The internal clock (Q1–Q4) of PIC Micro controller runs at a quarter of the oscillator frequency (FOSC/4).

My question is if i divide the frequency with 4 Then the timer period of each clock pulse will increase 4 times.

enter image description here

However this picture is confusing. it shows after the internal circuitry divides the clock into four even quadrature clocks. the timer period of each clock (i.e Q1, Q2, Q3, and Q4) less than the main primary oscillator(that is connected to the OSC1 and OSC2 pins). But i think time period of each clock pulse must be 4 times time period of main primary oscillator. enter image description here

  1. Can anybody explain the above??

2.What is oscillator cycle?? the frequency of oscillator cycle??

  1. What is machine cycle ??
  • 1
    \$\begingroup\$ FYI the upper diagram which you copied into your question, shows a divide-by-2, not a divide-by-4. \$\endgroup\$
    – SamGibson
    Nov 26, 2018 at 18:36

2 Answers 2


(1) What I believe it says, is that 1 instruction cycle (machine cycle) takes 4 oscillator cycles to complete.

(2) The oscillator cycle is the time taken for a complete revolution to occur, or to put it simply, the time it takes to go from low, to high and back to low. The oscillator frequency is how often this cycle will occur every second, so 4MHz will mean 4 million cycles per second.

(3) The machine cycle or instruction cycle consist of 4 Q cycles. These Q cycles are what you would have shown us above. The pic18 family manual that I’m using explains it better than I could. Try giving this a read to see if you understand. Excerpt from PIC18 Family Manual

And this Excerpt from PIC18 Family Manual

If you’re still confused about it, try to pinpoint where you got confused :) and I’m sure someone will be able to explain it to you in a way you’d easily get the concept.


The oscillator frequency is divided by four, so a machine cycle takes some integer number of 4 oscillator cycles. The oscillator cycle is just the reciprocal of the oscillator frequency, so if the oscillator is 16MHz, each oscillator cycle is 62.5ns.

In the case of the mid-range PIC, that number is 1 or 2, so that will be 4 or 8 oscillator cycles. In other words, if the oscillator is 16MHz, each instruction will take 250ns or 500ns.

The Q1-Q4 states are important when you want to know exactly when (against what oscillator edge) the outputs appear at the pins, and when the inputs are sampled, and for similar reasons. Usually you don't need to worry about the Q states and can simply consider the processor running at fosc/4.


Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.