EE 308 PWM Clock Selection on the HC12

HC12 - Two Choices of Clock for PWM

1.

PCLKx = 0 (Clock A or B):

${\rm Clk}_{\rm {x}} = 0.125 \mu{\rm {s}} \times 2^N$
${\rm Per}_{\rm {x}} = 0.125 \mu{\rm {s}} \times 2^N \times ({\tt {PWPERx+1}})$

2.

PCLKx = 1 (Clock S0 or S1):

${\rm Clk}_{\rm {x}} = 0.125 \mu{\rm {s}} \times 2^{(N+1)} \times (M+1)$
${\rm Per}_{\rm {x}} = 0.125 \mu{\rm {s}} \times 2^{(N+1)} \times (M+1) \times ({\tt {PWPERx+1}})$

N = PCKA 2:0 for PWM 1 and 0, $N = 0, 1, \cdots, 7$
N = PCKB 2:0 for PWM 3 and 2.

M = PWSCAL0 7:0 for PWM 1 and 0, $M = 0, 1, \cdots, 255$
M = PWSCAL1 7:0 for PWM 3 and 2.

Normally want PWPERx = 200 -> 255.

Example: Want 5 kHz PWM - Per $_{\rm {x}}$ = 200 $\mu$ s = 1,600 P clock cycles..

1.

Try PCLKx = 0 and PWPERx = 255

$\begin{displaymath}1600 = 2^N \times ({\tt {PWPERx+1}})\end{displaymath}$

$\begin{displaymath}2^N = \frac{1600}{256} = 6.25\end{displaymath}$

Not possible. Try 2^N = 8, N = 3

$\begin{displaymath}1600 = 2^N \times ({\tt {PWPERx+1}}) = 2^3 \times ({\tt {PWPERx+1}})\end{displaymath}$

$\begin{displaymath}{\tt {PWPERx+1}} = \frac{1600}{2^3} = 200\end{displaymath}$

$\begin{displaymath}{\tt {PWPERx}} = 199\end{displaymath}$

Exactly 5 kHz, get 1 part in 200 accuracy

2.

Try PCLKx = 1 and PWPERx = 255

$\begin{displaymath}1600 = 2^{(N+1)} \times (M+1) \times ({\tt {PWPERx+1}})\end{displaymath}$

$\begin{displaymath}2^{(N+1)} \times (M+1) = \frac{1600}{256} = 6.25\end{displaymath}$

Not possible. Use a slightly different frequency - let

$\begin{displaymath}2^{(N+1)} \times (M+1) = 6\end{displaymath}$

$\begin{displaymath}N = 0 {\rm\ and \ } M = 2\end{displaymath}$

This gives

$\begin{displaymath}{\rm Per}_{\rm {x}} = 2^{(0+1)} \times (2+1) \times (255+1) = 1536 {\rm ~P~clocks} = 192 \mu{\rm {s}}\end{displaymath}$

for a 5.2 kHz frequency

Bill Rison
2000-03-01