need help for calculating time!!!!

[mahmudur.rahman]

hi...

I am trying to understand something which is related with the CPU clock and ADC clock.

I don't know how to describe. I am just writing what I know.

My controller is PIC18F2550 and my crystal is 16MHz.

Now... I used the following line in my code:

[Ttelmah]

First of all, distinguish 'times' from 'frequencies'. Things beginning with 'T', are normally 'times', while things using 'F' are normally 'frequencies'. Using 'Tosc' for a frequency is confusing.....

One instruction of what?.
The processor will execute one instruction every 1/16000000th second. However one C instruction can be millions of CPU instructions in some cases. A search here will find a lot about this.

Your 'master clock' (Fosc), is the crystal*4, not the crystal frequency itself.
Look at figure 2.1 in the data sheet, and the two lines exiting centre right, for the CPU, and peripherals. Your clock with the current settings, will be taking the oscillator, dividing it by 4, through the 'PLL prescaler', feeding this into the PLL to generate 96MHz, then through the PLL postscaler, dividing it by 2, to give 48MHz, and this is then fed into the MUX, and on to your CPU.
The ADC clock, is fed from the CPU clock, _not_ directly from the external crystal.

Now, you run into a problem here. Technically, at 48MHz, there is _no_ available divider, that gives a legal clock rate for the ADC. Look at table 21-1 in the data sheet.

The ADC takes 11 cycles of it's clock to perform a conversion.
Overclocking the ADC, reduces accuracy. This may be acceptable - depends on the ADC accuracy you need.

There are two ways to run the ADC 'legally' on these chips with a 48MHz master clock:
1) Change clock speed just for the conversion.
2) Put the processor to sleep for the conversion.

Best Wishes