Programming The MCS-51: Subroutines (L12) - Webstaff.kmutt.ac.th ...

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ENE 334 Microprocessors 

Lecture 12: 

Serial Data Transmission 

Week #12 : Dejwoot KHAWPARISUTH 

http://webstaff.kmutt.ac.th/~dejwoot.kha/

ENE 334 MCS-51 Serial Data Transmission 

Page 2 

Week #12 

Outlines: 

➔ 

Character Transmission Using a Time Delay 

➔ 

Character Transmission by Polling 

➔ 

Interrupt-Driven Character Transmission 

➔ 

Polling for Received Data 

➔ 

Interrupt-Driven Data Reception

ENE 334 MCS-51 Serial Data Transmission Page 3 

Week #12 

Serial Data Tx: 

Asynchronous 8-bit Character: 

The 8051 contains serial data transmission/reception 

circuitry that can be programmed to use four asynchronous 

data communication modes numbered from 0 to 3. One of 

these, mode 1, is the standard UART mode (universal 

asynchronous receiver transmitters)

Serial Data Tx: Sendchar 

SENDCHAR: 

A program called Sendchar takes the character in the A 

register, transmits it, delays for the transmission time, and 

then returns to the calling program. 

- Timer 1 must be used to set the baud rate, which is 

2400 baud in this example. Using a 12 megahertz crystal, 

the reload number is 256 - 12E6/(32 x 12 x 2400), which 

is 242.98 or integer 243. This yields an actual rate of 

2404. (TH1 = 243d = 0F3h, SMOD = 0) 


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SENDCHAR: 

- Timer 1: the baud frequency (mode 2) 

= 12E6/(32 x 12 x (256-243)) 

= 2403.846 Hz. 

%error = (2403.846-2400)/2400 x 100% = 0.16% 

- The delay for one 10-bit character is (1000/2400) x 10 or 

4.16 milliseconds. The software delay softime is used for a 5 

millisecond delay. 


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7 6 5 4 3 2 1 0 

SMOD - - - GF1 GF0 PD IDL 

The Power Mode Control (PCON) 

7 6 5 4 3 2 1 0 

Gate C/T M1 M0 Gate C/T M1 M0 

[ Timer1 ] [ Timer0 ] 

The Timer Mode Control (TMOD) 

7 6 5 4 3 2 1 0 

SM0 SM1 SM2 REN TB8 RB8 TI RI 

The Serial Port Control (SCON)



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Character Transmission Using a Time Delay:


Week #12 


Character Transmission Using a Time Delay:



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Note: 

- If timer 1 and the serial port have different uses in the 

user program, then push and pop affected control 

registers. But remember, TI and SBUF can only be used 

for one function at any given time.


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Serial Data Tx: Xmit 

Character Transmission by Polling: 

An alternative to waiting a set time for transmission is to 

monitor the TI flag in the SCON register until it is set by the 

transmission of the last character written to SBUF. The polling 

routine must reset TI before returning to the call program. Failure 

to reset TI will inhibit all calls after the first, stopping all data 

transmission except the first character. 

This technique has the advantage of simplicity; less code is used, 

and the routine xmit, does not care what the actual baud rate is.


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Serial Data Tx: Xmit 

Character Transmission by Polling:


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Serial Data Tx: Sbufr 

SBUFR: 

An interrupt-driven data transmission routine for 

continuous character transmission, which is assembled 

at the interrupt vector location 0023h. A portion of the 

user program that activates the interrupt routine is 

shown.


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Interrupt-Driven Character Transmission:


Interrupt-Driven Character Transmission: 

7 6 5 4 3 2 1 0 

EA - ET2 ES ET1 EX1 ET0 EX0 

The Interrupt Enable (IE) 


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Serial Data Rx: 

Receiving Serial Data: 

Transmissions from outside sources to the 8051 are 

not predictable unless an elaborate time-of-day clock is 

maintained at the sender and receiver. Messages can 

then be sent at predefined times. A time-of-day clock 

generally ties up timers at both ends to generate the 

required “wake-up” calls.


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Serial Data Rx: Recv 

Polling for Received Data 

Polling involves periodically testing the Received Data 

flag RI and calling the data receiving subroutine when 

it is set. Care must be taken to remember to reset RI, 

or the same character will be read again. Reading 

SBUF does not clear the data in SBUF or the RI flag.


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RECV 

A polling-type program named Recv follows. Recv is a 

circular-type program that tests RI at the label here: 

and jumps to the receive subroutine if RI is set. If RI is 

not set the program continues with the section 

represented by the NOP instruction. At the end of the 

program a jump back to here: re-tests RI.


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Polling for Received Data:


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Polling for Received Data:


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Serial Data Rx: intdat 

Interrupt-Driven Data Reception 

When large volumes of data must be received, the 

data rate will overwhelm the polling approach unless 

the user program is extremely short, a feature not 

usually found in systems in which large amounts of 

data are interchanged. Interrupt-driven systems allow 

the program to run with brief pauses to read the 

received data.


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INTDAT 

Intdat demonstrate how RI may be enabled to 

automatically cause an interrupt call to location 0023h 

in the program. Once RI is enabled to interrupt the 

main program, data is read as it is received by the 

subroutine at 0023h. Received data could be then 

stored by the subroutine in the RAM locations (a data 

buffer) for later reading by the main program.


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Interrupt-Driven Data Reception:


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Interrupt-Driven Data Reception:


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Note 

- that a program that both transmits and receives would 

have to test both the TI and the RI flags to determine 

the proper response to an interrupt to 0023h. 

- If the receive subroutine that is called takes longer to 

execute than the character time, then data will be lost. 

Long subroutine times would be highly unusual; 

however, it is possible to overload any system by 

constant data reception.


Week #12 

Conclusions: 

Final: 

Friday, Dec 7 th , 2012 (9am-12pm)

Questions?: 


Week #12

Hw: 


Week #12

Programming The MCS-51: Subroutines (L12) - Webstaff.kmutt.ac.th ...

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