Functional Description and Application Information
PWM Control Module (PWM8B2C)
MM912F634
Freescale Semiconductor
93
4.13.3.1.1
PWM Enable (PWMEx)
NOTE
The first PWM cycle after enabling the channel can be irregular. If both PWM channels are
disabled (PWME1–0 = 0), the prescaler counter shuts off for power savings.
Each PWM channel has an enable bit (PWMEx) to start its waveform output. When any of the PWMEx bits are set (PWMEx = 1),
the associated PWM output is enabled immediately. However, the actual PWM waveform is not available on the associated PWM
output until its clock source begins its next cycle, due to the synchronization of PWMEx and the clock source.
4.13.3.1.2
PWM Polarity (PPOLx)
NOTE
PPOLx register bits can be written anytime. If the polarity changes while a PWM signal is
being generated, a truncated or stretched pulse can occur during the transition
The starting polarity of each PWM channel waveform is determined by the associated PPOLx bit. If the polarity bit is one, the
PWM channel output is high at the beginning of the cycle and then goes low when the duty count is reached. Conversely, if the
polarity bit is zero, the output starts low and then goes high when the duty count is reached.
4.13.3.1.3
PWM Clock Select (PCLKx)
NOTE
Register bits PCLK0 and PCLK1 can be written anytime. If a clock select changes while a
PWM signal is being generated, a truncated or stretched pulse can occur during the
transition.
Each PWM channel has a choice of two clocks to use as the clock source for that channel as described by the following.
Table 111. PWMCTL - Register Field Descriptions
Field
Description
7–6
CAE[1:0]
Center Aligned Output Modes on Channels 1–0
0 Channels 1–0 operate in left aligned output mode.
1 Channels 1–0 operate in center aligned output mode.
5
PCLK1
Pulse Width Channel 1 Clock Select
0 Clock B is the clock source for PWM channel 1.
1 Clock SB is the clock source for PWM channel 1.
4
PCLK0
Pulse Width Channel 0 Clock Select
0 Clock A is the clock source for PWM channel 0.
1 Clock SA is the clock source for PWM channel 0.
3–2
PPOL[1:0]
Pulse Width Channel 1–0 Polarity Bits
0 PWM channel 1–0 outputs are low at the beginning of the period, then go high when the duty count is reached.
1 PWM channel 1–0 outputs are high at the beginning of the period, then go low when the duty count is reached.
1-0
PWME[1:0]
Pulse Width Channel 1–0 Enable
0 Pulse width channel 1–0 is disabled.
1 Pulse width channel 1–0 is enabled. The pulse modulated signal becomes available at PWM, output bit 1 when its clock
source begins its next cycle.