Freescale Semiconductor MCF51QE128RM manual Freescale Semiconductor

Timer/PWM Module (S08TPMV3)

•Edge-aligned PWM mode

The value of a 16-bit modulo register plus 1 sets the period of the PWM output signal. The channel value register sets the duty cycle of the PWM output signal. You may also choose the polarity of the PWM output signal. Interrupts are available at the end of the period and at the duty-cycle transition point. This type of PWM signal is called edge-aligned because the leading edges of all PWM signals are aligned with the beginning of the period, which is the same for all channels within a TPM.

•Center-aligned PWM mode

Twice the value of a 16-bit modulo register sets the period of the PWM output, and the channel-value register sets the half-duty-cycle duration. The timer counter counts up until it reaches the modulo value and then counts down until it reaches zero. As the count matches the channel value register while counting down, the PWM output becomes active. When the count matches the channel value register while counting up, the PWM output becomes inactive. This type of PWM signal is called center-aligned because the centers of the active duty cycle periods for all channels are aligned with a count value of zero. This type of PWM is required for types of motors used in small appliances.

This is a high-level description only. Detailed descriptions of operating modes are in later sections.

17.1.6Block Diagram

The TPM uses one input/output (I/O) pin per channel, TPMxCHn (timer channel n), where n is the channel number (1-8). The TPM shares its I/O pins with general purpose I/O port pins (refer to I/O pin descriptions in full-chip specification for the specific chip implementation).

Figure 17-2shows the TPM structure. The central component of the TPM is the 16-bit counter that can operate as a free-running counter or a modulo up/down counter. The TPM counter (when operating in normal up-counting mode) provides the timing reference for the input capture, output compare, and edge-aligned PWM functions. The timer counter modulo registers, TPMxMODH:TPMxMODL, control the modulo value of the counter (the values 0x0000 or 0xFFFF effectively make the counter free running). Software can read the counter value at any time without affecting the counting sequence. Any write to either half of the TPMxCNT counter resets the counter, regardless of the data value written.

MCF51QE128 MCU Series Reference Manual, Rev. 3

336 Freescale Semiconductor

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