Hello all,

I am using pic18f4550 and i am interfacing a crystal of 20Mhz. I want to know the execution time of 1 instruction cycle. I am having a great confusion. Please help me.

Also, suggest me the method to do so please.

Thanking you in advance

 

From page 387 of the datasheet:

Instruction Cycle Time (Tcy) = 4*Tosc = 4/Fosc with a minimum of 83.3 ns.

Fosc for you is 20 MHz. I'm sure you can do the rest...

 

1 instruction cycle should take 4/(20*10^6) seconds.

 

I'll go even further: 200 ns

 

In case of doubt, using the simulator, MPLAB Sim, just by stepping through a simple series of NOPs, you could see how much time is spent in each instruction cycle.

Look for the "stopwatch" function.

 

Very common problem, there are so many clocking options to consider.
I usually test the clock frequency like this:

loop:
btg LATX,X ;toggle IO pin state
bra loop

Measure the frequency of the output to confirm the clock speed.
This saves much confusion about what is really going on, it's easy to get confused and set the clock bits wrong, then drive yourself insane trying to set a UART Baud rate.

 

Very common problem, there are so many clocking options to consider.
I usually test the clock frequency like this:

loop:
btg LATX,X ;toggle IO pin state
bra loop

Measure the frequency of the output to confirm the clock speed.
This saves much confusion about what is really going on, it's easy to get confused and set the clock bits wrong, then drive yourself insane trying to set a UART Baud rate.

But how long is a bra? I go a another step when pounding this out:

loop:
bfs LATX,X ;toggle IO pin state
bcs LATX,X ;toggle IO pin state
bfs LATX,X ;toggle IO pin state
bcs LATX,X ;toggle IO pin state
bfs LATX,X ;toggle IO pin state
bcs LATX,X ;toggle IO pin state
bfs LATX,X ;toggle IO pin state
bcs LATX,X ;toggle IO pin state
bfs LATX,X ;toggle IO pin state
bcs LATX,X ;toggle IO pin state
bfs LATX,X ;toggle IO pin state
bcs LATX,X ;toggle IO pin state
bfs LATX,X ;toggle IO pin state
bcs LATX,X ;toggle IO pin state
bfs LATX,X ;toggle IO pin state
bcs LATX,X ;toggle IO pin state
bra loop

Then I look with a scope, and it is right there in my face what the instruction rate is.

 

But how long is a bra?

Depends on the lady it's designed for .