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8051 Timers

Design Exercise

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Timer Registers

TL0 Timer0 low byte

TH0 Timer0 high byte

TL1 Timer1 low byte

TH1 Timer1 high byte

TMOD Timer Mode Register

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TMOD Register

Lower 4bits for timer0 & upper 4bits for timer1

In each case lower 2 bits to specify timer mode &

upper 2 bits to specify the operation

GATE:- Gating control (h/wor s/w control) C/T:- Timer or Counter operation

M0:- Mode bit0

M1:- Mode bit1

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TMOD Register

Gate= 0 Timer is enabledwhen TRx bit is set (SETB TR0)

Gate= 1 Timer is enabledonly while the INTx pin is high and

TRx control pin is set

C/T= 0-Timer Operation

C/T= 1-Counter Operation

M1 M0

0 0 -Mode0

0 1 -Mode1

1 0 -Mode2

1 1 -Mode3

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Timer in Mode-1 Programming Steps

Load TMOD register with value for selecting mode1 of timer0 or

timer1 or both

Load TL and TH with initial values

Start timer (SETB TR0, TR1)

Incase of polling

Keep Monitoring Timer Status

If Flag is high

Reload TL and TH

Else Keep Monitoring Timer Status

Incase of interrupt

Configure for interrupt detection and service (Configure IE)

wait in infinite loop

If interrupted

Reload TL and TH & return

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Finding values to be loaded into Timer

1. Convert the time to microsecond range. Divide the

desired time by 1 microseconds (12 MHz) or 1.085

microseconds (11.0592 MHz usedwith SBC-51)

and set it as n2. If the value is less than 16 bit maximum value then

perform 65536-n

3. Convert result of step2 to hex

4. Set TL=xx and TH=yy,where the result of step3 isyyxx

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Finding values to be loaded into Timer

For example if 50 milliseconds delay is

required then

Convert to microseconds (50 x1000) = 50000 s Divide by 1.085 s = 46083

65536- 46083 = 19453

Convert 19453 to hex -> 4BFD

TH0 = 4B and TL0 = FD

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Mode-0 Programming

Same as Mode1 except that it is a 13-bit timer instead

of 16-bit

13-bit can hold values up to 1FFFH

When timer reaches 1FFFH it rolls over to 0000H

TF flag is set or overflow interrupt will be occurring

at this event

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Mode2 Programming

It is a 8-bit Timer

allows values up to FFH to be loaded into TH register

Load the TMOD value register indicating which timer

is to be used, and the timer mode is selected

Load TH register with the initial count value

Start the timer (SETB TRx)

Keep monitoring TF or wait for interrupt

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; one second delay counter

org 8000h

start: mov tmod,#01h ;timer 0 mode 1 setup

mov a,#00hmov r0,#00h

mov ie,#82h

setb tr0 ;set tcon register( tr0=1)

back: sjmp back

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t_isr:

Loop1: inc r0

cjne r0,#0bh,rtnmov dptr,#7e00h

movx @dptr,a

inc a

mov r0,#00h

cjne a,#0ah,rtn

mov a,#00h

rtn: setb tr0

reti

org 0ffe4h

ljmp t_isr end

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