Analysis of the use of STC single chip microcomputer hardware PWM

How to achieve variable frequency PWM output for STC microcontroller (with PCA function)? Many friends are still using timers to do variable frequency PWM. Here I will introduce how to use hardware PWM and design PWM under the frequency division base of timer 0. 

First, let's look at the CMOD register. Here we mainly focus on CPS0, CPS1, and CPS2. These three bits control the selection of the PCA count pulse source. For the PWM function, it is to select the frequency. First, you can choose 6 fixed frequency divisions. As you can see from the picture, they are 1, 2, 4, 6, 8, and 12 system frequency divisions. We need to make a variable frequency PWM, so we need to select mode 2 and select the overflow of timer 0 as the system frequency division base. 

Then, let's see how to calculate and implement frequency division. Here we need the calculation rules in the picture above. Here I give an example to illustrate, for example, the crystal oscillator of our microcontroller uses 12MHZ, and we need to achieve a frequency of 300HZ. What if we get it? 

First, if we use the fixed frequency division of CMOD, we can find that no matter it is 1, 2, 4, 8, or 12, the frequency divided is not 300HZ. So here we can see the benefit of using timer 0 as the frequency division base. 

We continue to calculate, 12MHZ needs to be converted to 300HZ, then according to the above figure, first we need to determine the PCA clock input frequency, according to the formula 300*256=76800HZ, this value is the PCA clock input frequency we need. Now the question is, how do we convert 12MHZ to 76.8KHZ, 12000KHZ/76.8KHZ=156.25, this 156.25 is the frequency division base, and this frequency division base is set by our timer overflow parameter, which means that when our timer counts 156.25 overflows, the frequency division base can be 156.25, so we set the count start value of timer 0 to 65536-156=65380, corresponding to TH0=0XFF, TL0=0X64. 

Well, here is the introduction of how to calculate PWM to achieve any frequency we need under timer 0. If we want to dynamically change the frequency, we can dynamically adjust the counting base of timer 0 through communication (actually adjusting the frequency division base), and the frequency can be changed. In addition, it is explained that the timer 0 here does not need interrupts, and the timer is not used to generate PWM, but as a frequency division base!