5.8051 Timer programming Revision as of 12:19, 22 August 2016 by Explorer (Talk | contribs) (diff) ← Older revision | Latest revision (diff) | Newer revision → (diff)

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.

Timer Basics

As the name suggests these are used to measure the time or generate the accurate time delay. The microcontroller can also generate/measure the required time delays by running loops, but the timer/counter relieves the CPU from that redundant and repetitive task, allowing it to allocate maximum processing time for other tasks.

The timer is nothing but a simple binary counter that can be configured to count clock pulses(Internal/External). Once it reaches the Max value, it will roll back to zero setting up an OverFlow flag and generates the interrupt if enabled.

8051 Timer Module

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	0.2usec	13.107ms
TIMER1	16-bit	TMOD,TCON	TH1,TL1	0.2usec	104.857ms
TIMER2(8052 only)	16-bit	TMOD,TCON	TH1,TL1	0.2usec	104.857ms

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 Operation

{{Box|type=l_green_light|text= The Timer 0 is a 16 bit registers as shown. This can be accessed as 2 eight bit registers TL0 and TL1. Same applies to Timer 1.
The 8051 timer and counter is the same unit, but in this tutorial we will discuss only the timer unit to simplify the discussion.

fig 1:Basic Timer

Timer Tick

Fig 1, shows the basic 8051 timer unit. The registers TCON and TMOD affect the timer operation. The clock frequency is divided by 12 and used by the timer unit. Thus if a 11.0592MHz external crystal is used, the timer uses a frequency of 921KHz. Thus timer increments every 1.085μ seconds.

• The C/Ṫ = 0 bit of TMOD register selects operation of Timer/counter unit as timer.
• The TR bit of TCON register is used to start the timer.

Timer Registers

Timer Register T0/T1

Table. 1: Timer Register

T0
TH0								TL0
D15	D14	D13	D12	D11	D10	D9	D8	D7	D6	D5	D4	D3	D2	D1	D0

TMOD Register

The TMOD Register specifies the operational mode of the two timers. The higher nibble is used for Timer 1 and Lower for the timer 0 as shown below.

Table 3. Timer Modes

M1	M0	Operation
0	0	13 bit Timer
0	1	16 bit Timer
1	0	8 bit Auto Reload
1	1	Split Mode

Table 2. TMOD Register

TMOD
D7	D6	D5	D4	D3	D2	D1	D0
Gate	C/T	M1	M0	Gate	C/T	M1	M0
Timer1				Timer 0

TCON Register

Table 4. TCON Register

TCON
D7	D6	D5	D4	D3	D2	D1	D0
TF1	TR1	TF0	TR0	IE1	IT1	IE0	IT0
Timer1		Timer 0		Interrupts

fig 2:Mode 0

fig 3:Mode 1

fig 4:Mode 2

Timer Example

This example will demonstrate use of Timer 0 for generating delay. We will use Timer 0 in mode 1 to generate a delay of 50ms and will toggle pins of P3 every 50ms

#include<reg51.h>
void delay_t0(void);// function prototype
 
void delay_t0()
{
   TMOD = 0x01; //Timer zero mode 1  
   TH0 = 0X4B; 
   TL0 = 0XFF;
   TR0 = 1; //turn ON Timer zero
   while(TF0 == 0);
   TF0 = 0; //clear the timer
   TR0 = 0;
}
void main()
{
  P3 = 0x00; //set port as output  
  while(1)
  {
   P3 = 0XFF;
   delay_t0();
   P3 = 0X00;
   delay_t0();
  } 
}

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