EET203 MICROCONTROLLER SYSTEMS DESIGN Serial Port Interfacing

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1 EET203 MICROCONTROLLER SYSTEMS DESIGN Serial Port Interfacing

2 Objectives Explain serial communication protocol Describe data transfer rate and bps rate Describe the main registers used by serial communication of the PIC16 Program the PIC16 serial port in C language Interface the PIC16 with an RS232 connector

3 Communication There are two options to differentiate when speaking about transmission of information on the transmission lines: Serial Communication Parallel Communication

4 Types of Communication In addition to the serial and parallel communications, there are 2 types of communication we will explore: Synchronous communication When using the synchronous communication the information is transmitted from the transmitter to the receiver: in sequence bit after bit with fixed baud rate and the clock frequency is transmitted along with the bits That means that the transmitter and the receiver are synchronized between them by the same clock frequency. The clock frequency can be transmitted along with the information, while it is encoded in the information itself, or in many cases there is an additional wire for the clock. This type of communication is faster compare to the asynchronous communication since it is "constantly transmitting the information, with no stops.

5 Types of Communication Asynchronous communication When using the asynchronous communication - the transmitter and the receiver refraining to transmit long sequences of bits because there isn't a full synchronization between the transmitter, that sends the data, and the receiver, that receives the data. In this case, the information is divided into frames, in the size of byte. Each one of the frame has: Start bit marks the beginning of a new frame. Stop bit marks the end of the frame. Frames of information must not necessarily be transmitted at equal time space, since they are independent of the clock.

6 Enabling Serial Communication To communicate with external components such as computers or microcontrollers, the PIC micro uses a component called USART - Universal Synchronous Asynchronous Receiver Transmitter. This component can be configured as: a Full-Duplex asynchronous system that can communicate with peripheral devices, such as CRT terminals and personal computers a Half-Duplex synchronous system that can communicate with peripheral devices, such as A/D or D/A integrated circuits, serial EEPROMs, etc.

7 Enabling Serial Communication To enable the serial communication with PIC micro we must set different parameters within two registers: TXSTA - Transmit Status and Control Register RCSTA - Receive Status and Control Register

8 USART Transmit Block Diagram

9 USART Receive Block Diagram

10 Baud Rate Calculation BAUD -baud rate bps -units in which we are measuring pace of transmission PIC transfers and receives data serially at many different baud rates. The baud rate in the PIC16 is programmable The value loaded into the 8-bit register SPBRG decides the baud rate Depend on crystal frequency Baud Rate SPBRG (Hex Value) XTAL = 10MHz F SPBRG (Hex Value) XTAL = 20MHz

11 Baud Rate Calculation Desired Baud Rate = Fosc/64 (X+1) X: value loaded in SPBRG reg. XTAL oscillator Example: 10 MHz 20 MHz 4 Instruction cycle freq 2.5 MHz 5.0 MHz 16 by UART Desired Baud Rate = Fosc/64 (X+1) X = (Fosc/(64*Desired Baud Rate)) 1 To UART to set the baud rate 156,250 Hz Hz Note: Dividing the instruction cycle frequency by 16 is the setting upon Reset. Can get a higher baud rate by making bit BRGH = 1 in TXSTA register

12 Baud Rate Calculation With Fosc = 10 MHz, find the SPBRG value needed to have the following baud rates: (a) 9600 (b) 4800 (c) 2400 (d) 1200 Solution: Desired Baud Rate = Fosc/64 (X+1) X = (10MHz/(64*Desired Baud Rate)) 1 (a) X = (10M/(64*9600)) 1 = = 15 = F (hex) (b) X = (10M/(64*4800)) 1 = = 32 = 20 (hex) (c) X = (10M/(64*2400)) 1 = = 64 = 40 (hex) (d) X = (10M/(64*1200)) 1 = = 129 = 81 (hex)

13 Baud Rate Calculation With Fosc = 20 MHz, find the SPBRG value needed to have the following baud rates: (a) 9600 (b) 4800 (c) 2400 (d) 1200 Solution: Desired Baud Rate = Fosc/64 (X+1) X = (Fosc/(64*Desired Baud Rate)) 1 (a) X = (b) X = (c) X = (d) X =

14 Quadrupling the baud rate in PIC16 There are two ways to increase the baud rate of data transfer in PIC16: 1. Use a higher frequency crystal not feasible! 2. Change a bit in TXSTA register Option 2 can be used while the crystal frequency stays the same. Change bit of TXSTA<2>: BRGH bit Upon Reset, BRGH = 0 Low Speed When BRGH = 1 High Speed Baud rate is quadrupled with this method!

15 Quadrupling the baud rate in PIC16 (cont d) Baud rates for BRGH = 0 Default when PIC is powered up. PIC divides Fosc/4 by 16 once more. Use that frequency for UART to set the baud rate. Example: XTAL = 10 MHz Instruction cycle freq = 10 MHz/4 = 2.5 MHz And 2.5 MHz/16 = 156,250 Hz This is the frequency used by UART to set the baud rate for BRGH = 0 The baud rate for BRGH = 0 was discussed in previous slides.

16 Quadrupling the baud rate in PIC16 (cont d) Baud rates for BRGH = 1 With fixed crystal frequency, the baud rate can be quadrupled PIC divides Fosc/4 by 4 once more. Use that frequency for UART to set the baud rate. Example: XTAL = 10 MHz Instruction cycle freq = 10 MHz/4 = 2.5 MHz And 2.5 MHz/4 = 625,000 Hz This is the frequency used by UART to set the baud rate for BRGH = 1

17 Quadrupling the baud rate in PIC16 (cont d) X = (Fosc/(16*Desired Baud Rate)) 1 SPBRG Values for Various Baud Rates (BRGH = 1) SPBRG (Hex Value) SPBRG (Hex Value) Baud Rate XTAL = 10MHz XTAL = 20MHz A F X: value loaded in SPBRG reg.

18 Quadrupling the baud rate in PIC16 (cont d) For XTAL = 10 MHz, find the SPBRG value (in both decimal and hex) to set the baud rate to each of the following: (a) 9600 if BRGH = 1 (b) 4800 if BRGH = 1 Solution: X = (fosc/(16*desired Baud Rate)) 1 XTAL = 10 MHz, Fosc/4 = 2.5 MHz, BRGH = 1 UART frequency = 2.5 MHz/4 = 625,000 Hz (a) X = (625,500/9600) 1 = 64 D = 40 H (b) X = (625,500/4800) 1 = 129 D = 81 H XTAL oscillator Example: 10 MHz 4 Instruction cycle freq 2.5 MHz BRGH = 1 4 BRGH = ,500 Hz To UART to set the baud rate 156,250 Hz

19 Quadrupling the baud rate in PIC16 (cont d) For XTAL = 10 MHz, find the SPBRG value (in both decimal and hex) to set the baud rate to each of the following: (a) 9600 if BRGH = 1 (b) 4800 if BRGH = 1 Solution: X = (fosc/(16*desired Baud Rate)) 1 XTAL = 20 MHz, Fosc/4 = 5.0 MHz, BRGH = 1 UART frequency = 5.0 MHz/4 = 1,250,000 Hz (a) X = (b) X = XTAL oscillator Example: 20 MHz 4 Instruction cycle freq 5 MHz BRGH = 1 4 BRGH = ,250,000 Hz To UART to set the baud rate 312,500 Hz

20 PIC16 Serial Port Programming Only asynchronous mode and full-duplex serial data communication will be covered. In PIC uc, six major registers are associated with the USART (universal synchronous asynchronous receiver): SPBRG: Serial Port Baud Rate Generator TXREG: Transfer Register RCREG: Receive Register TXSTA: Transmit Status and Control Register RCSTA: Receive Status and Control Register PIR1: Peripheral Interrupt Request Register1

21 TXREG Register 8-bit register used for serial communication in the PIC16 For a byte of data to be transferred via the TX pin, it must be placed in the TXREG register The moment a byte is written into TXREG, the 8-bit data is framed with the START and STOP bits. The 10-bit data is transferred serially via TX pin.

22 RCREG Register 8-bit register used for serial communication in the PIC16 When the bits are received serially via the RX pin, the PIC16 de-frames them by eliminating the START and STOP bit. Making a byte out of data received and then placing in the RCREG register

23 TXSTA Register Transmit Status and Control Register 8-bit register Used to select the synchronous/asynchronous modes and data framing size. BRGH bit: select a higher speed for transmission. Default is lower baud rate transmission D6 of TXSTA: determines the framing of data by specifying the number of bits per character

25 RCSTA Register Receive Status and Control Register 8-bit register Used to enable the serial port to receive data.

27 PIR1 Register Peripheral Interrupt Request Register 1 8-bit register TWO (2) of PIR1 register bits are used by UART: TXIF (transmit interrupt flag) RCIF (receive interrupt flag) We monitor (poll) the TXIF flag bit to make sure that all the bits of the last byte are transmitted before we write another byte into TXREG. By the same logic, we monitor the RCIF flag bit to see if a byte of data has come in yet.

28 PIR1 Register

29 PIR1 Register (cont d)

30 Programming the PIC16 to Transfer Data Serially 1. Load TXSTA register = 20H Indicating asynchronous mode with 8-bit data frame, low baud rate and transmit enabled 2. Make TX pin (PORTC6) an output 3. Load SPBRG to set the baud rate 4. Enabled the serial port: RCSTA<7> (SPEN = 1) 5. Character byte to be transmit is written into TXREG 6. Keep monitor TXIF flag bit of PIR1 register to make sure UART is ready for next byte 7. To transmit next character, go to Step 5

31 Example 1 Write a C program for the PIC16 to transfer the letter 'G' serially at 9600 baud continuously. Assume XTAL = 10 MHz #include <htc.h> _CONFIG ; #define _XTAL PIC configuration void main (void) { TRISC = 0x00; // TX pin (PORTC6) as output TXSTA = 0x20; // 0b ; choose low baud rate, 8-bit, asynchronous mode SPBRG = 15; // or 0x0F; 9600 baud rate, XTAL = 10 MHz TXEN = 1; // enable transmit SPEN = 1; // enable serial port while(1) { TXREG = G ; // place value in buffer while(txif==0); // wait until transmitted } }

32 Write a C program to transfer the message YES serially Example 2 at 9600 baud, 8-bit data, and 1 STOP bit. Do this continuously. Assume XTAL = 10 MHz // PIC configuration void SerTx(unsigned char); // function for sending character void main(void) { TRISC = 0x00; // TX pin (PORTC6) as output TXSTA = 0x20; // 0b ; choose low baud rate, 8-bit, asynchronous mode SPBRG = 15; // or 0x0F; 9600 baud rate, XTAL = 10 MHz TXEN = 1; // enable transmit SPEN = 1; // enable serial port while(1) { SerTx( Y ); SerTx( E ); SerTx( S ); } } void SerTx(unsigned char c) { while(txif==0); // wait for transmitted TXREG = c; // place character in buffer }

33 Example 2 (cont d) // PIC configuration void main(void) { TRISC = 0x00; // TX pin (PORTC6) as output TXSTA = 0x20; // 0b ; choose low baud rate, 8-bit, asynchronous mode SPBRG = 15; // or 0x0F; 9600 baud rate, XTAL = 10 MHz TXEN = 1; // enable transmit SPEN = 1; // enable serial port unsigned char z; unsigned char Mess[] = YES ; while(1) { for (z=0; z<3; z++) // write message { while(txif==0); // wait for transmit TXREG = Mess[z]; // place char in buffer } } }

34 Example 3 Write a C program to send the two messages Normal Speed and High Speed to the serial port. Assuming that SW is connected to pin PORTB0, monitor its status and set the baud rate as follows: SW = 0 : 9600 baud rate SW = 1 : baud rate Assume that XTAL = 10 MHz for both cases. Solution: BRGH = 0 BRGH = 1 UART freq = 156,250 Hz UART freq = 625,000 Hz X = (156,250/9600) - 1 = 15 D X = (625,000/38400) - 1 = 15 D

35 Example 3 (cont d) // PIC configuration #define SW RB0; // PORTB0 as input switch SW void main (void) { TRISB = 0x01; TRISC = 0x00; TXSTA = 0x20; SPBRG = 15; TXEN = 1; SPEN = 1; // PORTB0 as input switch // TX pin (PORTC6) as output // 0b ; choose low baud rate, 8-bit, asynchronous mode // 9600 baud rate, XTAL = 10 MHz // enable transmit // enable serial port unsigned char z; unsigned char Mess1[] = Normal Speed ; unsigned char Mess2[] = High Speed ;

36 Example 3 (cont d) } if (SW==0) { for (z=0; z<12; z++) { while(txif==0); TXREG = Mess1[z]; } } else { TXSTA = TXSTA 0x04; for (z=0; z<10; z++) { while(txif==0); TXREG = Mess2[z]; } } while(1); // stay in loop // wait for transmit // place value in buffer // for high speed (BRGH=1) // wait for transmit // place value in buffer

37 Programming the PIC16 to Receive Data Serially 1. Load RCSTA register = 90H: To enable the continuous receive in addition to the 8-bit data size option 2. Load TXSTA register = 00H: To choose the low baud rate option 3. Load SPBRG to set the baud rate 4. Make RX pin (PORTC7) an input 5. Keep monitor RCIF flag bit of PIR1 register for a HIGH to see if an entire character has been received yet 6. When RCIF = 1, RCREG has a byte, its contents are moved into a safe place 7. To receive next character, go to step 5

38 Example 4 Write a C program to receive bytes of data serially and put them on PORTB. Set the baud rate at 9600, 8-bit data, and 1 STOP bit. // PIC configuration void main(void) { TRICS = 0x00; // set all pin at port B as output TRISC = 0x80; // RX pin (PORTC7) as input, TX pin (PORTC6) as output RCSTA = 0x90; // 0b ; Usart Enable, Continuous receive enable TXSTA = 0x00; // 0b ; choose low baud rate, 8-bit, asynchronous mode SPBRG = 15; // or 0x0F; 9600 baud rate, XTAL = 10 MHz TXEN = 0; // disable transmit SPEN = 1; // enable serial port unsigned char z; while(1) { while(rxif==0); // wait for receive z = RCREG; PORTB = z; // read data // send data to port B } }

39 Hardware Interfacing To communicate over UART or USART, we just need three basic signals which are namely, i) RXD (receive) ii) TXD (transmit) iii) GND (common ground)

40 Hardware Interfacing

41 Summary Serial Communication- data is sent one bit a time, is used in situations where data is sent over significant distances because in parallel communications, where data is sent a byte or more a time, great distances can cause distortion of the data. Additional advantage- allowing transmission over phone lines Asynchronous communication - data is sent one byte at a time and Synchronous communication - data is sent in blocks of bytes Simplex - can sent but cannot receive, Half duplex - can send and receive, but not at the same time and Full duplex - can send and receive at the same time RS232 - standard foe serial communication connectors

42 End of Chapter Life is not so short but that there is always time enough for courtesy Quote by Ralph Waldo Emerson A river cuts the rock not because of its power but because of its consistency. Never lose your hope and keep walking towards your vision!

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