NXP Semiconductors MC9S12VRP64, Owner Reference Manual

Page: 294 / 555

Pulse-Width Modulator (S12PWM8B8CV2)
MC9S12VRP Family Reference Manual Rev. 1.3
294 NXP Semiconductors
NOTE
Changing the PWM output mode from left aligned to center aligned output
(or vice versa) while channels are operating can cause irregularities in the
PWM output. It is recommended to program the output mode before
enabling the PWM channel.
Figure 9-17. PWM Left Aligned Output Waveform
To calculate the output frequency in left aligned output mode for a particular channel, take the selected
clock source frequency for the channel (A, B, SA, or SB) and divide it by the value in the period register
for that channel.
• PWMx Frequency = Clock (A, B, SA, or SB) / PWMPERx
• PWMx Duty Cycle (high time as a% of period):
— Polarity = 0 (PPOLx = 0)
Duty Cycle = [(PWMPERx-PWMDTYx)/PWMPERx] * 100%
— Polarity = 1 (PPOLx = 1)
Duty Cycle = [PWMDTYx / PWMPERx] * 100%
As an example of a left aligned output, consider the following case:
Clock Source = bus clock, where bus clock = 10 MHz (100 ns period)
PPOLx = 0
PWMPERx = 4
PWMDTYx = 1
PWMx Frequency = 10 MHz/4 = 2.5 MHz
PWMx Period = 400 ns
PWMx Duty Cycle = 3/4 *100% = 75%
The output waveform generated is shown in Figure 9-18 .
PWMDTYx
Period = PWMPERx
PPOLx = 0
PPOLx = 1