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Embedded System Spring, 2011 PIC18 Serial Port Programming in Assembly. Eng. Wazen M. Shbair

Embedded System Spring, 2011 PIC18 Serial Port Programming in Assembly. Eng. Wazen M. Shbair. Today’s Lecture. Explain serial communication protocol Describe data transfer rate and bps rate Interface the PIC18 with an RS232 connector

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Embedded System Spring, 2011 PIC18 Serial Port Programming in Assembly. Eng. Wazen M. Shbair

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  1. Embedded SystemSpring, 2011PIC18 Serial Port Programming in Assembly. Eng. Wazen M. Shbair

  2. Today’s Lecture • Explain serial communication protocol • Describe data transfer rate and bps rate • Interface the PIC18 with an RS232 connector • Describe the main registers used by serial communication of the PIC18 • Program the PIC18 serial port in Assembly IUG- Embedded System

  3. Introduction • Computers transfer data in two ways: Parallel and Serial. • Parallel: Eight or more data lines, few feet only, short time • Serial: Single data line, long distance • The PIC18 has serial communication capability built into it.

  4. Basics of Serial Communication • The byte of data must be converted to serial bits using a parallel-in-serial-out shift register Serial versus Parallel Data Transfer

  5. Basics of Serial Communication (cont’d) • The receiving end must be a serial-in-parallel-out shift register and pack them into a byte. • Two methods of serial data communication: Asynchronous and Synchronous Transfers a block of data at a time Transfers a single byte at a time

  6. Half-and Full-Duplex Transmission IUG-Embedded System

  7. Start and Stop Bits • In the asynchronous method, each character is placed between start and stop bits (framing) MSB LSB Framing ASCII ‘A’ (41H)

  8. Data Transfer Rate • Rate of data transfer: bps(bits per second) • Another widely used terminology for bps is baud rate • For Asynchronous serial data communication, the baud rate is generally limited to 100,000bps

  9. RS232 Standard • Standard for serial comm (COM port) 1: -3V to -25V; 0: +3V to +25V • Reason: for long distance wired line • Input-output voltage are not TTL compatible • So, we need MAX232/233 for voltage converter. Commonly known as line drivers

  10. DB-25 25-Pin Connector DB-9 9-Pin Connector RS232 Pins Connectors: Minimally, 3 wires: RxD, TxD, GND Could have 9-pin or 25-pin

  11. Data in Data out DB-9 9-Pin Connector RS232 Pins (cont’d) IBM PC DB-9 Signals Pin 1 – Data Carrier Detect (DCD) Pin 2 – Received Data (RxD) Pin 3 – Transmitted Data (TxD) Pin 4 – Data Terminal Ready (DTR) Pin 5 – Signal Ground (GND) Pin 6 – Data Set Ready (/DSR) Pin 7 – Request to Send (/RTS) Pin 8 – Clear to Send (/CTS) Pin 9 – Ring Indicator (RI)

  12. PIC18 Connection to RS232 Line driver (a) Inside MAX232 (b) its Connection to the PIC18

  13. Null Modem Connection • Null modem is a communication method to connect two DTEs (computer, terminal, printer etc.) directly using a RS-232 serial cable. • With a null modem connection the transmit and receive lines are cross linked. • Depending on the purpose, sometimes also one or more handshake lines are crosslinked.

  14. PIC18 Serial Port Programming in Assembly • We need to show how to program PIC18 to transfer and receive data using asynchronous mode. • The USART (universal synchronous asynchronous receiver) has both : • Synchronous • Used to transfer data between the PIC and external peripherals (ADC, EEPROMs). • Asynchronous (we concern this type) • Used to connect PIC18 to IBM PC serial port for the purpose of full-duplex. IUG-Embedded System 2011IUG- Embedded System

  15. PIC18 Serial Port Programming in Assembly • In the PIC there are 6 major register used for serial communication • SPBRG ( Serial Port Baud rate Generator ) • TXREG (Transfer Register ) • RCREG (Receiver Register ) • TXSTA (Transmit status and control register) • RCSTA (Receive status and control register) • PIR1 (Peripheral interrupt request register1) IUG-Embedded System 2011IUG- Embedded System

  16. SPBRG Register and Baud Rate in the PIC18 • The baud rate in is programmable • loaded into the SPBRG decides the baud rate • Depend on crystal frequency • BR = FFosc4*16*(X+1)( *For XTAL = 10MHz only!

  17. Baud rate Formula If Fosc = 10MHz X = (156250/Desired Baud Rate) - 1 Example: Desired baud rate = 1200, Clock Frequency = 10MHz X = (156250/1200) – 1 X = 129.21 = 129 = 81H

  18. TXREG Register • 8-bit register used for serial communication in the PIC18 • For a byte of data to be transferred via the Tx pin, it must be placed in the TXREG register first. • The moment a byte is written into TXREG, it is fetched into a non-accessible register TSR MOVFFPORTB, TXREG • The frame contains 10 bits

  19. RCREG Register • 8-bit register used for serial communication in the PIC18 • When the bits are received serially via the Rx pin, the PIC18 deframes them by eliminating the START and STOP bit, making a byte out of data received and then placing it in the RCREG register MOVFFRCREG, PORTB

  20. TXSTA (Transmit Status and Control Register)

  21. RCSTA register • Its 8- bit register used to enable the serial port to receive data, among other things. • We use 8-bit data frame. IUG-Embedded System 2011IUG- Embedded System

  22. PIR1 (Peripheral Interrupt Request Register 1)

  23. Programming the PIC18 to Transfer Data Serially • TXSTA register = 20H: Indicating asynchronous mode with 8-bit data frame, low baud rate and transmit enabled • Set Tx pin an output (RC6) • Loaded SPBRG for baud rate • Enabled the serial port (SPEN = 1 in RCSTA) • The character byte to transmit must be written into TXREG • Keep Monitor TXIF bit • To transmit next character, go to step 5

  24. Example 10.2 ;Write a program for the PIC18 to transfer the letter 'G' serially ;at 9600 baud continuously. Assume XTAL = 10 MHz MOVLW B'00100000' MOVWF TXSTA MOVLW D'15'; 9600 bps MOVWF SPBRG BCF TRISC, TX BSF RCSTA, SPEN OVER MOVLW A'G' S1 BTFSS PIR1, TXIF BRA S1 MOVWF TXREG BRA OVER

  25. TXSTA: Transmit Status and Control Register

  26. Programming the PIC18 to Receive Data Serially • RCSTA register = 90H: To enable the continuous receive in addition to the 8-bit data size option • The TXSTA register = 00H: To choose the low baud rate option • Loaded SPBRG for baud rate • Set Rx pin an input • Keep Monitor RCIF bit • Move RCREG into a safe place • To receive next character, go to step 5

  27. Example 10.4 ;Write a program for the PIC18 to receive data serially and ;put them on PORTB. Set the baud rate at 9600, 8-bit data ;and 1 stop bit MOVLW B'10010000' MOVWF RCSTA MOVLW D'15' MOVWF SPBRG BSF TRISC, RX CLRF TRISB R1 BTFSS PIR1, RCIF BRA R1 MOVFF RCREG, PORTB BRA R1

  28. References • Jie Hu , ECE692 Embedded Computing Systems , Fall 2010. • PIC Microcontroller And Embedded Systems: using Assembly and C for PIC 18, M. Mazidi, R. McKinlay and D. Causey, Prentice Fall, 2008. • Eng. Husam Alzaq, Embedded System Course, IUG, 2010 IUG- Embedded System 28

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