This document discusses serial communication using the 8051 microcontroller. It begins by introducing serial vs parallel data transfer, communication modes, and framing. It then describes the RS-232 protocol and pinout when using the 8051 UART. Setting the baud rate using Timer 1 is explained. The document provides details on the Serial Control Register and transmitting and receiving data with the serial port. It concludes by discussing using interrupts with the serial port and providing an example of transmitting data from Port 1 and receiving to Port 0 using interrupts.

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

2. 26-Sep-02 2
Serial Comm. - Introduction
l Serial Vs Parallel Transfer of data
l Simplex, Duplex and half-Duplex modes
l Synchronous, Asynchronous, UART, USART
l 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
l Data transfer rate
– Bps, baud
l RS232 protocol
– Non TTL compatible logic levels (-3 to –25 for 1 and +3 to +25
for 0)

3. 26-Sep-02 3
RS232 Pins
l Too many signals, but most are unused in the 8051
UART
l In non-handshaking mode, only three signals
– Pin2 : RxD – received data
– Pin3 : TxD – Transmitted data
– Pin5 : GND
l 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)

4. 26-Sep-02 4
RS232 Line driver
l RS232 uses TLL-incompatible logic levels
l Need Line drivers to interface 8051 to Rs232
protocol
l MAX232, MAX233 most commonly used line
drivers
– Dual channels
– Single supply, +5V operation
– MAX233 needs no external
capacitors
8051 MAX233
DB-9
TxD
RxD
11
10
5
4
2
3
5

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

6. 26-Sep-02 6
Setting the Baud rate
l Timer 1is the timing controller for serial port in 8051
l Clock for the Timer1 in the UART is
– XTAL /12 /32 = 28,800Hz (for XTAL = 11.0592MHz)
l Set SMOD bit (PCON.7) to program 8051 to use 1/16
multiplier
l XTAL / 12 / 16 = 56,700Hz
l Effectively doubles the baud rate
l Timer1 has to be programmed in
– Mode 2, 8bit auto reload mode
– Load TH1 with the required value

7. 26-Sep-02 7
Setting the Baud rate
l TH values (XTAL = 11.0592MHz)
– Baud Rate: 9600 = 28800/3 è TH1 = -3 = 0xFD
– Baud Rate: 4800 = 28800/6 è TH1 = -6 = 0xFA
– Baud Rate: 2400 = 28800/12 è TH1 = -12 = 0xF4
– Baud Rate: 1200 = 28800/24 è TH1 = -24 = 0xE8
l If SMOD (PCON.7) is set then the same values
for TH1 will give
– 19200 etc

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

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

10. 26-Sep-02 10
Examples: Transmit a character
l 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)

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

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

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

14. 26-Sep-02 14
Serial Ports with Interrupts
l General Case
– 8051 is in full duplex mode, I.e. receives and
transmits data continuously
– Both Transmit and Receive is interrupt driven
l 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.

15. 26-Sep-02 15
Example : Simple case
l 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 ;avoid the IVT
org 23H ;serial port ISR
ljmp SERIAL
org 30H
MAIN: mov P1, #0FFH ;P1 as input port
mov TMOD, #20 ;T1 in mode 2
mov TH1, #-3 ;9600 baud
mov SCON, #50H ; 8b, 1start, 1stop
mov IE, #10010000B ;enable S0 interrupt
setb TR1 ;enable T1
BACK: mov A, P1
mov P2, A
sjmp BACK
org 100H
SERIAL: jb TI, TRANS
mov A, SBUF ;copy received data
mov P0, A ;display it on P0
clr RI ;clear RI
RETI
TRANS: clr TI ;do nothing
RETI ;ISR does not handle TX
end

16. 26-Sep-02 16
8051 Dev System Survey
l Paul’s 8051 Dev Board [http://www.pjrc.com]
– 87C52, 22MHz, 2 8255 devices, 50 I/O lines, LCD header, 30K Flash,
32K SRAM, proto area, 2 serial ports, ~80$ assembled
l Axman CME552 and CME562 [http://www.axman.com/]
– S80C552-12 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$
l New Micros [http://www.newmicros.com/]
– Many products, all have proto area, Cost from 40$ to 180$
l Bipom [http://www.bipom.com/]
– Many products, very small, no proto area, uses atmel devices with
onboard Flash and SRAM, ISP, LCD and keypad interface, 40 to 70$
l Tecel [http://www.tecel.com/t1/]
– Many products. Very small, upto 32K SRAM and EEPROM, many I/O
lines, No proto area, $40 to $70. T1 bare board is 12$ !

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