Calculation of Frequency

[malu79]

Hello,
I am a guy who recently approached the planning of the pic, in this regard I would ask yourself a question. I refer to the example in the CCS C Compiler (EX_PATG.c) and the question is as follows:
how can I determine the frequencies starting from periods? I'll explain: For example, assuming you have a clock 4000000 Hz, a time of 25600 microseconds (each period is a multiple of 400) and an interrupt of 200 microseconds, with an oscilloscope, I get that the frequency corresponding to that period is 7.9 Hz If you use the formula f = 1 / T can not get that value and then how can I determine the value of frequency?
Thanks

[PCM programmer]

You want to run the Ex_patg.c program at 4 MHz instead of 20 MHz.
You can do it, but only with one output (not with 7 outputs). The reason
is that the code inside the #int_timer1 routine takes too long to run at
4 MHz. I measured it on the oscilloscope with the PIC running at 4 MHz,
and with all 7 outputs being generated, the code in the isr takes 500 us.
The isr overhead takes about 50us at 4MHz, so that's a total of 550 us
per interrupt, and it's supposed to be 200 us. It can't do it. The code
in the isr takes too long to execute at 4 MHz if you try to run 7 channels.

Here are the changes I made to run it at 4 MHz with only 1 channel.
The output is on Pin B0 only: