I/O Emulated UART Baud Rate Calibration Application Note

Transcription

1 I/O Emulated UART Baud Rate Calibration Application Note 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. 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 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. UART VDD TX RX GND UART VDD RX TX GND. 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: START Bit + Data Bits + STOP Bit, as shown below: AN0475E / January, 0

2 LSB MSB START Bit 0 Bit Bit Bit Bit 4 Bit 5 Bit 6 Bit 7 STOP. 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 400, 9600, 900 and 400 in bit/sec, which can convert to a bit transmission time as shown in the following table: Baud Rate Bit Transmission Time (µs) 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 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. When preparing to transmit, TX is at a high level.. When data transmission begins, TX will be pulled low for a period of bit. (START Bit). The data is transferred in sequence, with the least significant bit (Bit 0) first, followed by Bit, Bit, Bit, Bit 4, Bit 5, Bit 6 and Bit After the data transmission has completed, TX will be pulled high for a period of bit. (STOP Bit) 5. Then the following data can be transferred. Waiting or receiving data bits LSB MSB START Bit 0 Bit Bit Bit Bit 4 Bit 5 Bit 6 Bit 7 STOP AN0475E / January, 0

3 I/O Emulated RX Data Reception Steps. When preparing to receive, RX is at a high level.. 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, otherwise increase the ONE count by. After three consecutive decisions, check whether the ZERO count is greater than or equal to. If it is, indicate that this signal refers to the correct data, otherwise to other signals.. If the correct data has been confirmed, the data will be received in sequence after a bit transmission time delay, with the least significant bit (Bit 0) first, followed by Bit, Bit, Bit, 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 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. Waiting or receiving data bits LSB MSB START Bit 0 Bit Bit Bit Bit 4 Bit 5 Bit 6 Bit 7 STOP RX Receiving Baud Rate Steps Method. When preparing to receive, RX is at a high level.. When RX receives a low signal, start counting and record the count as COUNT.. 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 START Bit 0 Bit Bit Bit Bit 4 Bit 5 Bit 6 Bit 7 STOP AN0475E / January, 0

4 Method. When preparing to receive, RX is at a high level.. When RX receives a low signal, which stands for an incoming START signal, prepare to start counting.. 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 COUNT. 5. When RX receives a high signal, start counting and record the count as COUNT. 6. When RX receives a low signal, start counting and record the count as COUNT. 7. When RX receives a high signal, start counting and record the count as COUNT4.. When RX receives a low signal, start counting and record the count as COUNT5. 9. When RX receives a high signal, start counting and record the count as COUNT6. 0. When RX receives a low signal, start counting and record the count as COUNT7.. When RX receives a high signal, stop counting.. 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. COUNT COUNT COUNT5 COUNT7 START Bit 0 Bit Bit Bit Bit 4 Bit 5 Bit 6 Bit 7 STOP COUNT0 COUNT COUNT4 COUNT6 Method Use the Time Base or the TM to count the bit transmission time, and calculate the data transmission or reception time. Note: The following sample program takes Method as an example. Other methods are not further explained and demonstrated here. AN0475E 4 / January, 0

5 Hardware Description The MCU VDD and GND pins are connected to the MCU VDD and GND pins. The MCU PB5 and PA4 pins, known as the TX and RX pin functions, are connected to the MCU RX and TX pins. MCU VDD PB5 PA4/INT0 GND MCU VDD RX TX GND Software Description The MCU initial setups are shown in the table: Step Operation Content Register Setup Bits Functional Description Setup HIRC operating frequency Setup pins SCC SCC HIRCC HIRCC PAS PBS PAC PBC PAPU PBPU Setup WDT WDTC 4 Setup interrupts INTC0 INTC0 INTC0 INTEG CKS[:0] FHS HIRC[:0] HIRCEN PAS[:0] PBS[:] PAC4 PBC5 PAPU4 PBPU5 WE[4:0] WS[:0] INT0E INT0F EMI INT0S[:0] System clock selection: f H CKS[:0] = 000 High frequency clock selection: HIRC FHS = 0 HIRC frequency selection: MHz HIRC[:0] = 0 HIRC oscillator control: enable HIRCEN = PA4 pin-shared function selection: INT0/PA4 PAS[:0] = 00 PB5 pin-shared function selection: PB5 PBS[:] = 00 PA4 pin type control: input PAC4 = PB5 pin type control: output PBC5 = 0 PA4 pin pull-high resistor control: enable pull-high resistor PAPU4 = PB5 pin pull-high resistor control: enable pull-high resistor PBPU5 = WDT control: enable WE[4:0] = 000 WDT time-out period selection: /f LIRC WS[:0] = External interrupt 0 control: enable INT0E = External interrupt 0 request flag: 0 INT0F = 0 Global interrupt control: enable EMI = External interrupt 0 edge selection: falling edge INT0S[:0] = 0 AN0475E 5 / January, 0

6 The user defined registers are described below: Item Register Bit Number Functional description Note RX_DATA The received data RX_DATA_ITEMS The number of the received data TX_DATA The transmitted data 4 TX_DATA_ITEMS The number of the transmitted data 5 ONE The counting value of keeping "" for half cycle 6 ZERO The counting value of keeping "0" for half cycle 7 RX_FG Reception completion flag COUNT0 Bit 0 counting width 9 COUNT Bit counting width 0 COUNT Bit counting width COUNT Bit counting width COUNT4 Bit 4 counting width COUNT5 Bit 5 counting width 4 COUNT6 Bit 6 counting width 5 COUNT7 Bit 7 counting width 6 COUNT 6 7 BR_ONE_BIT One cycle width BR_HALF_BIT Half cycle width Points to consider: Summation and average of Bit 0 ~ Bit 7 in counting width COUNT_L COUNT_H. 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 00. Additionally, in the receiver and transmitter programs, 0 instructions are required, thus special care must be taken. The reference selections are as follows: System Frequency fsys System Period t SYS(µs) Instruction Cycle Time Instruction Cycle (µs)(4t) Available Instruction Cycles for Different Baud Rates 400 (46.67µs) 9600 (04.7µs) 900 (5.0µs) 400 (6.04µs) 500 (.6µs) M * 4* 4M * M * 6M ** Note: * is limited to 0 instruction cycles and thus cannot be used. ** is limited to 00 instruction cycles and thus cannot be used.. 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: () Correct Signals: AN0475E 6 / January, 0

7 () Error Signals: Flowchart Global Flowchart Step. Initialisation: Clear the RAM data, setup the system frequency and the Watchdog Timer, initialise the I/O and then go to Step. Step. Read the data width and then go to Step. Step. 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 7 / January, 0

8 START INT0 ISR INIT RAM Read data from RX INIT SYSTEM RET INIT IO Measure_BR Calaulate_BR INIT INT INIT UART MAIN Read Data Width Flowchart Step. Determine whether the UART_RX_PIN is "". If yes, indicate that the UART_RX_PIN is ready and then go to Step. If no, indicate that the UART_RX_PIN is not ready and then repeat Step. Step. Determine whether the UART_RX_PIN is "0". If yes, indicate that the UART_RX_PIN is ready and then go to Step. If no, then repeat Step. Step. Determine whether the UART_RX_PIN is "". If yes, then go to Step 4. If no, indicate that the START bit has not transmitted completely and then repeat Step. Step 4. Increase COUNT0 by and then go to Step 5. Step 5. Determine whether the UART_RX_PIN is "0". If yes, then go to Step 6. If no, indicate that Bit 0 has not transmitted completely and then go to Step 4. Step 6. Increase COUNT by and then go to Step 7. Step 7. Determine whether the UART_RX_PIN is "". If yes, then go to Step. If no, indicate that Bit has not transmitted completely and then go to Step 6. Step. Increase COUNT by and then go to Step 9. AN0475E / January, 0

9 Step 9. Determine whether the UART_RX_PIN is "0". If yes, then go to Step 0. If no, indicate that Bit has not transmitted completely and then go to Step. Step 0. Increase COUNT by and then go to Step. Step. Determine whether the UART_RX_PIN is "". If yes, then go to Step. If no, indicate that Bit has not transmitted completely and then go to Step 0. Step. Increase COUNT4 by and then go to Step. Step. Determine whether the UART_RX_PIN is "0". If yes, then go to Step 4. If no, indicate that Bit 4 has not transmitted completely and then go to Step. Step 4. Increase COUNT5 by and then go to Step 5. Step 5. Determine whether the UART_RX_PIN is "". If yes, then go to Step 6. If no, indicate that Bit 5 has not transmitted completely and then go to Step 4. Step 6. Increase COUNT6 by and then go to Step 7. Step 7. Determine whether the UART_RX_PIN is "0". If yes, then go to Step. If no, indicate that Bit 6 has not transmitted completely and then go to Step 6. Step. Increase COUNT7 by and then go to Step 9. Step 9. Determine whether the UART_RX_PIN is "". If yes, then go to Step 0. If no, indicate that Bit 7 has not transmitted completely and then go to Step. Step 0. Return to the main program. AN0475E 9 / January, 0

10 Measure_BR COUNT = COUNT + UART_RX_PIN = =? UART_RX_PIN = =? UART_RX_PIN = = 0? COUNT4 = COUNT4 + UART_RX_PIN = =? UART_RX_PIN = = 0? COUNT5 = COUNT5 + COUNT0 = COUNT0 + UART_RX_PIN = = 0? UART_RX_PIN = =? COUNT6 = COUNT6 + COUNT = COUNT + UART_RX_PIN = =? UART_RX_PIN = = 0? COUNT7 = COUNT7 + COUNT = COUNT + UART_RX_PIN = = 0? UART_RX_PIN = =? RET AN0475E 0 / January, 0

11 Calculate Baud Rate Flowchart Step. Add up all the COUNT0 ~ COUNT7, store the result in COUNT, and then go to Step. Step. Average the COUNT, store the result in COUNT, and then go to Step. Step. Subtract 5 from the COUNT, store the result in BR_ONE_BIT, and then go to Step 4. Step 4. Subtract from the COUNT and divide by, store the result in BR_HALF_BIT, and then go to Step 5. Step 5. Return to the main program. Note:. During data transmission or reception, 0 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.. During a half cycle, three decisions will made to determine whether the START bit is correct. This will take instruction cycles, and in turn the conversion time is. Therefore, the BR_HALF_BIT value will be correct when is subtracted from COUNT. Calaulate_BR COUNT = COUNT0 + COUNT + COUNT + COUNT + COUNT4 + COUNT5 + COUNT6 + COUNT7 COUNT = COUNT / BR_ONE_BIT = COUNT - 05H BR_HALF_BIT =( COUNT - 0H) / RET AN0475E / January, 0

12 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: Versions and Modification Information Date Author Issue Release and Modification 陳淑娟 (Chen, Shu-Juan) First Version Disclaimer All information, trademarks, logos, graphics, videos, audio clips, links and other items appearing on this website ('Information') are for reference only and is subject to change at any time without prior notice and at the discretion of Holtek Semiconductor Inc. (hereinafter 'Holtek', 'the company', 'us', 'we' or 'our'). Whilst Holtek endeavors to ensure the accuracy of the Information on this website, no express or implied warranty is given by Holtek to the accuracy of the Information. Holtek shall bear no responsibility for any incorrectness or leakage. Holtek shall not be liable for any damages (including but not limited to computer virus, system problems or data loss) whatsoever arising in using or in connection with the use of this website by any party. There may be links in this area, which allow you to visit the websites of other companies. These websites are not controlled by Holtek. Holtek will bear no responsibility and no guarantee to whatsoever Information displayed at such sites. Hyperlinks to other websites are at your own risk. AN0475E / January, 0

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