Question about PIC16F877A PWM calculation

When I first saw this picture, I felt a little confused ~ Later I understood it a lot better.

For PWM experiments, we usually only care about frequency and duty cycle. Let me explain this picture:

First, assume that the microcontroller is connected to an external 4MHz crystal oscillator;

The period of the PWM wave is determined by the PR2 register, and the duty cycle is composed of CCPR1L+2 bits to form a 10-bit resolution;

But the period is 8-bit resolution, and the numerator is larger than the denominator. Can this PWM still work normally?

I believe that many people may have the same doubts as me at the beginning!

Let's take a look at how the cycle is calculated in the official data manual:

PWM cycle = (PR2+1)*4*Tosc*TMR2 frequency division number

There are two things to note in this formula:

① Tosc is 4MHz, not 1MHz

② Multiply by 4, which means CCPR1L<<2 (shift right 2 bits)

Suppose I want to implement a 10kHz square wave with a duty cycle of 90, how should I fill in the values ​​of the relevant registers?

First calculate the 10kHz period, which is 100us

100=(PR2+1)*4*0.25*1(frequency division number)

Then PR2=99

CCPR1L:X:Y=90*4*0.25*1

You can take X:Y=0:0 (both sides of the equation are shifted right by 2 bits)

Then CCPR1L=90

So far I haven't answered why the numerator is larger than the denominator? In fact, both the numerator and the denominator are 10 bits, but the lowest 2 bits of the denominator are always 0;

The 10 bits of the numerator can be set, which has the advantage that the desired duty cycle can be accurately designed, such as 27%, 61%, 84%...

The experimental procedure is as follows:

#include

__CONFIG(0xFF32);

void init_ccp1(void)

{

TRISC2=0; //RC2/CCP1 is set as output

#if 0

PR2=61; //Period is 1ms, duty cycle is 40%

T2CON=0x06; //TMR2 pre-divided to 1:16, start working

CCPR1L=0x19;

#else

PR2=99; //Period is 100us, duty cycle is 90%

T2CON=0x04; //TMR2 pre-divided to 1:1, start working

CCPR1L=90;

#endif

CCP1CON=0x0c; //PWM mode

}

void main()

{

init_ccp1();

while(1);

}