how to code 25ns delay in ccs

[syamin0712]

Hi,

Does anybody know how to code the delay of 25ns each in ccs? So that, I can get the 20MHz for 1 cycle.
I use the crystal 10MHz and use HSPLL so that I can get 40MHz clock.
I have tried to use the fraction, but when I burn the code into PIC18F4580 and test in real probe...not pulse appeared. But, when I change into low frequency for example 500Hz or 50kHz..it was successfully appeared in the probe.

This is my code:

[temtronic]

re:

delay_us(1/40); //set 25ns each

You need to read the CCS C manual !
Press f11 when your project is open and read the section on
delay_us()

knowledge is power

jay

[Ttelmah]

None of the delay instructions can accept fractional delays. Int16 values for delay_us.

The shortest delay you can code, is delay_cycles(1);
Note 'cycles', not 'us'.
This gives a single machine instruction delay.

The processor at 40MHz clock, is executing 10mips. 100nSec/instruction. You cannot do anything faster than this. delay_cycles(1) gives 100nSec.
Then the output's each take an instruction, and the loop, two.

The processor cannot do a delay of 25nSec.
Even if you code immediately output_high, followed by output_low, there will be one machine cycle between the operations.

However you need to be using 'fast_io', or there will be two extra instructions inserted to control the TRIS.

Then the branch instruction to loop back, will take two instruction clocks, so the fastest possible for the processor is:

[Mike Walne]

You could use the PWM to generate either 25ns high or 25ns low pulses.
As Mr T. has already said the maximum frequency will be 10MHz.

To get your 20MHz, 50% duty ratio signal, you'll need an 80MHz clock.

Mike

[temtronic]

Thinking 'outside the PIC' you could use a 'crystal clock module'. Just buy one of the correct frequency. Simple, cheap and it works.
We don't know your application..continous pulses or clocked?
Using the module, frees up the PIC (even a lower-end one) to do other tasks.....

Jay