微算機原理與實驗 - National Chiao Tung...

Principle of Microcomputer Serial Port 1

Principle of Microcomputer（UEE 2301/1071 ）

Chap 10. MCS-51 Serial Port Operation

微算機原理與實驗

宋開泰

Office：EE709 Phone：5731865（校內分機：31865）

E-mail：[email protected] URL：http://isci.cn.nctu.edu.tw


Serial Port Operation

Outlines:

• Serial I/O Protocols

• Asynchronous Protocols (RS-232)

• MCS-51 Serial Port Operation &

Programming


Terminal or

Computer Data Terminal

Equipment Modem

Modem DTE

Terminal or

Computer

Transmitter

Receiver

( Phone line)

Digital Data

(Protocol is needed)

Analog Data

Digital Data

DTE

DCE

DTE DCE

System architecture of serial communication


RS-232 Interface

• DTE: Data Terminal Equipment (資料終端裝置)

• DCE: Data Communication Equipment(資料通訊裝置)

• MODEM: Modulator/ Demodulator

• RS-232 is a standard originally for providing a

specification for connecting remote devices

over the telephone networks through

modems. Later (as now in wide use) it is used

for terminal and equipment without using

modems.


RS-232 Signals(DB-25連結器)

Pin 2: TxD Transmit Data, transmits data from the DTE to DCE

Pin 3: RxD Receive Data, transmits data from the DCE to DTE

Pin 7: Signal Ground (Common)

Pin 4: RTS* Request To Send, general purpose output

Pin 5: CTS* Clear To Send, general purpose input

Pin 6: DSR* Data Set Ready, general purpose input, to signal to

the DTE that the DCE has been powered up and is ready

to go

Pin 20:DTR* Data Terminal Ready, general purpose output

Pin 8 :DCD* Data Carrier Detect, on DTE, it is frequently used

to disable data reception

Pin 1: Shield Ground,

if it is safe to connect the chassis ground

together

DB-9型連結器

DB-25連結器


Interfacing a PC and a Modem


Null Modem Connection


Examples of RS-232 Connection


Logic Level of RS-232 and TTL Logic


Serial link transceiver MAX232


Serial link structure


ASCII Code


Asynchronous Protocols

Electrical conventions: RS-232

• Timing convention :

used frequency: 300, 600, …, 9600, 19200 = BAUD rate

↓ ↓

↑

LSB MSB

Idle or

prior stop Start bit 7 data bits 1 parity bit One or two

stop bits



• Least significant bit (LSB) first

pattern (in time) 10000010

recognized as 01000001 = 41H = “A” ASCII code

• Start and stop bit: identify the beginning and end of each transfer; permit a receiver to resynchronize a local clock to each character.

• Receiver samples bits at the center of the bit period,

plus a good estimation of the leading transition on the start bit, plus both clocks to be accurate within 5﹪error margin (no problem with crystal clocks).

• To make the leading edge discoverable, the start bit is made different from both the idle state and the stop bits.

• Fast clock is used to determine the center of the start bit, 16× clock time base.



• Use a start and stop bit of opposite polarity

→ 20 ﹪ overhead

8 bits 16 bits 16 bits 16 bits

start bit


Serial port clocking


Parameters for setting up a serial port

1. The number of data bits/character

2. The number of stop bits

3. The parity bit

4. Baud rate


8051 Serial Port Operation

• MCS-51 has an on-chip serial port, the internal

structure of this serial port is shown below:

8051 Internal bus



• Full duplex, can simultaneous transmission and

reception.

• Receive buffering, allowing one character to be

received and held in a buffer while a second

character is received. The first byte (character) has to

be read before the second byte is received (with

MOV A, SBUF).

• Two special function registers provide software

access to the serial port SBUF and SCON.

• Both receiving and transmission of serial data are

through serial port buffer (SBUF, 99H).



• Write to SBUF loads data to be transmitted.

MOV SBUF, A

• Reading SBUF accesses received data

MOV A, SBUF

Note: SBUF in these instructions has the same name

and address, but they are two separate and distinct

registers, the transmit write-only register and the

receive read-only register.


SCON (98H): Serial Port Control Register (1/3)

The mode of the operation of the 8051 serial port is set

by writing to the serial port control register SCON (98H).


SCON (98H): Serial Port Control Register(2/3)

• RI (SCON. 0): Receive Interrupt flag

– Set by hardware and must be cleared by software

– Set at the end of character reception

– Indicate “receive buffer full”

– This condition is tested by software or

programmed to cause an interrupt

WAIT: JNB RI, WAIT ; Check RI until set

CLR RI

; Clear RI

MOV A, SBUF

; Read character


SCON (98H): Serial Port Control Register(3/3)

• TI (SCON. 1): Transmit Interrupt flag

– Set by hardware and must be cleared by software

– Set at the end of character transmission

– Indicate “transmit buffer empty”

– If a previous character was sent, wait until

transmission is finished before sending the next

character. WAIT: JNB TI, WAIT ; Check TI until set

CLR TI

; Clear TI

MOV SBUF, A

; Send character


Setting of the Transmission Interrupt (TI) flag


SCON (98H): Serial Port Control Register

• RB8 (SCON. 2) Receive bit 8

– In mode 2 and mode 3, store the ninth data

bit received.

• TB8 (SCON. 3) Transmit bit 8

– In mode 2 and mode 3, the to be

transmitted ninth data bit is placed in TB8

– The bit content is set by software


SCON (98H): Serial Port Control Register

• REN (SCON. 4): Receive enable control bit

– 0: receive disabled

– 1: receive enable

– The bit content is set by software

• SM2 (SCON. 5): Serial port mode control bit 2

– Enables multiprocessor communications in modes

2 and 3.

– In mode 2 and mode 3,

when SM2 = 1

RI = 0 if RB8 = 0,

RI = 1 if RB8 = 1.


Mode of Operation

• The 8051 serial port has four modes of operation,

selectable by writing 1s or 0s to the SM0 (SCON. 7)

and SM1 (SCON. 6) bits in SCON.


8-bit Shift Register (Mode 0)

• Synchronous operation mode.

• 8-bit receiving and transmission serial data, no start bit and stop bit.

• Serial data enter and exit through RxD (P3.0, pin 10)

• TxD (P3.1, pin 11) outputs the shift clock (= fosc/12)


Mode 0 & a shift register


Clocking of MCS-51 Serial Port


8-bit UART with Variable Band Rate (Mode1)

• In mode 1, 8051 serial port operates as an 8-bit UART

(Universal asynchronous receiver/transmitter) with

variable baud rate.

• TxD (P3.1, pin 11) transmits serial data, RxD (P3.0, pin

10) receives serial data.

• 10 bits are transmitted or received. This consists of a

start bit (always 0), 8 data bits (LSB first) and a stop bit

(always 1).


9-bit UART with Fixed Baud Rate (Mode 2)

• When SM1 = 1, SM0 = 0, the serial port operates in mode 2, a

9-bit UART with a fixed baud rate.

• Serial data consists of 11 bits

– 1 start bit

– 8 data bits

– 1 programmable 9th data bit (TB8)

– 1 stop bit.

• On transmission, the ninth data bit is whatever has been put in

TB8 (SCON. 3)

• On reception, the ninth data bit received is put into RB8(SCON.

2)

• The baud rate in mode 2 is either 1/32 or 1/64 of the on-chip

oscillator frequency.


9-Bit UART with Variable Baud Rate (Mode 3)

• The operation of Mode 3 is the same as

Mode 2, except the baud rate is

programmable and provided by the timer.

• Baud rate: variable

– 8051: programmed by the overflow rate of

Timer 1

– 8052: programmed by the overflow rate of

Timer1 or Timer2


Serial Port Baud Rates(1/4)

1. Mode 0 Baud rate

• Fixed baud rate = fosc/12

For example: 12 MHz crystal 8051

Baud rate = 12 MHz/12 = 1MHz = 1Mbps

2. Mode 2 Baud rate

The baud rate is affected by a bit in the power control register PCON, PCON.7 (SMOD bit)

• SMOD = 1, Baud rate = fosc/32

• SMOD = 0, Baud rate = fosc/64

By default, following a system reset, the mode 2 baud rate = fosc/64

for example, MOV A, PCON

SETB ACC.7

MOV PCON, A ; setting SMOD.



3. Mode 1 and Mode 3 Baud rate

• Determined by the Timer 1 overflow rate

• The overflow is divided by 32 (16 if SMOD = 1) before providing the baud rate clock to the serial port.

• Timer works in Mode 2 (Auto-reload mode)

For example, 1200 baud rate

1200 = Timer 1 overflow rate ÷ 32

→ Timer 1 overflow rate = 38.4 KHz

if 12 MHz crystal, Timer 1 clock rate = 1 MHz

Overflow is required every 1000 KHz ÷38.4 KHz = 26.04 (round to 26) clocks

)]1TH(256[12

fosc

32

2rate Baud

SMOD

rate Baud384

fosc22561TH

SMOD



Timer 1 counts up and overflow on FFH-to-00H

transition of the count, 26 counts less than “0” is the

required reload for TH1.

MOV TH1, # - 26

or MOV TH1, # 0E6H

• There is a slight error in the resulting baud rate, due

to rounding

• A 5﹪error is tolerable using asynchronous protocol

• If 11.059 MHz crystal is used, exact baud rates are

possible.


Example: Initialize the Serial Port

• To initialize the serial port to operate as an 8-bit UART at 2400 Baud, using Timer 1.

– Four registers must be initialized: SMOD, TMOD, TCON, TH1

– SM0/SM1 = 0/1 → 8-bit UART mode

– REN = 1 enable the serial port to receive characters.

– Setting TI = 1, transmission of the first character indicating that the transmit buffer is empty

– For TMOD, Timer 1 in 8-bit auto-reload mode, →M0/M1 = 1/0.

– TR1 = 1 turns the Timer 1 on.

– TH1 value provides the required overflow rate, 2400× 32 = 76.8 KHz

12 MHz crystal for 8051, Timer 1 is clocked at 1 MHz.

1000÷76.8 = 13.02 (round to 13) → the reload value = -13 (0F3H)


Example 1: Initialize the Serial Port

ORG 8100H

INIT: MOV SCON, 52H ; serial port mode1

MOV TMOD, # 20H ; timer1, mode2

MOV TH1, #-13 ; reload count for 2400 baud

SETB TR1 ; start timer1

(to be

continued)


Output Character Subroutine

(From previous page)

OUTCHAR: MOV C,P ;Put parity bit in C Flag

CPL C ;Change to odd parity

MOV ACC.7,C ;Add to character code, mode 1

;8 data bits transmitted

AGAIN: JNB TI, AGAIN ;TxD buffer empty?

CLR TI ;Yes, Clear flag and

MOV SBUF, A ; send character

CLR ACC.7 ;Strip off parity

RET ;do nothing

END


Example 2: Serial Port Interrupts

ORG 0000H ;Reset

LJMP MAIN

ORG 0023H ;Serial port vector

LJMP SPISR

ORG 0030H ;Main program entry

MAIN: MOV TMOD, #20H ;timer1,mode 2

MOV TH1, # -26 ;1200 Baud reload

SETB TR1 ;Start timer1

MOV SCON, #42H ; Mode 1. Set TI

MOV A, #20H ; Send ASCII space

MOV IE, #90H ;enable serial port intrrrupt

SJMP $ ;do nothing


SPISR: CJNE A, #7FH, SKIP ;if finished ASCII set

MOV A, #20H ;reset to space

SKIP: MOV SBUF, A ; send character

INC A ;Increment ASCII code

CLR TI ;clear inter. Flag

RETI

END

Interrupt Service Routine

LAB#8

SERIAL PORT 實驗

實驗原理

• 8051串列傳輸為全雙工式，亦即傳送及接收可同時進行

• 對串列埠接收及傳送暫存器之存取都是在特殊功能暫存器SBUF進行，

• 寫入SBUF(MOV SBUF,A)等於資料載入傳送暫存器

• 讀取SBUF(MOV A,SBUF)實際上是由另一分開的接收暫存器取出資料。

• 8051串列埠有4種操作模式，此次實驗主要以模式1(mode 1)為主

SERIAL PORT與中斷

• 每當8051完成傳輸後，TI(傳輸中斷旗標)會被設為1，如有Enable中斷則會跳至中斷向量23H

• 每當8051收完一筆資料後，RI(接收中斷旗標)會被設為1，如有Enable中斷則亦會跳至中斷向量23H

• 因此在中斷服務程式內要先用RI/TI判斷是接收完成還是傳輸完成而產生此中斷

鮑率（BAUD RATE）

• 常用計時器1(Timer 1) 之自動重載（Auto-reload）方式產生鮑率。

• 鮑率( 以Timer 1 之mode2 產生)

=

=

2

32 12 [256-TH1]

SMOD

石英震盪器頻率

2 1

32 [256-TH1]

SMOD

機器週期

暫存器設定

SCON

PCON

SM0 SM1 SM2 REN TB8 RB8 TI RI

SMOD GF1 GF0 PD IDL

• 設定Serial port工作於Mode 1 => MOV SCON,#01010000B

• 設定SMOD為0 => ANL PCON, #01111111B

Mode 1：每次10個位元，首先為啟始位元（0），8個資

料位元（先為LSB），和一個停止位元（1）。

接線

1.連接DB-F53的P01到電腦的COM1。

2.P3.0（RXD）連接到JP06的R1，P3.1（TXD

）連接到T1。

JP01

JP06

PC端連線程式操作說明(1/3)

STEP2:按下Connect

STEP1:執行MSCOMM.exe (請從課程網站下載)

PC端測試程式操作說明(2/3)

STEP3:設定COMM

COM PORT編號

鮑率

PC端測試程式操作說明(3/3)

在此輸入資料以傳送到8051

8051送出的資料會以ASCII字元顯示在此

勾選此選項可改以 16進位顯示

DEMO

1.利用模式1傳送鍵盤資料至PC。

2.利用模式1接收PC端資料再將同樣的資料回送至PC。

0 1 2 3

4 5 6 7

8 9 A B

C D E F

DEMO2參考流程圖:

利用模式1接收PC端資料再將同樣的資料回送至PC

設定傳輸模式與鮑率

主程式

設定欲傳輸資料初值

空迴圈等待

接收資料

SERIAL PORT中斷

RI==1?

關閉中斷

RETI

RETI

傳輸資料

Ｎ

Ｙ

結報

• 若現在需要更快的傳輸速率，例如：19200 BAUD，請問原有的12MHz之振盪器還能使用嗎？請提出您的看法並解釋。

微算機原理與實驗 - National Chiao Tung...

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