EE 308 Pulse Widt Modulation on the HC12

Using the HC12 PWM

1.

Choose 8-bit mode (PWCLK 7:6 = 00)

2.

Choose left-aligned (PWCTL 3 = 0)

3.

Choose polarity = 1 (PPOL 3:0 = 1111)

4.

Select clock mode in PWPOL: PCLKx = 0 for 2^N,
PCLKx = 1 for $2^{(N+1)} \times (M+1)$ ,

5.

Select N in PWCLK register: PCKA 2:0 for channels 1, 0; PCKB 2:0 for channels 3, 2.

6.

If PCLKx = 1, select M: PWSCAL0 7:0 = M for channels 1, 0; PWSCAL1 7:0 = M for channels 3, 2.

7.

Select PWPERx, normally between 200 and 255.

8.

Enable desired PWM channels: PWEN 3:0.

9.

Select PWDTYx, normally between 0 and PWPERx. Then

$\begin{displaymath}{\rm Duty\ Cycle\ x} = \frac{{\tt PWDTYx}+1}{{\tt PWPERx}+1} \times 100 \%\end{displaymath}$

Change duty cycle to control speed of motor or intensity of light, etc.

10.

For 0% duty cycle, choose PWDTYx = 0xFF.

Bill Rison
2000-03-01