I/O Emulated UART Baud Rate Calibration Application Note · I/O Emulated UART Baud Rate Calibration Application Note . AN0475E 2 / 13 January 11, 2018 . START Bit 0 Bit 1 Bit 2 Bit

I/O Emulated UART Baud Rate Calibration Application Note

AN0475E 1 / 13 January 11, 2018


D/N: AN0475E

Introduction

Not every HOLTEK MCU contains a Universal Asynchronous Receiver/Transmitter

function, otherwise known as a UART. If this function is required, it can be emulated using

software. However, the software accuracy will change according to the system frequency

error. This application note will illustrate how to use a software method to calibrate the

baud rate and how to avoid errors between the two communicating parties. It will also

show how to readjust to the same transmission frequency of each other when

transmission errors occur.

Taking the HT66F4540 as an example, this application note describes the UART baud

rate calibration method.

Functional Description

1. UART data transmission mode: The UART transmission is implemented in a

time-sharing way. The transmitter sends a data bit to the receiver at fixed time

intervals, until all 8 data bits have been transmitted. Note that the receiving speed of

the receiver should be the same as the transmitter; otherwise the received data will be

incorrect.

UART1 UART2

GND

RX

TX

VDD

GND

TX

RX

VDD

2. During data transmission, the UART will send a START (low) signal first to

synchronise with the receiver, after which it will begin to transfer data from the LSB

until the entire byte has been transmitted. After this, it will send a STOP (high) signal

to inform the receiver that the data transmission has finished.

UART data transmission format: 1 START Bit + 8 Data Bits + 1 STOP Bit, as shown

below:


AN0475E 2 / 13 January 11, 2018

START Bit 0 Bit 1 Bit 2 Bit 3 Bit 4 Bit 5 Bit 6 Bit 7 STOP

LSB MSB

3. UART transmission speed: This is measured in the number of transmitting bits per

second, with a unit of bit/sec, commonly known as baud rate or bit rate.

The common baud rates are 2400, 9600, 19200 and 38400 in bit/sec, which can

convert to a 1 bit transmission time as shown in the following table:

Baud Rate 1 Bit Transmission Time (µs)

2400 416.67

9600 104.16

19200 52.08

38400 26.04

Operating Principle

When the UART is waiting for data to be received or transmitted, the RX and TX pins will

remain at a high level. When data reception or transmission begins, these pins are pulled

low for a period of 1 bit, which is used to synchronise with each other as a START bit.

After this the data transmission will start from bit 0. After the completion of an entire data

transmission, the RX and TX pins will go high. Therefore, when the I/O emulated UART

function is used, the RX pin function should be emulated using an external interrupt pin.

This will start to receive data as a falling edge incoming signal. The TX pin function can

be emulated using general I/O pins. However, the RX pin function can use general I/O

pins for their emulation for baud rate detection.

I/O Emulated TX Data Transmission Steps

1. When preparing to transmit, TX is at a high level.

2. When data transmission begins, TX will be pulled low for a period of 1 bit. (START Bit)

3. The data is transferred in sequence, with the least significant bit (Bit 0) first, followed

by Bit 1, Bit 2, Bit 3, Bit 4, Bit 5, Bit 6 and Bit 7.

4. After the data transmission has completed, TX will be pulled high for a period of 1 bit.

(STOP Bit)

5. Then the following data can be transferred.


LSB MSBWaiting or receiving

data bits


AN0475E 3 / 13 January 11, 2018

I/O Emulated RX Data Reception Steps

1. When preparing to receive, RX is at a high level.

2. When the RX receives a falling edge signal, it begins to count for about a 0.4 bit

transmission time to determine whether the signal is low. If it is, increase the ZERO

count by 1, otherwise increase the ONE count by 1. After three consecutive decisions,

check whether the ZERO count is greater than or equal to 2. If it is, indicate that this

signal refers to the correct data, otherwise to other signals.

3. If the correct data has been confirmed, the data will be received in sequence after a 1 bit

transmission time delay, with the least significant bit (Bit 0) first, followed by Bit 1, Bit 2,

Bit 3, Bit 4, Bit 5, Bit 6 and Bit 7. A certain delay time, which includes about 0.5~0.6 bit

time at the beginning and a 1 bit time at the end, is allowed for to capture data at the

center and to avoid fetching data before the present data or after.

4. After the data reception has completed, RX will be pulled high.

5. After this the following data is ready to be received.


LSB MSBWaiting or receiving

data bits

1 2 3

RX Receiving Baud Rate Steps

Method 1


2. When RX receives a low signal, start counting and record the count as COUNT.

3. When RX receives a high signal, stop counting.

4. After the data reception has completed, calculate the data transmission or reception

time using this counting value.

Note: a. This method has to ensure that Bit 0 is high, so that the received data is correct.

b. The counting loop is a group of four instructions.


COUNT


AN0475E 4 / 13 January 11, 2018

Method 2


2. When RX receives a low signal, which stands for an incoming START signal, prepare

to start counting.

3. When RX receives a high signal, start counting and record the count as COUNT0.

4. When RX receives a low signal, start counting and record the count as COUNT1.







11. When RX receives a high signal, stop counting.

12. After the data reception has completed, calculate the data transmission or reception

time using the summation and average values of these counting values

COUNT0~COUNT7.

Note: a. This method must ensure that the data is 55H, so that the received data is correct.

b. The counting loop is a group of four instructions.


COUNT1 COUNT3 COUNT5 COUNT7

COUNT0 COUNT2 COUNT4 COUNT6

Method 3

Use the Time Base or the TM to count the 1 bit transmission time, and calculate the data

transmission or reception time.

Note: The following sample program takes Method 2 as an example. Other methods are

not further explained and demonstrated here.


AN0475E 5 / 13 January 11, 2018

Hardware Description The MCU1 VDD and GND pins are connected to the MCU2 VDD and GND pins.

The MCU1 PB5 and PA4 pins, known as the TX and RX pin functions, are connected to

the MCU2 RX and TX pins.

MCU1 MCU2

GND

PA4/INT0

PB5

VDD

GND

TX

RX

VDD

Software Description

The MCU initial setups are shown in the table:

Step Operation Content Register Setup Bits Functional Description

1 Setup HIRC operating frequency

SCC CKS[2:0] System clock selection: fH CKS[2:0] = 000

SCC FHS High frequency clock selection: HIRC FHS = 0

HIRCC HIRC[1:0] HIRC frequency selection: 8MHz HIRC[1:0] = 10

HIRCC HIRCEN HIRC oscillator control: enable HIRCEN = 1

2 Setup pins

PAS1 PAS1[1:0] PA4 pin-shared function selection: INT0/PA4 PAS1[1:0] = 00

PBS1 PBS1[3:2] PB5 pin-shared function selection: PB5 PBS1[3:2] = 00

PAC PAC4 PA4 pin type control: input PAC4 = 1

PBC PBC5 PB5 pin type control: output PBC5 = 0

PAPU PAPU4 PA4 pin pull-high resistor control: enable pull-high resistor PAPU4 = 1

PBPU PBPU5 PB5 pin pull-high resistor control: enable pull-high resistor PBPU5 = 1

3 Setup WDT WDTC WE[4:0] WDT control: enable

WE[4:0] = 01010

WS[2:0] WDT time-out period selection: 218/fLIRC WS[2:0] = 111

4 Setup interrupts

INTC0 INT0E External interrupt 0 control: enable INT0E = 1

INTC0 INT0F External interrupt 0 request flag: 0 INT0F = 0

INTC0 EMI Global interrupt control: enable EMI = 1

INTEG INT0S[1:0] External interrupt 0 edge selection: falling edge INT0S[1:0] = 01


AN0475E 6 / 13 January 11, 2018

The user defined registers are described below:

Item Register Bit Number Functional description Note 1 RX_DATA 8 The received data 2 RX_DATA_ITEMS 8 The number of the received data 3 TX_DATA 8 The transmitted data 4 TX_DATA_ITEMS 8 The number of the transmitted data 5 ONE 8 The counting value of keeping "1" for half cycle 6 ZERO 8 The counting value of keeping "0" for half cycle 7 RX_FG 1 Reception completion flag 8 COUNT0 8 Bit 0 counting width 9 COUNT1 8 Bit 1 counting width

10 COUNT2 8 Bit 2 counting width 11 COUNT3 8 Bit 3 counting width 12 COUNT4 8 Bit 4 counting width 13 COUNT5 8 Bit 5 counting width 14 COUNT6 8 Bit 6 counting width 15 COUNT7 8 Bit 7 counting width

16 COUNT 16 Summation and average of Bit 0 ~ Bit 7 in counting width

COUNT_L COUNT_H

17 BR_ONE_BIT 8 One cycle width 18 BR_HALF_BIT 8 Half cycle width

Points to consider:

1. When using this program, special attention must be made to the system frequency

and baud rate choices. In the software design only one user defined register is used to

store the baud rate parameters and in the calculations the execution of one LOOP

takes four instruction cycles. As a result, the maximum allowable instruction cycles are

1020. Additionally, in the receiver and transmitter programs, 20 instructions are

required, thus special care must be taken. The reference selections are as follows:

System Frequency

System Period

1 Instruction Cycle Time Available Instruction Cycles for Different Baud Rates

fsys tSYS(µs) Instruction Cycle (µs)(4T)

2400 (416.67µs)

9600 (104.17µs)

19200 (52.08µs)

38400 (26.04µs)

115200 (8.68µs)

2M 0.5 2 208 52 26 13* 4* 4M 0.25 1 416 104 52 26 8* 8M 0.125 0.5 833 208 104 52 17* 16M 0.0625 0.25 1666** 416 208 104 34

Note: * is limited to 20 instruction cycles and thus cannot be used.

** is limited to 1020 instruction cycles and thus cannot be used.

2. When receiving data, a low state bit, known as START, will first be received. However,

there may be noise on the receiver input, so that this low signal is not a START bit. For

this reason, it is necessary to make three decisions in the START bit reception

process. If two or more ZEROs are detected, determine if this signal is correct, and

continue to receive the subsequent signals. But if only one ZERO is detected,

determine that the signal is incorrect and then stop reception. Then jump out and wait

for the next signal. The diagram is shown below:

(1) Correct Signals:

Start Bit 01

2

3

4 6

5 7

8

9 11

10

13

1412

15

Start Bit 01

2

3

4 6

5 7

8

9 11

10

13

1412

15


AN0475E 7 / 13 January 11, 2018

Start Bit 01

2

3

4 6

5 7

8

9 11

10

13

1412

15

Start Bit 01 3 5 7 9 11 13 15

(2) Error Signals:

Start Bit 01

2

3

4 6

5 7

8

9 11

10

13

1412

15

Start Bit 01

2

3

4 6

5 7

8

9 11

10

13

1412

15

Start Bit 01

2

3

4 6

5 7

8

9 11

10

13

1412

15

Start Bit 01

2

3

4 6

5 7

8

9 11

10

13

1412

15

Flowchart

Global Flowchart

Step 1. Initialisation: Clear the RAM data, setup the system frequency and the Watchdog

Timer, initialise the I/O and then go to Step 2.

Step 2. Read the data width and then go to Step 3.

Step 3. Calculate the baud rate and then go to Step 4.

Step 4. Setup the external interrupt 0 enable control bit and the global interrupt enable

control bit, then go to Step 5.

Step 5. Setup the UART related parameters and go to Step 6.

Step 6. Determine whether the RX_FG is set high.

If yes, then go to Step 7.

If no, then go to Step 6.

Step 7. Send back the received data as well as additional data of 0xAA, then go to Step 6.

INT0 ISR: When a falling edge on the INT0 pin occurs, determine whether the received

START bit is correct. If it is, start to receive the data, store the data into RX_DATA and set

RX_FG high. Otherwise, jump out from the interrupt.


AN0475E 8 / 13 January 11, 2018

START

INIT RAM

INIT SYSTEM

INIT IO

Measure_BR

Calaulate_BR

INIT INT

MAIN

INIT UART

INT0 ISR

Read data from RX

RET

Read Data Width Flowchart

Step 1. Determine whether the UART_RX_PIN is "1".

If yes, indicate that the UART_RX_PIN is ready and then go to Step 2.

If no, indicate that the UART_RX_PIN is not ready and then repeat Step 1.


If yes, indicate that the UART_RX_PIN is ready and then go to Step 3.

If no, then repeat Step 2.



If no, indicate that the START bit has not transmitted completely and then repeat

Step 3.

Step 4. Increase COUNT0 by 1 and then go to Step 5.



If no, indicate that Bit 0 has not transmitted completely and then go to Step 4.







AN0475E 9 / 13 January 11, 2018
























Step 20. Return to the main program.


AN0475E 10 / 13 January 11, 2018

UART_RX_PIN = = 1?

UART_RX_PIN = = 0?

YES

NO

Measure_BR

YES

NO

UART_RX_PIN = = 1?

UART_RX_PIN = = 0?

YES

NO

YES

NO

COUNT0 = COUNT0 + 1

UART_RX_PIN = = 1?

YES

NO

COUNT1 = COUNT1 + 1

UART_RX_PIN = = 0?

YES

NO

COUNT2 = COUNT2 + 1

UART_RX_PIN = = 1?

YES

NO

COUNT3 = COUNT3 + 1

UART_RX_PIN = = 0?

YES

NO

COUNT4 = COUNT4 + 1

UART_RX_PIN = = 1?

YES

NO

COUNT5 = COUNT5 + 1

UART_RX_PIN = = 0?

YES

NO

COUNT6 = COUNT6 + 1

UART_RX_PIN = = 1?

YES

NO

COUNT7 = COUNT7 + 1

RET


AN0475E 11 / 13 January 11, 2018

Calculate Baud Rate Flowchart

Step 1. Add up all the COUNT0 ~ COUNT7, store the result in COUNT, and then go to

Step 2.

Step 2. Average the COUNT, store the result in COUNT, and then go to Step 3.

Step 3. Subtract 5 from the COUNT, store the result in BR_ONE_BIT, and then go to Step 4.

Step 4. Subtract 8 from the COUNT and divide by 2, store the result in BR_HALF_BIT,

and then go to Step 5.

Step 5. Return to the main program.

Note:

1. During data transmission or reception, 20 instruction cycles are required, and in turn

the conversion time is 5. Therefore, the BR_ONE_BIT value will be correct when 5 is

subtracted from COUNT.

2. During a half cycle, three decisions will made to determine whether the START bit is

correct. This will take 32 instruction cycles, and in turn the conversion time is 8.

Therefore, the BR_HALF_BIT value will be correct when 8 is subtracted from COUNT.

Calaulate_BR

COUNT = COUNT0 + COUNT1 + COUNT2 + COUNT3 + COUNT4 +

COUNT5 + COUNT6 + COUNT7

COUNT = COUNT / 8

BR_ONE_BIT = COUNT - 05H

BR_HALF_BIT =( COUNT - 08H) / 2

RET


AN0475E 12 / 13 January 11, 2018

Conclusion

This application note has mainly introduced how to calibrate the baud rate to ensure

correct data transmission when using I/O to emulate the UART function. When UART

data transmission begins, the signal will first be pulled low as a START signal and thus

used as the data transmission width. However, problems will occur if the detection of the

LSB is not high. Therefore, two methods are provided to compensate, one is to set the

LSB high; the other is to set the data bits to 55H. Both of these have advantages and

disadvantages. The first method will create a large error. The second method has a

smaller error due to the detection of eight data bits, but because there is no fixed baud

rate, the other communicating party must first send data to settle its transmission speed

when in use.

References

Reference document: HT66F45x0 Data Sheet.

For more information, refer to the Holtek’s official website: http://www.holtek.com/en/.

Versions and Modification Information

Date Author Issue Release and Modification

2017.12.01 陳淑娟 (Chen, Shu-Juan) First Version

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AN0475E 13 / 13 January 11, 2018

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Documents

I/O Emulated UART Baud Rate Calibration Application Note · I/O Emulated UART Baud Rate Calibration Application Note . AN0475E 2 / 13 January 11, 2018 . START Bit 0 Bit 1 Bit 2 Bit