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Chapter 8 Pulse-Width Modulator (S12PWM8B8CV1)

MC9S12XDP512 Data Sheet, Rev. 2.21

Freescale Semiconductor

377

NOTE

Writing to these registers when in special modes can alter the PWM

functionality.

8.3.2.12

PWM Channel Counter Registers (PWMCNTx)

Each channel has a dedicated 8-bit up/down counter which runs at the rate of the selected clock source.

The counter can be read at any time without affecting the count or the operation of the PWM channel. In

left aligned output mode, the counter counts from 0 to the value in the period register - 1. In center aligned

output mode, the counter counts from 0 up to the value in the period register and then back down to 0.

Any value written to the counter causes the counter to reset to $00, the counter direction to be set to up,

the immediate load of both duty and period registers with values from the buffers, and the output to change

according to the polarity bit. The counter is also cleared at the end of the effective period (see

Section 8.4.2.5, “Left Aligned Outputs” and Section 8.4.2.6, “Center Aligned Outputs” for more details).

When the channel is disabled (PWMEx = 0), the PWMCNTx register does not count. When a channel

becomes enabled (PWMEx = 1), the associated PWM counter starts at the count in the PWMCNTx

Counters”.

In concatenated mode, writes to the 16-bit counter by using a 16-bit access or writes to either the low or

high order byte of the counter will reset the 16-bit counter. Reads of the 16-bit counter must be made by

16-bit access to maintain data coherency.

NOTE

Writing to the counter while the channel is enabled can cause an irregular PWM cycle to occur.

Read: Anytime

Write: Anytime (any value written causes PWM counter to be reset to $00).

8.3.2.13

PWM Channel Period Registers (PWMPERx)

There is a dedicated period register for each channel. The value in this register determines the period of

the associated PWM channel.

The period registers for each channel are double buffered so that if they change while the channel is

enabled, the change will NOT take effect until one of the following occurs:

•

The effective period ends

•

The counter is written (counter resets to $00)

•

The channel is disabled

76543210

Bit 7

Bit 0

00000000

Reset

00000000

Figure 8-14. PWM Channel Counter Registers (PWMCNTx)

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