Các vi điều khiển 8051 pps

Scott Mackenzie, The 8051 Microcontroller Serial Port • RXDP3.0 and TXDP3.1 pins • Full Duplex: simultaneous transmission and reception • 2 special function registers: SCONand SBUF • SCO

Chapter 5 Serial Port Operation

The 8051 Microcontroller

Lê Chí Thông Ref I Scott Mackenzie, The 8051 Microcontroller

Serial Port

• RXD(P3.0) and TXD(P3.1) pins

• Full Duplex: simultaneous transmission and reception

• 2 special function registers: SCONand SBUF

• SCON: status bits and control bits

• SBUF: same address but 2 buffers; 1 buffer for transmission and 1 buffer for reception

• Baud rate (serial port frequency of operation) is supplied and programmed by Timer1

Serial port block diagram

Writing to SBUF loads data to be transmitted Reading SBUF accesses received data

P→ → S buffer S→ → P buffer

SCON Register

SCON Register

Mode 0: 8-Bit Shift Register

• RXD is used for both data input and output

• Serial data enter and exit (LSB first) through RXD

• TXD line serves as the clock

• TXDoutputs the shift clock

• Baud rate = 1/12 fOSC

Mode 0: 8-Bit Shift Register

• Transmissionis initiated by instruction that writes data to SBUF (eg MOV SBUF,A)

Mode 0: 8-Bit Shift Register

• Receptionis initiated when REN is 1 and RI is 0

• Set REN at the beginning of a program

• Clear RI to begin a data input operation

Mode 0: 8-Bit Shift Register

• One application of shift register mode is to expand the out capability of the 8051

• A serial-to-parallel shift register IC can be connected to the 8051 TXD and RXD lines to provide an extra output lines

Mode 1: 8-Bit UART with Variable Baud Rate

• UART: Universal Asynchronous Receiver/Transmitter

• A data frame includes a start bit (low), data bits, and a stop bit (high)

• A parity bitis sometimes inserted between the last data bit and the stop bit

• Mode 1:10 bits are transmitted on TXD or received on RXD, including a start bit (0), 8 data bits (LSB first), and a stop bit (1)

• The stop bit goes into RB8 in SCON

• Baud rate is set by the Timer 1 overflow rate

Mode 1: 8-Bit UART with Variable Baud Rate

• Transmission is initiated by writing to SBUF.

• TI is set at the end of character transmission and

indicates “transmit buffer empty”.

WAIT:JNB TI,WAIT ;Check TI until set

MOV SBUF,A ;Send character

synchronization event stop

Mode 1: 8-Bit UART with Variable Baud Rate

• Reception is initiated by a 1-to-0 transition on RXD.

1 The stop bit goes into RB8 in SCON

2 SBUF is loaded with 8 data bits

3 RI is set at the end of character reception and indicates “receiver buffer full”.

• Conditions for reception:

1 RI=0, and

2 SM2=0, or SM2=1 and the received stop bit = 1

WAIT:JNB RI,WAIT ;Check RI until set

MOV A,SBUF ;Read character

Serial Port Baud Rates

1MHz (12 MHz crystal)

To set SMOD:

375K/187.5K (12 MHz crystal)

Using Timer 1 as the Baud Rate Clock

• Usually use Timer 1 Mode 2 to provide baud rate clock

• Baud Rate = Timer 1 overflow rate / 32 (SMOD=0)

• Baud Rate = Timer 1 overflow rate / 16 (SMOD=1)

• Eg Calculate Timer 1 overflow rate to provide 1200 baud operation (12 MHz crystal)

• fOSC= 12 MHz
fCLK= 1 MHz
TCLK= 1 μs

• Assume SMOD=0: Timer 1 overflow rate = 1200 x

32 = 38.4 KHz
Toverflow= 1/38.4 kHz = 26.04 μs

• An overflow requires Toverflow/TCLK≈ 26 clocks

The reload value for Timer 1 is -26

Error in Baud Rate

• Due to rounding, there is a slight error Generally, a 5%

error is tolerable

• Exact baud rates are possible using an 11.0592 MHz crystal

• Eg Calculate Timer 1 overflow rate to provide 1200 baud operation (11.0592 MHz crystal)

• fOSC= 11.0592 MHz
TCLK= 12/11.0592 μs

• Assume SMOD=0: Timer 1 overflow rate = 1200 x

32 = 38.4 KHz
Toverflow= 1/0.0384 [μs]

• An overflow requires Toverflow/TCLK= 24 clocks

The initial value for Timer 1 is -24

Baud Rate Summary

Initialize the Serial Port

ORG 0000H MOV SCON,#01010010B ;Serial port mode 1 MOV TMOD,#00100000B ;Timer 1 mode 2 MOV TH1,#-26 ;reload count for 1200 baud

…

Initialize the Serial Port (SMOD=1)

ORG 0000H MOV SCON,#01010010B ;Serial port mode 1 MOV A,PCON

MOV PCON,A MOV TMOD,#00100000B ;Timer 1 mode 2 MOV TH1,#-26 ;reload count for 2400 baud

…

Example 1: Transmission

ORG 0000H MOV SCON,#01010010B ;Serial port mode 1 MOV TMOD,#00100000B ;Timer 1 mode 2 MOV TH1,#-24 ;reload count for 1200 baud

MOV R2,#10 ;number of loops MOV R0,#30H ;starting address

DJNZ R2,LOOP ;loop 10 times SJMP DONE

SEND: JNB TI,$ ;transmit buffer empty? No:check again

MOV A,SBUF ; send data

DONE: NOP

END

Assume a 10-byte string of data is stored in the internal RAM from the location 30H.

Write a program that sends this string to the 8051 serial port (1200 baud, crystal 11.0592 MHz)

19

Lê Chí Thông Ref I Scott Mackenzie

Example 2: Reception

ORG 0000H MOV SCON,#01010010B ;Serial port mode 1 MOV TMOD,#00100000B ;Timer 1 mode 2 MOV TH1,#-12 ;reload count for 2400 baud

MOV R2,#20 ;number of loops MOV R0,#40H ;starting address LOOP: ACALL RECEIVE ;receive data

DJNZ R2,LOOP ;loop 10 times SJMP DONE

RECEIVE:

JNB RI,$ ;receive buffer full? No: check again

MOV SBUF,A ; send data

DONE: NOP

Write a program that receives a 20-byte string from the 8051 serial port (2400 baud, crystal 11.0592 MHz) and then stores in the internal RAM from the location 40H.

Mode 2: 9-Bit UART with Fixed Baud Rate

• Mode 2:11 bits are transmitted on TXD or received on RXD, including a start bit (0), 9 data bits (LSB first), and a stop bit (1)

• On transmission, the 9thbit is whatever has been put in TB8 in SCON

• On reception, the 9thbit received is placed in RB8

in SCON

• Baud rate is either fOSC/64 (SMOD=0)

or fOSC/32 (SMOD=1)

Mode 3: 9-Bit UART with Variable Baud Rate

• 9-bit UART: same as mode 2

• Variable baud rate: same as mode 1

Adding a Parity Bit

• A common use for the 9thbit is to add parity to a character

• The P bit in PSW register is set or cleared to establish even paritywith 8 bits in A register

• Eg Put even parity bit in TB8, which becomes the 9th

data bit to be transmitted:

MOV C,P ;put even parity bit in C flag MOV TB8,C ;and move to the 9 th data bit MOV SBUF,A;move from A to SBUF to transmit

Adding a Parity Bit

• Eg Put odd parity bit in TB8, which becomes the 9th

data bit to be transmitted:

MOV C,P ;put even parity bit in C flag CPL C ;convert to odd parity

MOV TB8,C ;and move to the 9 th data bit MOV SBUF,A;move from A to SBUF to transmit

Example 3

Assume a 10-byte string of 8-bit ASCII codes is stored in internal RAM from the location 30H Write a program that transmits this string out the 8051 serial port (4800 baud, crystal 11.0592 MHz) with odd parity added as the 9 th bit

ORG 0000H MOV SCON,#11010010B ;Serial port mode 3 (9-bit) MOV TMOD,#00100000B ;Timer 1 mode 2

MOV TH1,#-6 ;reload count for 4800 baud

MOV R2,#10 ;number of loops MOV R0,#30H ;starting address

DJNZ R2,LOOP ;loop 10 times SEND: JNB TI,$ ;check TI empty? No: check again

MOV A,SBUF ; send data

END

Example 4

Assume a 10-byte string of 7-bit ASCII codes is stored in internal RAM from the location 30H Write a program that transmits this string out the 8051 serial port (4800 baud, crystal 11.0592 MHz) with odd parity added as the 8 th bit

ORG 0000H MOV SCON,#01010010B ;Serial port mode 1 (8-bit) MOV TMOD,#00100000B ;Timer 1 mode 2

MOV TH1,#-6 ;reload count for 4800 baud

MOV R2,#10 ;number of loops MOV R0,#30H ;starting address

DJNZ R2,LOOP ;loop 10 times SEND: JNB TI,$ ;check TI empty? No: check again

MOV A,SBUF ; send data

Multiprocessor Communications

• When SM2=1, reception is done only if RB8=1.

• The master first sends out an address byte that has 1 in the 9 th bit So all slave can receive the address byte and examine it to test if it is being addressed.

• The addressed slave will clear its SM2 bit and prepare to receive the data bytes that follow The 9 th bit in data byte is 0

• The slaves that were not addressed leave their SM2 bits set and ignore the incoming data bytes

References

• I Scott Mackenzie, The 8051 Microcontroller

• Các tài liệu trên Internet không trích dẫn hoặc không ghi tác giả