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ASM delay subroutines for PIC16 mcu [Programming]
posted June 6 2014 10:38.17 by spic0m

These are the delay subroutines Benoit Frigon uses in his PIC projects. It was written for the PIC16 family but should be compatible with the 18F instruction set as well.

The simplest way to produce delay is to write a subroutine that waste instructions cycles for the desired amount of time. This method should not be used for application that require a high level of precision since the delay subroutine may be halted by interrupts. The delay is offset by the time it takes to execute the interrupt service routine. For short delays however, this should not be a problem.

How many cycles are required

Let's say that the processor clock runs at 8 MHZ. It takes 4 clock pulse to execute one instruction cycle. At 8 MHZ, the processor is able to execute 2 millions instructions per seconds. It also means that each instructions takes 0.0000005 seconds (0.5 micro seconds) to execute. To produce a 50 milliseconds delay, you then need to execute 100,000 instruction cycles.

Here is the formula to find out the number of instruction required :

i_time = 1 / (fosc / 4)
0.0000005 = 1 / (8000000 / 4)

n = time / i_time
100000 = 0.05 / 0.0000005

The delay subroutines

There are 4 delay subroutines :
delay10cty WREG x 10 cycles
delay100cty WREG x 100 cycles
delay1kcty WREG x 1000 cycles
delay10kcty WREG x 10000 cycles

Each routines can be looped 'n' times specified by WREG. For instance, if you put D'50 in WREG before calling delay10cty, the routine will waste 500 instruction cycles. The routines takes into account the time it takes to call the routine itself except for delay10kcty. The 3 cycles it takes to call the routine is negligible over 10000 cycles.

The maximum possible delay using these routines is 2,550,000 instruction cycles or 1.275 second @ 8 MHZ

Read the rest and see the code in Benoit's site.

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