Different ways to generate delays in 8051

The delay length in 8051 microcontroller depends on three factors:

1. The crystal frequency
2. the number of clock per machine
3. the C compiler.

The original 8051 used 1/12 of the crystal oscillator frequency as one machine cycle. In other words, each machine cycle is equal to 12 clocks period of the crystal frequency connected to X1-X2 pins of 8051. To speed up the 8051, many recent versions of the 8051 have reduced the number of clocks per machine cycle from 12 to four, or even one. The frequency for the timer is always 1/12th the frequency of the crystal attached to the 8051, regardless of the 8051 version. In other words, AT89C51, DS5000, and DS89C4x0 the duration of the time to execute an instruction varies, but they all use 1/12th of the crystal's oscillator frequency for the clock source.

8051 has two different ways to generate time delay using C programming, regardless of 8051 version.

The first method is simply using Loop program function in which Delay() function is made or by providing for(); delay loop in Embedded C programming. You can define your own value of delay and how long you want to display. For example- for(i=0;i<"any decimal value";i++); this is the delay for loop used in embedded C.

Code to generate 250 ms delay on Port P1 of 8051:

#include "REG52.h"
void MSDelay(unsigned int);

void main(void)
{
while (1) //repeat forever
{
P1=0x55;
MSDelay(250);

P1=0xAA;
MSDelay(250);
}
}
void MSDelay(unsigned int itime)
{
unsigned int i,j;
for (i=0;i<itime;i++) // this is For(); loop delay used to define delay value in Embedded C
{
for (j=0;j<1275;j++);
}
}

The second method is using Timer registers TH, TL and TMOD that are accessible in embedded C by defining header file reg52.h Both timers 0 and 1 use the same register, called TMOD (timer mode), to set the various timer operation modes in 8051 C programming. There are four operating modes of timer 0 and 1. To generate Time delay using timer registers:

1. Load the TMOD value register indicating which timer (timer 0 or timer 1) is to be used and which timer mode (0 or is selected
2. Load registers TL and TH with initial count value
3. Start the timer
4. Keep monitoring the timer flag (TF) until it rolls over from FFFFH to 0000.
5. After the timer reaches its limit and rolls over, in order to repeat the process - TH and TL must be reloaded with the original value, and TR is turned off by setting value to 0 and TF must be reloaded to 0.

Code generating delay using timer register:

#include <REG52.h>
void T0Delay(void);
void main(void){

while (1) 
{
P1=0x55;
T0Delay();

P1=0xAA;
T0Delay();
}
}
void T0Delay(){

TMOD=0x01;     // timer 0, mode 1
TL0=0x00;      // load TL0

TH0=0x35;      // load TH0
TR0=1;         // turn on Timer0

while (TF0==0);           // wait for TF0 to roll over

TR0=0;      // turn off timer
TF0=0;      // clear TF0
}