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Interfacing

Input/Output mechanisms

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What does interfacing mean?

Interfacing is the process of connecting devices together so that they can exchange information

An interface consists of: physical connections and circuits (hardware) a set of rules and procedures (protocol)

The software routines responsible to send information from one device to another are called software or device drivers

A driver translates a “high-level” command into detailed hardware signals that drives the device

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Interface Standards

An interface standard is a document that dictates some interface details

The purpose and benefits of a standard are to ensure that equipment from one manufacturer can communicate with that of another

Standards are issued by technical associations made up of representatives from government, universities and different companies in industry

IEEE, ANSI , ISO, … Sometimes an interface developed by one

manufacturer becomes so popular that a technical association decide to adopt it as a de facto standard.

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input/output (I/O)

The process of reading input signals and sending output signals is called I/O

The I/O “direction” is defined with respect to the microprocessor

input = read = receive output = write = transmit Generally, the memory interface is not classified as

I/O since memory is considered to be an integral part of the system.

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Memory or input/output mapping

There are two ways to identify and select an external device:memory mappingI/O mapping

In memory mapped I/O, each device has an address just like a memory location.

The 68HC11 uses memory mapped I/O.

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I/O built-in “subsystems”

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The 68HC11 subsystems

Parallel I/O system Serial systems

Synchronous serial I/O subsystemAsynchronous serial I/O subsystem

Programmable Timer I/O subsystem Analog to Digital I/O subsystem

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I/O ports

Each subsystem has chip pins for the I/O associated with it

An I/O port is a collection of pins used to input or output data

Each port has an associated data register An I/O data register holds the most recently read data

from an input port. And for an output port the data register stores the next

data to be sent out to the port

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I/O Subsystem Registers

• The I/O subsystems have three types of registers:

• Control registers, • Status registers,• Data registers

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Generic I/O Subsystem Registers

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Port Replacement Unit (PRU)

For expanded multiplexed mode, ports B and C are used for interfacing memory (addresses and data bus).

As a consequence when the 68HC11 operates in expanded multiplexed mode, it loses the I/O port functions for port B and C

By using a special chip called a PRU the microcontroller recovers the use of ports B and C for I/O.

The MC68HC24 PRU integrated circuit was specifically designed for the 68HC11 operating in expanded mode

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There are two ways of handling the I/O of data among devices: Polling or Interrupts.

Although the two approaches are quite different the steps followed are similar

Interfacing using Polling or Interrupts

• Configuration• Request• Polling• Servicing

• Configuration• Request• Interrupt• Servicing

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Polling Description (1)

Polling is the process of periodically checking a peripheral device to determine whether or not it has requested servicing.

When it finds that a peripheral has requested service, the microcontroller performs the data transfer operation (either read or write).

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Configuration & Request (2)

Configuration: Configuration refers to the step to set up a subsystem initially. Each subsystem has its own control register. A bit in the control register selects polled I/O mode, usually by masking a specific

interrupt. Another control bit may determine how the subsystem recognizes a request. Some subsystems may require additional control bit settings to specify further

variations.

Request: An external peripheral may request service by pulsing an I/O request line. The

request may be for either a read or a write operation. Either a low-to-high (rising edge) transition or a high-to-low (falling edge) transition

on the I/O request line causes a bit called the REQUEST flag in the status register to set (or reset).

It may be possible to program one of the control register bits to determine whether a rising or falling edge causes the action. This would have been done during the configuration stage.

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Polling and Servicing (3)

Polling: To determine whether or not a peripheral actually requires service, the

MCU reads (polls) a status register periodically to check the corresponding REQUEST flag.

If it indicates that an I/O request occurred, the MCU clears (or sets) the bit. This rearms the status register to detect the next request.

Servicing: The MCU executes an I/O service routine to service a peripheral device. For an input or read operation, the peripheral sent data to the I/O port.

This data is passed on to the data register. When an output or write request occurred, the MCU will eventually write

data to the data register. The data register contents are then sent to the I/O port.

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Interrupt Description (1)

When a peripheral device requests service, an interrupt is generated.

The MCU suspends current processing to service the interrupt. The current state of the CPU is stored on the stack.

The interrupt service routine handles the required I/O processing.

Upon completion of the service routine, the MCU resumes the suspended task after restoring the state of the CPU.

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Configuration and Request (2)

Configuration A control register bit selects the interrupt I/O mode. This is

usually done by enabling a specific interrupt. Another bit may specify whether a falling or rising edge triggers

the interrupt (request). Other subsystems may require other control bit settings to be

specified due to other operations.

Request Like polling, an active transition on the request line sets the

REQUEST flag. It also asserts an interrupt.

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Interrupt and Servicing

Interrupt: The MCU stacks the CPU registers. The I bit is set in the condition code register (CCR). If required, it resolves any interrupt priorities between multiple interrupts

and fetches the vector for the highest-priority interrupt. Typically, each peripheral may have an associated interrupt vector.

Servicing: The MCU executes the service routine addressed by the vector. If more than one device could cause the same interrupt, the MCU will poll

other status bits to determine which device caused the interrupt. The MCU clears the REQUEST flag. It then completes the I/O operation, depending on the requirements. After completion, the MCU executes the return from interrupt (RTI)

instruction, and returns to the interrupted program.

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Interfacing using Polling or Interrupts

When an I/O peripheral device requests service, sets a bit called a flag, in a status register.

If the subsystem was configured for interrupts, an interrupt is generated automatically each time the flag sets.

If it is configured for polling, the software must read the status register periodically (poll it) to find out whether or not the flag has set

Although interrupt-driven I/O and polling detect a request differently, they both handle the request the same way

When a request is detected, the service routine access the data register and clears the flag by writing to the status register. It reads the data register for an input operation or writes the data register for an output operation. The writing of the flag can be done before or after accessing the data register.

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Parallel I/O (1)

Parallel I/O refers to capability of sending or receiving multiple data bits simultaneously

Typically, eight bits (one byte) of data, are sent or received at the same time

The 68HC11 has five parallel ports: A through E.

Some ports are permanently configured as either input or output, while others can be configured by the programmer.

Each port has a data register associated with it. (e.g. PORTA: $1000, PORTB: $1004, PORTC: $1003 registers).

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Parallel I/O (2)

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Serial System

Serial I/O refers to the use of a single line to transmit or receive data one bit at a time.

The use of serial I/O reduces the number of lines required to transfer data, but it also reduces the rate of data transfer.

Serial I/O is most often used when transferring data over relatively long distances, such as over phone lines or a network.

There are two basic types of serial systems: synchronous asynchronous

Serial I/O systems have three possible modes of operation: full-duplex – data is received and transmitted at the same time half-duplex – data is received and transmitted but not at the same time simplex – data can only be transmitted or received but not both

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A synchronous serial I/O subsystem is one in which all devices use the same clock.

Synchronous serial systems are typically used when all devices are local, such as on the same circuit board or the same machine.

One device called master controls the data transfer. The other devices are called slaves.

Synchronous Serial I/O (1)

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Synchronous Serial I/O (2)

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In an asynchronous serial I/O subsystem, each device has its own clock.

Asynchronous serial I/O is typically used when the devices are remote. Although the transmitter and receiver use separate clocks, they must run

at the same rate. The rate of bit transfer is known as the baud rate. For each byte of data sent, the transmitter adds two extra bits:

start bit stop bit

The data together with the start and stop bits is called a frame. By convention, the data line is high when idle. The start bit, being a 0,

brings the data line low, signaling the start of transmission. Eight data bits are then sent, followed by the stop bit. The stop bit is a 1, which leaves the data line high, ready to receive the next byte.

In some cases, a parity bit is transmitted along with the eight data bits. The parity bit is used for error checking of the data byte.

Asynchronous Serial I/O (1)

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Prior to transmitting any data, the sender and receiver must agree on a specific serial protocol.

A serial protocol needs to specify the following parameters: baud rate number of data bits start and stop bits parity bit

There are many standards for serial protocols. One of the most common is the RS-232 interface.

Typical applications include computer communication to peripherals such as mice, instruments, printers, or other computers

Asynchronous Serial I/O (2)

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Asynchronous Serial I/O (3)

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Timer I/O Subsystem (1)

By using a timer the CPU does not have to execute software time-delay loops keep track of time

This also allows to measure time more accurately because interrupts will have no effect

The timer subsystem can generate a stream of pulses at different frequencies and pulse widths. It can also generate single pulses.

The system can act as an internal timer without an output to start and stop a task at set times.

The timer inputs are used to measure period (or frequency) and the pulse width of signals.

Another application is to control the speed of a DC motor using a technique called PWM

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Timer I/O Subsystem (2)

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A signal that can be of any value between some maximum and minimum levels is called an analog signal.

Many applications of microcontrollers involve measuring or generating analog signals.

For a microcontroller to use an analog signal, the signal must be converted into digital form.

The 68HC11 has eight built-in A/D, but it does not have a built-in D/A An analog-to-digital converter (A/D) reads an analog voltage or current

and converts it to a binary number to represent its value in volts. Despite the fact that conversions take time, the software that control A/D

converters is usually quite simple

A/D applications: read analog sensors used to measure pressure, temperature, light, battery voltage, etc.

Analog/Digital I/O Subsystem

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Analog/Digital I/O Subsystem

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Manipulating I/O mapped registers

I/O interfacing is usually done using bit-manipulation instructions. However these instructions use only direct or indexed addressing. A common example is to use an index register to point to memory mapped registers

Example:…

BIT0 EQU $01 ;bit 0BIT6 EQU $40 ;bit 6REGBAS EQU $1000 ;start address of register blockPORTC EQU $03 ;port C offsetPORTA EQU $04 ;port A offset

LDX #REGBAS ;point to register blockBRCLR PORTC,X BIT0 NO_LEFTBSET PORTB,X BIT6

…