Using the HC12 Pulse Width Modulation Subsystem

1.

Determine the PWM period in P-clock cycles:
NC = Period/(0.125 $\mu$s) = 8 $\times 10^6$ cycles/second / Frequency

For example, to get a 500 Hz PWM signal:

NC = $8 \times 10^6 / 500 = 16000$

2.

Try to find N and PWPER such that

$$2^N \times {{\rm {PWPER}} + 1} = NC$$

where $0 \le N \le 7$ and $199 \le$ PWPER $\le 255$

For 500 Hz PWM,

$$2^N = \frac{16000}{{\rm PWPER} + 1}$$

For PWPER = 249, you get 2^N = 64, N = 6.

3.

If no value of N works in the above, try to find N and M such that

$$2^(N+1) \times (M+1) \times {{\rm {PWPER}} + 1} = NC$$

where $0 \le N \le 7$, $0 \le M \le 255$ and $199 \le$ PWPER $\le 255$

For 500 Hz PWM,

$$2^N = \frac{16000}{(M+1)({\rm PWPER} + 1)}$$

For PWPER = 199,

$$2^N = \frac{80}{(M+1)}$$

There are several values which will work: N = 1, M = 39; N = 2, M = 19 and N = 3, M = 9

4.

Once you find N and M, program the following registers (assuming 8-bit mode, left-aligned, high polarity). Note that if you are using both channel 0 and 1, they both must have the same N. If you use M for both channel 0 and 1, they both must have the same M.

If using N only:

Action	Register	Channel 0	Channel 1	Channel 2	Channel 3
8-bit mode	PWCLK	Bit 6 = 0	Bit 6 = 0	Bit 7 = 0	Bit 7 = 0
High Polarity	PWPOL	Bit 0 = 1	Bit 1 = 1	Bit 2 = 1	Bit 3 = 1
Left Aligned	PWCTL	Bit 3 = 0	Bit 3 = 0	Bit 3 = 0	Bit 3 = 0
Clock Select	PWPOL	Bit 4 = 0	Bit 5 = 0	Bit 6 = 0	Bit 7 = 0
N	PWCLK	Bits 5:3 = N	Bits 5:3 = N	Bits 2:0 = N	Bits 2:0 = N
Enable PWM	PWEN	Bit 0 = 1	Bit 1 = 1	Bit 2 = 1	Bit 3 = 1

If using N and M:

Action	Register	Channel 0	Channel 1	Channel 2	Channel 3
8-bit mode	PWCLK	Bit 6 = 0	Bit 6 = 0	Bit 7 = 0	Bit 7 = 0
High Polarity	PWPOL	Bit 0 = 1	Bit 1 = 1	Bit 2 = 1	Bit 3 = 1
Left Aligned	PWCTL	Bit 3 = 0	Bit 3 = 0	Bit 3 = 0	Bit 3 = 0
Clock Select	PWPOL	Bit 4 = 1	Bit 5 = 1	Bit 6 = 1	Bit 7 = 1
N	PWCLK	Bits 5:3 = N	Bits 5:3 = N	Bits 2:0 = N	Bits 2:0 = N
M	PWSCALx	PWSCAL0 = M	PWSCAL0 = M	PWSCAL1 = M	PWSCAL1 = M
Enable PWM	PWEN	Bit 0 = 1	Bit 1 = 1	Bit 2 = 1	Bit 3 = 1

5.

Now program the PWPERx register for the used channel(s) with the value calculated in Part 2 or 3.

6.

Set the duty cycle by programming the PWPDTYx for the used channel(s) using the formula:

PWDTYx = (duty cycle $\times$ (PWPERx + 1)) - 1

For example, with PWPERx = 199, and a duty cycle of 25%,

PWDTYx = (0.25 $\times$ (199 + 1)) - 1 = 49

To progam Channel 0 for 500 Hz PWM with a 25% duty cycle (using N = 3, M = 9, and PWPER = 199 from Part 3):

 
    /* Choose 8-bit mode */
    PWCLK = PWCLK & ~0x40;
    /* Choose high polarity */
    PWPOL = PWPOL | 0x01;
    /* Choose left-aligned */
    PWCTL = PWCTL & ~0x08;
    /* Select clock mode 1 (use N and M) for Channel 0 */
    PWPOL = PWPOL | 0x10;
    /* Select N = 2 and M = 2 for Channels 1 and 0 */
    PWCLK = (PWCLK | 0x18) & ~0x20;   /* PWCLK = X X 0 1 1 X X X */
    /* Select and M = 9 for Channels 1 and 0 */
    PWSCAL0 = 9; 
    /* Select period of 199 for Channels 1 and 0 */
    PWPER0 = 199;
    /* Enable PWM on Channels 1 and 0 */
    PWEN = PWEN | 0x03;
    /* 25 % duty cycle on Channel 0 */
    PWDTY0 = 49;