Generate PWM pulses using the PCA function of SST microcontroller

1. Program Description:

SST microcontroller has 5-channel PCA module. PCA function provides stronger timing than ordinary microcontrollers, and less CPU intervention. It can reduce software overhead and improve accuracy. PCA pulse width modulation (PWM) mode can generate an 8-bit PWM, which is generated by comparing the low bit (CL) of the PCA timer and the low bit (CCAPnL) of the compare register. When CL=CCAPnL, the output is high. To enter this mode, you can set the PWM and ECOM bits of CCAPMn.

In PWM mode, the frequency of the output is determined by the source of the PCA timer. Since there is only one pair of CH and CL registers, all modules share the PCA timer and frequency. The duty cycle of the output is controlled by the value loaded into the high bit CCAPnH. Since writing to the CCAPnH register is not synchronous, the value written to the high bit will not be immediately loaded into CCAPnL for comparison until another cycle of the output (when CL flips from 255 to 0). Use the following formula to calculate any duty cycle CCAPnH value:

CCAPnH=256 (1-duty cycle)

CCAPnH is an 8-bit integer and the duty cycle is a fraction.

Pulse Width Modulation Frequency

2. Introduction to related registers:

PCA Timer/Counter Mode Register (CMOD)

Flag function

CIDL counter idle control, when CIDL=0, PCA continues to work in idle mode. When CIDL=1, PCA shuts down in idle mode.

WDTE Watchdog timer enable: When WDTE=0, the watchdog function of PCA module 4 is disabled. When WDTE=1, it is enabled.

CPS1 PCA count pulse width selection bit 1

CPS0 PCA count pulse width selection bit 0