8051 Serial Port. EE4380 Fall02 Class 10. Pari vallal Kannan. Center for Integrated Circuits and Systems University of Texas at Dallas

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1 8051 Serial Port EE4380 Fall02 Class 10 Pari vallal Kannan Center for Integrated Circuits and Systems University of Texas at Dallas

2 Serial Comm. - Introduction Serial Vs Parallel Transfer of data Simplex, Duplex and half-duplex modes Synchronous, Asynchronous, UART, USART Framing Start bit, Stop bit, mark (1 no signal), space (0 no signal) Start bit (0), LSB, MSB, Stop bit (1) Optional parity bit Stop bit can be one or two bits Data transfer rate Bps, baud RS232 protocol Non TTL compatible logic levels (-3 to 25 for 1 and +3 to +25 for 0) 26-Sep-02 2

3 RS232 Pins Too many signals, but most are unused in the 8051 UART In non-handshaking mode, only three signals Pin2 : RxD received data Pin3 : TxD Transmitted data Pin5 : GND For 8051 to PC serial port (COMx) connection, use null-modem connection RxD of 8051 system to TxD of PC TxD of 8051 system to RxD of PC GND to GND Need to set transfer mode to use software flow control (XON/XOF) 26-Sep-02 3

4 RS232 Line driver RS232 uses TLL-incompatible logic levels Need Line drivers to interface 8051 to Rs232 protocol MAX232, MAX233 most commonly used line drivers 8051 MAX233 Dual channels Single supply, +5V operation MAX233 needs no external capacitors TxD RxD DB- 26-Sep-02 4

5 8051 Serial Port 8051 has an internal UART Baud rate is set by Timer1 Control Registers SBUF : Serial Buffer Register Data moved to SBUF is Transmitted serially Serial data Rx-ed is stored by 8051 in SBUF SCON : Serial Control Register Program the mode (start bit, stop bit, data bits length) Only Mode 1 (8, 1, 1) is of interest, as others are obsolete Receive enable REN RI and TI Receive & Transmit Interrupt flags 26-Sep-02 5

6 Setting the Baud rate Timer 1is the timing controller for serial port in 8051 Clock for the Timer1 in the UART is XTAL /12 /32 = 28,800Hz (for XTAL = MHz) Set SMOD bit (PCON.7) to program 8051 to use 1/16 multiplier XTAL / 12 / 16 = 56,700Hz Effectively doubles the baud rate Timer1 has to be programmed in Mode 2, 8bit auto reload mode Load TH1 with the required value 26-Sep-02 6

7 Setting the Baud rate TH values (XTAL = MHz) Baud Rate: 9600 = 28800/3 TH1 = -3 = 0xFD Baud Rate: 4800 = 28800/6 TH1 = -6 = 0xFA Baud Rate: 2400 = 28800/12 TH1 = -12 = 0xF4 Baud Rate: 1200 = 28800/24 TH1 = -24 = 0xE8 If SMOD (PCON.7) is set then the same values for TH1 will give etc 26-Sep-02 7

8 SCON Register SCON.0 = RI Receive interrupt flag. Valid byte is received in SBUF SCON.1 = TI Transmit interrupt flag. Byte in SBUF was completely transmitted. SCON.4 = REN Receive enable. Set to enable reception. Clr for transmit only SCON.7:SCON.6 = SM0:SM1 Serial mode setting 01 = Mode 1 is the widely used mode 8bit data, 1start bit and 1 stop bit All other bits to be set to 0 26-Sep-02 8

9 Steps to Transmit a Byte 1. Program T1 for Mode2 (TMOD 0x20) 2. Load TH1 with the initial value (baud rate dependant) (TH1 FD / FA / F4 / E8) 3. Program SCON for Mode1 (SCON 0x50) 4. Start Timer1 (setb TR1) 5. Clear TI 6. Load SBUF with the byte to be transferred (SBUF byte) 7. Wait until TI becomes 1 (jnb TI, not_done) 8. Go back to Step5 for next byte 26-Sep-02 9

10 Examples: Transmit a character Transfer ASCII A serially at 9600 baud continuously START: mov TMOD, #20H ;T1 is mode2 mov TH1, #-3 ;9600 baud mov SCON, #50H ;8b, 1stop, 1start, REN enabled setb TR1 ;start timer T1 AGAIN: clr TI ;ready to transmit mov SBUF, # A ;letter A is to be transmitted HERE: jnb TI, HERE ;poll TI until all the bits are transmitted sjmp AGAIN ;while(1) loop (forever loop) 26-Sep-02 10

11 Steps to Receive a Byte 1. Program T1 for Mode2 (TMOD 0x20) 2. Load TH1 with the initial value (baud rate dependant) (TH1 FD / FA / F4 / E8) 3. Program SCON for Mode1 (SCON 0x50) 4. Start Timer1 (setb TR1) 5. Clear RI 6. Wait until RI becomes 1 (jnb RI, not_done) 7. Store SBUF (A SBUF) 8. Go back to Step5 for next byte 26-Sep-02 11

12 Example: Receive Data Receive bytes serially and display them on P1, continuously. START: mov TMOD, #20H ;T1 in mode 2 mov TH1, #-3 mov SCON, #50H setb TR1 ;9600 baud ;8b, 1start, 1stop ;start T1 AGAIN: clr RI ;ready to receive a byte HERE: jnb RI, HERE ;wait until one byte is Rx-ed mov A, SBUF mov P1, A ;read the received byte from SBUF ;display on P1 sjmp AGAIN ;while (1) 26-Sep-02 12

13 Serial Ports with Interrupts Using serial port with interrupts is THE way it was intended to be used. Both the RI and TI flags raise the Serial interrupt (S0), if S0 is enabled in IE. ISR for S0 is at 0x0023 Simple Case Transmit is polling based (Poll TI flag) and Receive is interrupt driven Transmit is interrupt driven and Receive is polling based (Poll RI flag) In these cases, the ISR of S0 will check for the appropriate flag and either copy data to or from SBUF 26-Sep-02 13

14 Serial Ports with Interrupts General Case 8051 is in full duplex mode, I.e. receives and transmits data continuously Both Transmit and Receive is interrupt driven Write the ISR for S0 such that ISR must first check which one of RI and TI raised the S0 interrupt If RI is set, then read data from SBUF to a safe place and clear RI If TI is set, then copy the next character to be transmitted onto SBUF and clear TI. 26-Sep-02 14

15 Example : Simple case 8051 gets data from P1 and sends it to P2 continuously while receiving from Serial port. Serial port data is to be displayed on P0 org 0 ljmp MAIN org 23H ljmp SERIAL org 30H ;avoid the IVT ;serial port ISR MAIN: mov P1, #0FFH ;P1 as input port BACK: mov TMOD, #20 ;T1 in mode 2 mov TH1, #-3 mov SCON, #50H ;9600 baud ; 8b, 1start, 1stop mov IE, # B ;enable S0 interrupt setb TR1 mov A, P1 mov P2, A sjmp BACK ;enable T1 SERIAL: org 100H jb TI, TRANS mov A, SBUF ;copy received data mov P0, A clr RI RETI ;display it on P0 ;clear RI TRANS: clr TI ;do nothing RETI end ;ISR does not handle TX 26-Sep-02 15

16 8051 Dev System Survey Paul s 8051 Dev Board [ 87C52, 22MHz, devices, 50 I/O lines, LCD header, 30K Flash, 32K SRAM, proto area, 2 serial ports, ~80$ assembled Axman CME552 and CME562 [ S80C CPU, 32K each of EPROM, EEPROM and SRAM, 8 A/D inputs, 2 PWM outputs, LCD and keypad interface, 20 I/O lines, proto area, ~120$ New Micros [ Many products, all have proto area, Cost from 40$ to 180$ Bipom [ Many products, very small, no proto area, uses atmel devices with onboard Flash and SRAM, ISP, LCD and keypad interface, 40 to 70$ Tecel [ Many products. Very small, upto 32K SRAM and EEPROM, many I/O lines, No proto area, $40 to $70. T1 bare board is 12$! 26-Sep-02 16

17 Next Class Interfacing analog sensors Interfacing Stepper Motors Embedded System Diagnostics 26-Sep-02 17

8051 Timers and Serial Port

8051 Timers and Serial Port EE4380 Fall 2001 Class 10 Pari vallal Kannan Center for Integrated Circuits and Systems University of Texas at Dallas Timer: Mode 1 Operation (recap) 16 bit counter. Load the