NXP Semiconductors MC9S08LG16, Справочное руководство

Страница: 309 / 373

Chapter 13 Timer/Pulse-Width Modulator (S08TPMV3)
MC9S08LG32 MCU Series, Rev. 5
Freescale Semiconductor 309
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 can 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 that is 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.
16.1.7 Block 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 16-2 shows 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.