CoE3DJ4 Digital Systems Design Chapter 5: Serial Port Operation
Serial port 8051 includes an on-chip serial port Hardware access to the port is through TXD and RXD (Port 3 bits 1 and 0) Serial port is full duplex (simultaneous transmission and reception) Note: In Mode 0, it is half duplex Serial port has a receiving buffering allowing one character to be received and held in a buffer while a second character is received Two registers SBUF and SCON provide access to the serial port Serial buffer (SBUF) is in fact two buffers one write only for transmission and one read-only for receiving
The SCON Register MSB LSB SM0 SM1 SM2 REN TB8 RB8 TI RI Bit SCON.7 SCON.6 SCON.5 SCON.4 SCON.3 SCON.2 SCON.1 SCON.0 Name SM0 SM1 SM2 REN TB8 RB8 TI RI Description Serial Port Mode bit 0 Serial Port Mode bit 1 Multiprocessor Communication Enable Receive Enable Set to enable reception. CLR to disable reception. Bit 8 of message to transmit Used to transmit optional parity bit Bit 8 of received message Receives optional parity bit Transmit Interrupt Flag Set when Byte in SBUF is completely transmitted. Receive Interrupt Flag Set when a valid byte is received into SBUF
The SCON Register SMO, SM1, SM2 Serial Mode Control Bits SM0 SM1 Mode Baud Rate Description 0 0 0 f osc /12 8-bit shift register 0 1 1 variable 8-bit UART 1 0 2 f osc /32 or f osc /64 9-bit UART 1 1 3 variable 9-bit UART SM2 Multiprocessor Mode Control Bit 1 = Multi-processor mode 0 = Normal mode REN Receiver Enable Bit 1 = Receive Enable 0 = Receive Disabled TB8 9th Transmit Bit Enabled only in modes 2 and 3 RB8 9th Bit Received Used in modes 2 and 3 RI, TI Serial Interrupts RI is set to indicate receipt of a serial word and TI is set to indicate completion of a serial transmission.
Modes Mode of operation of serial port is set by writing to SCON. Serial port has four modes of operation selected by writing 1s and 0s to SM0 and SM1 bits in SCON Mode 0 puts the serial port in 8-bit shift register mode Serial data enter and exit through RXD. TXD serves as clock (outputs shift clock) Eight bits are transmitted or received with the LSB first Baud rate is fixed at 1/12 th of on-chip oscillator Transmission is initiated by any instruction that writes data to SBUF Data are shifted out on RXD line with clock pulses sent out the TXD
Mode 0 Each transmitted bit is valid on RXD pin for one machine cycle Reception is initiated as soon as REN bit is set to 1 and the receive interrupt (RI) bit is cleared. Usually, REN is set at the beginning of the program to initialize the serial port, then RI is cleared to start a data input operation. As soon as RI is cleared, the shift clock will be produced on the TxD pin. At the beginning of the following machine cycle, data will be clocked in from the RxD line. The clocking occurs on the rising edge of the TxD line. After the 8 th clocking cycle, the data is copied to SBUF and the RI bit is set. It is up to the attached circuitry to provide data on RXD line synchronized by the clock signal on TXD.
Mode 1 Mode 1 is an 8-bit UART with variable baud rate UART (universal asynchronous receiver/transmitter): a device that receives and transmits serial data with each data character preceded by a start bit (low) and followed by a stop bit (high) In mode 1, 10 bit are transmitted on TXD or received on RXD These consist of a start bit (always 0), eight data bits (LSB first) and stop bit (always 1) For receive operation, stop bit goes into RB8 in SCON Baud rate is set by timer 1.
Framing An 8-bit message needs to be framed so that the receiver can detect correctly its beginning and end. Standard framing: Start bit always 0, Stop bit always 1. Optional parity bit The message now becomes: Start bit (1 0), LSB,, MSB, <parity bit>, Stop bit (1), Start 0 1 2 3 4 5 6 7 <P> Stop <St> Time
Mode 1 Transmission is initiated by writing to SBUF Shifted data are outputted on the TXD line beginning with start bit and followed by eight bit data bits then stop bit The transmit interrupt (TI) flag is set as soon as the stop bit appears on TxD.
Mode 1 Reception is initiated by a 1 to 0 transition on RXD (assuming REN is 1). The start bit is skipped and eight data bits are clocked into the serial port shift register When all eight bits have been clocked in, the following occur: 1. Ninth bit (stop bit) is clocked into RB8 in SCON 2. SBUF is loaded with eight data bits 3. Receiver interrupt flag (RI) is set These only occur if following conditions exist: 1. RI=0 (ensures software has read the previous character) 2. SM2=0 (applies in multiprocessor communications)
Modes 2 & 3 Mode 2: 9-bit UART with fixed baud rate Eleven bits are transmitted or received: a start bit, eight data bits, a programmable ninth bit and a stop bit On transmission, the ninth bit is whatever has been put in TB8 in SCON On reception, the ninth bit received is placed in RB8 Baud rate in mode 2 is either1/32 nd or 1/64 th on-chip oscillator Mode 3: 9-bit UART with variable baud rate Mode 3 is same as Mode 2 except baud rate is programmable and provided by Timer 1
Initialization and Accessing Receiver enable bit REN in SCON must be set by software to enable reception of characters This is done at the beginning of a program when port is initialized SETB REN MOV SCON, #xxx1xxxxb Ninth data bit transmitted in modes 2 and 3 must be loaded into TB8 by software Ninth bit received is placed in RB8
Initialization and Accessing A common use for ninth data bit is to add parity to a character P bit in PSW is set or cleared every machine cycle to establish even parity with bits in accumulator Example: transmitting eight bits in accumulator with an even parity added in ninth bit: MOV C,P MOV TB8,C MOV SBUF,A Odd parity MOV C,P CPL C MOV TB8,C MOV SBUF, A
Initialization and Accessing RI and TI in SCON play an important role in 8051 serial communications Both bits are set by hardware but must be cleared by software Typically RI is set at the end of character reception and indicates receive buffer full If software wishes to input a character from the device connected to serial port, it must wait until RI is set, then clear RI and read character from SBUF WAIT: JNB RI, WAIT CLR RI MOV A, SBUF
Initialization and Accessing TI is set at the end of character transmission and indicates transmit buffer empty. If software wishes to send a character to the device connected to the serial port, it must first check that the serial port is ready. WAIT: JNB TI, WAIT CLR TI MOV SBUF, A
Baud rate Baud rate is fixed in modes 0 and 2 In mode 0 it is always on-chip oscillator frequency divided by 12 Default baud rate for mode 2 (after system reset) is oscillator frequency divided by 64 Bit 7 of PCON is the SMOD bit Setting SMOD doubles the baud rate in mode 2 to 1/32 nd of oscillator frequency Since PCON is not bit-addressable, setting SMOD has to be done using the following approach: MOV A, PCON SETB ACC.7 MOV PCON,A
Baud rate The baud rates in Modes 1 and 3 are determined by Timer1overflow rate Since timer operates at a relatively high frequency, it is further divided by 32 (or 16 if SMOD=1) before driving the serial port The usual technique for baud rate generation is to initialize TMOD for 8-bit auto-reload mode (mode 3) and put the correct reload value in TH1 to yield proper overflow rate for baud rate
Example: 1200 baud operation: Serial port BAUD RATE=TIMER 1 OVERFLOW RATE / 32 TIMER 1 OVERFLOW RATE =1200*32=38.4kHz If a 12 MHz oscillator is used, timer 1 is clocked at 1 MHZ (1000 khz). Since the timer must overflow at a rate of 38.4 khz and the timer is clocked at 1000 khz, an overflow is required every 1000/38.4=26.04 clocks. Since timer counts up and overflows on FF to 00 transition, 26 counts less than 0 is the required reload value for TH1: MOV TH1, #-26 Table 5-3 gives reload values for different baud rates
Steps to Transmit a Byte 1. Program T1 for Mode2 (TMOD 0x20) 2. Load TH1 and TL1 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 (a wait loop using: JNB TI) 8. Go back to Step5 for next byte
Examples: Transmit a character Transfer ASCII A serially at 9600 baud continuously START: MOV TMOD, #20H ;Put T1 in mode2 MOV TH1, #-7 ;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 ;loop (forever loop)
Steps to Receive a Byte 1. Program T1 for Mode2 (TMOD 0x20) 2. Load TH1 and TL1 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 (a wait loop using JNB RI) 7. Store SBUF (A SBUF) 8. Go back to Step5 for next byte
Example: Receive Data Receive bytes serially and display them on P1, continuously. START: MOV TMOD, #20H ;T1 in mode 2 MOV TH1, #-7 ;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 ;loop forever