Timers on the 8051 Microcontroller

Aim:

In this tutorial, we are going to discuss the Timer module of 8051. First, we will see what are timers, their working and later we will configure the 8051 timers to generate the delay of 100ms and 500ms respectively. At the end, we will see how to use the ExploreEmdedded Timer library.

Description:

8051 has two indepenndent timer which can be used as timer(to generate delays)/Counters(count external events).
Timer 1 is also used for generating baud rate in serial communication, which we will discuss in the next tutorial
Below table provides the details of the 8051 Timers.

TIMER	SIZE	CONTROL REGISTER	COUNT REGISTER	MIN DELAY	MAX DELAY
TIMER0	16-bit	TMOD,TCON	TH0,TL0	1.085µs	71.107ms
TIMER1	16-bit	TMOD,TCON	TH1,TL1	1.085µs	71.107ms
TIMER2(8052 only)	16-bit	T2CON	RCAP2H,RCAP2L	1.085µs	71.107ms

Timer Registers

TMOD
TIMER1				TIMER 0
7	6	5	4	3	2	1	0
Gate	C/T	M1	M0	Gate	C/T	M1	M0

• Gate Control

0 = Timer enabled
1 = Timer enabled if INTx is high

• C/T:Counter or Timer Selector

0 = Internal count source (clock/12)
1 = External count source T0/T1(P3.4/P3.5) pin.

• M1-M0:Mode Control

00-Mode 0, 13 bit count mode
01-Mode 1, 16 bit count mode
10-Mode 2, Auto reload mode
11-Mode 3, Split Timer mode

TCON
7	6	5	4	3	2	1	0
TF1	TR1	TF0	TR0

• TRx: Timer x run control

0 = Timer not running
1 = Timer running

• TFx: Timer x OverFlow flag

0 = Timer has not overflowed/rolled over
1 = Timer has overflowed/rolled over

Timer Calculation

8051 Oscillator frequency is divided by 12 and then fed to the controller, Time to increment the Timer count by one(timer tick) can be determined as below.
tick = (1/(Fosc/12)
$$tick = 12/Fosc$$ For Fosc == 11.0592Mhz, the tick time will be
tick = 12/11.0592M = 1.085069444us = 1.085us

Now the Timer value for the required delay can be calculated as below.
Delay = TimerCount * tick
Count = (Delay/tick)
RegValue = TimerMax- Count
RegValue = TimerMax-(Delay/tick) = TimerMax – (Delay/1.085us)
$$RegValue = TimerMax-((Delay/1.085) * 10^6)$$

Timer Mode1

The timer in Mode-1 can be used as a 16-bit timer to count from 0000 to FFFFH thus allowing to generate a wide range of delay. The timer value for the required delay needs to be loaded into Timer Count registers TH & TL. After loading the values to the register, the timers must be started. Now the Timer starts counting up and once it reaches the max value(0xffff), it rolls back to zero setting the overflow flag. At this point, the timer values must be reloaded and the overflow flag should also be cleared.

Timer Calculation for 50ms delay

Fosc = 11.0592Mhz
Delay = 50ms
$$RegValue = TimerMax-((Delay/1.085) * 10^6)$$ RegValue = 65536 – (50ms/1.085)*10^6 = 65536 – 46082 = 19453 = 0x4BFD

Block Diagram

Schematic

Code

// **********************************************
// Project: Interfacing 8-bit timer0 to 8051
// Author: Code Bloges
// Module description: Operate array of LED's
// **********************************************
#include<reg51.h>
sbit LED = P2^0;

void timerDelay()
{
    TH0 = 0X4B;        //Load the timer value
    TL0 = 0XFD;
    TR0 = 1;           //turn ON Timer zero
    while(TF0 == 0);   // Wait for Timer Overflow
    TF0 = 0;           //clear the timer Over flow flag
    TR0 = 0;
}
void main()
{
    TMOD = 0x01; //Timer0 mode 1
    while(1)
    {
        LED = 1;
        timerDelay();
        LED = 0;
        timerDelay();
    }
}

Downloads:

The code was compiled in Keil uvision4 and simulation was made in Proteus v7.7.
To download code and proteus simulation click here.

Further Reading suggestions:

 You may also like,

• Interfacing keypad with 8051
• Interfacing with UART of 8051 controller
• Interfacing SPI communication with 8051
• 8051 Displaying Custom Characters on LCD 
• 8051 Graphical LCD
• RTC interfacing using I2C in 8051
• Interfacing ultrasonic sensor with 8051
• Interfacing GPS Modu with 8051
• Interfacing GSM Module with 8051
• ADC Module interfacing with 8051
• Interfacing PWM in 8051