Time Delay for 8051

Time Delay for 8051

Many times, it is necessary to generate time delays. For example, to generate a square wave we have to generate a delay of T/2 period so that we can make output 1 for T/2 period and make output 0 for T/2 period. Let us see how to calculate exact delays for 8051.

For 8051 operating frequency is one-twelfth (1/12) of the crystal frequency. Therefore, one machine cycle lasts for 12 oscillator periods.

ஃ Machine cycle period = 12 / Crystal frequency

For crystal frequency 11.0592 MHz,

Machine cycle period =  12 / 11.0592 × 10⁶ = 1.085 µs

For 8051, we know that how much machine cycle/s are required to execute the particular instruction. Therefore, we can calculate the exact time for execution of that instruction, as shown below.

Example 15.9.1 Calculate the time delay produced by the following subroutine.

Delay : MOV R1, #30

HERE : DJNZ R1, HERE

NOP

NOP

RET

Solution : Let us assume the crystal frequency of 8051 is 11.0592 MHz. Therefore, the period of the machine cycle will be

Time delay = [1 + (2 × 30) + 1 + X + 1] × 1.085 µsec = 69.44 µs

Here, (2 × 30) indicates that the instruction DJNZ RI, HERE is executed 30 times.