return lines 4.SOLUTION:

B7 B6 B5 B4 B3 B2 B1 B0

1 1 0 1 0 1 0 0

CNTL=1 SHIFT=1 Scan mode=010 (Scan line 2) Return mode=100 (Return line 4) Key code =D4 HDECODED SCAN: In this mode ,internal decoder decodes the least significant

bits of scan lines (SC3-SC0). That is provide the four combination such as 1110,1101,1011

and 0111. So the maximum size of keyboard is 8*4=32. The key code is similar to encoded code , only bit 5 (B5) is

always zero.

2-KEY LOCKOUT: In this mode, the two key depression is not allowed. When any key is depressed, the debounce logic is set and

8279 checks for any key depress next two scans. Three possible condition to avoid debouncing: Condition 1: If other key depression is not found during next two scan, it is

a single key is depressed .Then the status of key code is entered into FIFO RAM along with the status of CNTL and SHIFT lines

If FIFO RAM is empty , The CPU is entry the data. If FIFO RAM is full , The CPU does not entry the data.Condition 2: If any other key depress is encountered , no entry to the FIFO

can occur. When the key is released after that only Entry will be allowed.Condition3: If the two key is pressed in simultaneously in a debounce

cycle, both depression is not considered.

• N-KEY ROLLOVER: Each key is depression is treated as independently from all

others.SCANNED SENSOR MATRIX: In this mode , image of the sensor matrix is kept in the sensor

RAM. The status of sensor switches are input directly to the sensor

RAM. 8279 scans row one by one and store the status of each row

in the corresponding memory location. STROBED INPUT MODE: The data is entered from Returned lines.

Display modes:• It is basically two types,1. Left entry (Type writer mode).2. Right entry (Calculator mode).LEFT ENTRY: In this mode , 8279 display characters from left to right. Like a typewriter.AUTOINCREMENT IN LEFT ENTRY: In left entry mode , Autoincrement flag is set after each

operation display RAM address is incremented.

RIGHT ENTRY: In this mode , 8279 display characters from Right to left. Like a Calculator.AUTOINCREMENT IN RIGHT ENTRY: In right entry mode , Auto increment flag is set after each

operation display RAM address is incremented.

Interfacing with 8279 with 8085

Interfacing Analog to Digital DataConverters

• In most of the cases, the PIO 8255 is used for interfacing the analog to digital converters with microprocessor.• We have already studied 8255 interfacing with 8086 as an I/O port, in previous section. This section we will only emphasize the interfacing techniques of analog to digital converters with 8255.• The analog to digital converters is treaded as an input device by the microprocessor, that sends an initialising signal to the ADC to start the analogy to digital data conversation process. The start of conversation signal is a pulse of a specific duration.

Interfacing Analog to Digital DataConverters (cont..)

• General algorithm for ADC interfacing contains the

following steps:

1. Ensure the stability of analog input, applied to the ADC.

2. Issue start of conversion pulse to ADC

3. Read end of conversion signal to mark the end of

conversion processes.

4. Read digital data output of the ADC as equivalent digital

output.


• Analog input voltage must be constant at the input of the• ADC right from the start of conversion till the end of the• conversion to get correct results. This may be ensured by a• sample and hold circuit which samples the analog signal• and holds it constant for a specific time duration. The• microprocessor may issue a hold signal to the sample and• hold circuit.• • If the applied input changes before the complete• conversion process is over, the digital equivalent of the• analog input calculated by the ADC may not be correct.


ADC 0808/0809 :• The analog to digital converter chips 0808 and 0809 are 8- bit CMOS, successive approximation converters. This

technique is one of the fast techniques for analog to digital conversion. • The conversion delay is 100μs at a clock frequency of 640

KHz, which is quite low as compared to other converters. • These converters do not need any external zero or full scale

adjustments as they are already taken care of by internal circuits.

• These converters internally have a 3:8 analog multiplexer so that at a time eight different analog conversion by using address lines


• ADD A, ADD B, ADD C. Using these address inputs, multichannel data acquisition system can be designed using a single ADC.

• The CPU may drive these lines using output port lines in case of multichannel applications.

• In case of single input applications, these may be hardwired to select the proper input.• There are unipolar analog to digital converters, i.e. they are able to convert only positive analog input voltage to their digital equivalent. • These chips do no contain any internal sample and hold

circuit.



• If one needs a sample and hold circuit for the conversion of fast signal into equivalent digital quantities, it has to be externally connected at each of the analog inputs.

• Vcc Supply pins +5V

• GND GND

• Vref + Reference voltage positive +5 Volts maximum.

• Vref_ Reference voltage negative 0Volts minimum.


• I/P0

• –I/P7 Analog inputs

• ADD A,B,C Address lines for selecting analog inputs.

• O7 – O0 Digital 8-bit output with O7 MSB and O0 LSB

• SOC Start of conversion signal pin

• EOC End of conversion signal pin

• OE Output latch enable pin, if high enables output

• CLK Clock input for ADC



Interfacing Analog to Digital Data

Converters (cont..)• Example: Interfacing ADC 0808 with 8086 using 8255 ports. Use port A of 8255 for transferring digital data output of ADC to the CPU and port C for control signals.• Assume that an analog input is present at I/P2 of the ADC and a clock input of suitable frequency is available for ADC.• • Solution: The analog input I/P2 is used and therefore• address pins A,B,C should be 0,1,0 respectively to select• I/P2. The OE and ALE pins are already kept at +5V to• select the ADC and enable the outputs. Port C upper acts• as the input port to receive the EOC signal while port C• lower acts as the output port to send SOC to the ADC.


• Port A acts as a 8-bit input data port to receive the digital data output from the ADC. The 8255 control word is written

as follows:• D7 D6 D5 D4 D3 D2 D1 D0• 1 0 0 1 1 0 0 0• The required ALP is as follows:• MOV AL, 98h ;initialise 8255 as• OUT CWR, AL ;discussed above.• MOV AL, 02h ;Select I/P2 as analog• OUT Port B, AL ;input.


• MOV AL, 00h ;Give start of conversion• OUT Port C, AL ; pulse to the ADC• MOV AL, 01h• OUT Port C, AL• MOV AL, 00h• OUT Port C, AL• WAIT: IN AL, Port C ;Check for EOC by• RCR ; reading port C upper and• JNC WAIT ;rotating through carry.• IN AL, Port A ;If EOC, read digital equivalent;in AL• HLT ;Stop.


BUS INTERFACES

BUS INTERFACING WITH 8085

• There are three buses in Microprocessor:• 1.Address Bus• 2.Data Bus• 3.Control Bus

Bus interfacing with 8085 diagram

• 1.Address Bus:- Genearlly, Microprocessor has 16 bit address bus. The bus over which the

CPU sends out the address of the memory location is known as Address bus. The address bus carries the address of memory location to be written or to be read from.

• The address bus is unidirectional. It means bits flowing occurs only in one direction, only from microprocessor to peripheral devices.

• We can find that how much memory location it can using the formula 2^N. where N is the number of bits used for address lines.

here, 2^16 = 65536bytes or 64KbSo we can say that it can access upto 64 kb memory location.

• 2.Data Bus:- 8085 Microprocessor has 8 bit data bus. So it can be used to carry the 8

bit data starting from 00000000H(00H) to 11111111H(FFH). Here 'H' tells the Hexadecimal Number. It is bidirectional. These lines are used for data flowing in both direction means data can be transferred or can be received through these lines. The data bus also connects the I/O ports and CPU. The largest number that can appear on the data bus is 11111111.

It has 8 parallel lines of data bus. So it can access upto 2^8 = 256 data bus lines.

• 3.Control Bus:-The control bus is used for sending control signals to the memory and I/O devices. The CPU sends control signal on the control bus to enable the outputs of addressed memory devices or I/O port devices.

Some of the control bus signals are as follows:1.Memory read2.Memory write3.I/O read4.I/O write.

8251A programmable Communication Interface

Introduction 8251A is a USART (Universal Synchronous

Asynchronous Receiver Transmitter) for serial data communication.

Programmable peripheral designed for synchronous /asynchronous serial data communication, packaged in a 28-pin DIP.

Receives parallel data from the CPU & transmits serial data after conversion.

Also receives serial data from the outside & transmits parallel data to the CPU after conversion.

# Block diagram of the 8251 USART

# Sections of 8251A Data Bus buffer Read/Write Control Logic Modem Control Transmitter• Receiver

1. Data Bus Buffer D0-D7 : 8-bit data bus used to read or write status,

command word or data from or to the 8251A

2. Read/Write Control logic

• Includes a control logic, six input signals & three buffer registers: Data register, control register & status register.

• Control logic : Interfaces the chip with MPU, determines the functions of the chip according to the control word in the control register & monitors the data flow.

Input signals

CS – Chip Select : When signal goes low, the 8251A is selected by the MPU for communication.

C/D – Control/Data : When signal is high, the control or status register is addressed; when it is low, data buffer is addressed. (Control register & status register are differentiated by WR and RD signals)

WR : When signal is low, the MPU either writes in the control register or sends output to the data buffer.

RD : When signal goes low, the MPU either reads a status from the status register or accepts data from data buffer.

RESET : A high on this signal reset 8252A & forces it into the idle mode.

CLK : Clock input, usually connected to the system clock for communication with the microprocessor.

Control Register

16-bit register for a control word consist of two independent bytes namely mode word & command word.

Mode word : Specifies the general characteristics of operation such as baud, parity, number of bits etc.

Command word : Enables the data transmission and reception.

Register can be accessed as an output port when the Control/Data pin is high.

Status register

• Checks the ready status of the peripheral.

• Status word in the status register provides the information concerning register status and transmission errors.

Data register Used as an input and output port when the

C/D is lowCS

C/D WR

RD

Operation

00001

0011×

1001×

011 0 ×

MPU reads data from data bufferMPU writes data from data bufferMPU writes a word to control registerMPU reads a word from status registerChip is not selected for any operation

3. Modem Control DSR - Data Set Ready : Checks if the Data Set is

ready when communicating with a modem. DTR - Data Terminal Ready : Indicates that the

device is ready to accept data when the 8251 is communicating with a modem.

CTS - Clear to Send : If its low, the 8251A is enabled to transmit the serial data provided the enable bit in the command byte is set to ‘1’.

RTS - Request to Send Data : Low signal indicates the modem that the receiver is ready to receive a data byte from the modem.

4. Transmitter section Accepts parallel data from MPU & converts them

into serial data. Has two registers:

Buffer register : To hold eight bits Output register : To convert eight bits into a stream of

serial bits.

Transmit control

Output Register

Transmitter Buffer

The MPU writes a byte in the buffer register. Whenever the output register is empty; the contents of

buffer register are transferred to output register. Transmitter section consists of three output & one

input signals TxD - Transmitted Data Output : Output signal to transmit

the data to peripherals TxC - Transmitter Clock Input : Input signal, controls the

rate of transmission. TxRDY - Transmitter Ready : Output signal, indicates the

buffer register is empty and the USART is ready to accept the next data byte.

TxE - Transmitter Empty : Output signal to indicate the output register is empty and the USART is ready to accept the next data byte.

5. Receiver Section

Accepts serial data on the RxD pin and converts them to parallel data.

Has two registers : Receiver input register Buffer register

Receive Buffer

Receive control

Input Register

RxRDY

RxC

RxD

When RxD goes low, the control logic assumes it is a start bit, waits for half bit time, and samples the line again. If the line is still low, the input register accepts the following data, and loads it into buffer register at the rate determined by the receiver clock.

RxRDY - Receiver Ready Output: Output signal, goes high when the USART has a character in the buffer register & is ready to transfer it to the MPU.

RxD - Receive Data Input : Bits are received serially on this line & converted into a parallel byte in the receiver input register.

RxC - Receiver Clock Input : Clock signal that controls the rate at which bits are received by the USART.

# Pin diagram

• D 0 to D 7 (l/O terminal)• This is bidirectional data bus which receive control words and

transmits data from the CPU and sends status words and received data to CPU.

• RESET (Input terminal)• A "High" on this input forces the 8251 into "reset status." The

device waits for the writing of "mode instruction." The min. reset width is six clock inputs during the operating status of CLK.

• RD (Input terminal)• This is the "active low" input terminal which receives a signal

for reading receive data and status words from the 8251.

• TXC (Input terminal)

• This is a clock input signal which determines the transfer speed of transmitted data. In "synchronous mode," the baud rate will be the same as the frequency of TXC. In "asynchronous mode", it is possible to select the baud rate factor by mode instruction. It can be 1, 1/16 or 1/64 the TXC. The falling edge of TXC sifts the serial data out of the 8251.

• CLK (Input terminal)• CLK signal is used to generate internal device timing. CLK

signal is independent of RXC or TXC. However, the frequency of CLK must be greater than 30 times the RXC and TXC at Synchronous mode and Asynchronous "x1" mode, and must be greater than 5 times at Asynchronous "x16" and "x64" mode.

• WR (Input terminal)• This is the "active low" input terminal which receives a signal

for writing transmit data and control words from the CPU into the 8251.

• TXEMPTY (Output terminal)• This is an output terminal which indicates that the 8251 has

transmitted all the characters and had no data character. In "synchronous mode," the terminal is at high level, if transmit data characters are no longer remaining and sync characters are automatically transmitted. If the CPU writes a data character, TXEMPTY will be reset by the leading edge of WR signal. Note : As the transmitter is disabled by setting CTS "High" or command, data written before disable will be sent out. Then TXD and TXEMPTY will be "High". Even if a data is written after disable, that data is not sent out and TXE will be "High".After the transmitter is enabled, it sent out. (Refer to Timing Chart of Transmitter Control and Flag Timing)

• TXRDY (output terminal)• This is an output terminal which indicates that the 8251is

ready to accept a transmitted data character. But the terminal is always at low level if CTS = high or the device was set in "TX disable status" by a command. Note: TXRDY status word indicates that transmit data character is receivable, regardless of CTS or command. If the CPU writes a data character, TXRDY will be reset by the leading edge or WR signal.

• C/D (Input terminal)• This is an input terminal which receives a signal for selecting

data or command words and status words when the 8251 is accessed by the CPU. If C/D = low, data will be accessed. If C/D = high, command word or status word will be accessed.

• CS (Input terminal)• This is the "active low" input terminal which selects the 8251

at low level when the CPU accesses. Note: The device won’t be in "standby status"; only setting CS = High.

• TXD (output terminal)• This is an output terminal for transmitting

data from which serial-converted data is sent out. The device is in "mark status" (high level) after resetting or during a status when transmit is disabled. It is also possible to set the device in "break status" (low level) by a command.

• RXD (input terminal)• This is a terminal which receives serial data.• RXRDY (Output terminal)• This is a terminal which indicates that the 8251 contains a

character that is ready to READ. If the CPU reads a data character, RXRDY will be reset by the leading edge of RD signal. Unless the CPU reads a data character before the next one is received completely, the preceding data will be lost. In such a case, an overrun error flag status word will be set.

• RXC (Input terminal)• This is a clock input signal which determines the transfer speed of

received data. In "synchronous mode," the baud rate is the same as the frequency of RXC. In "asynchronous mode," it is possible to select the baud rate factor by mode instruction. It can be 1, 1/16, 1/64 the RXC.

• SYNDET/BD (Input or output terminal)• This is a terminal whose function changes according to mode. In

"internal synchronous mode." this terminal is at high level, if sync characters are received and synchronized. If a status word is read, the terminal will be reset. In "external synchronous mode, "this is an input terminal. A "High" on this input forces the 8251 to start receiving data characters.

• In "asynchronous mode," this is an output terminal which generates "high level"output upon the detection of a "break" character if receiver data contains a "low-level" space between the stop bits of two continuous characters. The terminal will be reset, if RXD is at high level. After Reset is active, the terminal will be output at low level.

• DSR (Input terminal)• This is an input port for MODEM interface. The input status of

the terminal can be recognized by the CPU reading status words.

• DTR (Output terminal)• This is an output port for MODEM interface. It is possible to

set the status of DTR by a command.• CTS (Input terminal)• This is an input terminal for MODEM interface which is used

for controlling a transmit circuit. The terminal controls data transmission if the device is set in "TX Enable" status by a command. Data is transmitable if the terminal is at low level.

• RTS (Output terminal)• This is an output port for MODEM interface. It is possible to

set the status RTS by a command.

• Control Words• There are two types of control word.• 1. Mode instruction (setting of function)• 2. Command (setting of operation)

1) Mode Instruction

• Mode instruction is used for setting the function of the 8251. Mode instruction will be in "wait for write" at either internal reset or external reset. That is, the writing of a control word after resetting will be recognized as a "mode instruction."

• Items set by mode instruction are as follows:• • Synchronous/asynchronous mode• • Stop bit length (asynchronous mode)• • Character length• • Parity bit• • Baud rate factor (asynchronous mode)• • Internal/external synchronization (synchronous mode)• • Number of synchronous characters (Synchronous mode)

Command

• Command is used for setting the operation of the 8251. It is possible to write a command whenever necessary after writing a mode instruction and sync characters.

• Items to be set by command are as follows:• • Transmit Enable/Disable• • Receive Enable/Disable• • DTR, RTS Output of data.• • Resetting of error flag.• • Sending to break characters• • Internal resetting• • Hunt mode (synchronous mode)

Status Word

Documents

Serial Communication Data Communications Data communications refers to the ability of one computer to exchange data with another computer or a peripheral