NXP Semiconductors MC9S12VRP64, Owner Reference Manual

Page: 296 / 555

Pulse-Width Modulator (S12PWM8B8CV2)
MC9S12VRP Family Reference Manual Rev. 1.3
296 NXP Semiconductors
To calculate the output frequency in center 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 twice the value in the period
register for that channel.
• PWMx Frequency = Clock (A, B, SA, or SB) / (2*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 center 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/8 = 1.25 MHz
PWMx Period = 800 ns
PWMx Duty Cycle = 3/4 *100% = 75%
Shown in Figure 9-20 is the output waveform generated.
Figure 9-20. PWM Center Aligned Output Example Waveform
9.4.2.7 PWM 16-Bit Functions
The scalable PWM timer also has the option of generating up to 8-channels of 8-bits or 4-channels of
16-bits for greater PWM resolution. This 16-bit channel option is achieved through the concatenation of
two 8-bit channels.
The PWMCTL register contains four control bits, each of which is used to concatenate a pair of PWM
channels into one 16-bit channel. Channels 6 and 7 are concatenated with the CON67 bit, channels 4 and
5 are concatenated with the CON45 bit, channels 2 and 3 are concatenated with the CON23 bit, and
channels 0 and 1 are concatenated with the CON01 bit.
NOTE
Change these bits only when both corresponding channels are disabled.
When channels 6 and 7 are concatenated, channel 6 registers become the high order bytes of the double
byte channel, as shown in
Figure 9-21 . Similarly, when channels 4 and 5 are concatenated, channel 4
E = 100 ns
DUTY CYCLE = 75%
E = 100 ns
PERIOD = 800 ns