Datasheet

Chapter 15 Timer/PWM Module (S08TPMV3)
MC9S08DN60 Series Data Sheet, Rev 3
Freescale Semiconductor 277
In output compare mode, values are transferred to the corresponding timer channel registers only after both
8-bit halves of a 16-bit register have been written and according to the value of CLKSB:CLKSA bits, so:
• If (CLKSB:CLKSA = 0:0), the registers are updated when the second byte is written
• If (CLKSB:CLKSA not = 0:0), the registers are updated at the next change of the TPM counter
(end of the prescaler counting) after the second byte is written.
The coherency sequence can be manually reset by writing to the channel status/control register
(TPMxCnSC).
An output compare event sets a flag bit (CHnF) which may optionally generate a CPU-interrupt request.
15.4.2.3 Edge-Aligned PWM Mode
This type of PWM output uses the normal up-counting mode of the timer counter (CPWMS=0) and can
be used when other channels in the same TPM are configured for input capture or output compare
functions. The period of this PWM signal is determined by the value of the modulus register
(TPMxMODH:TPMxMODL) plus 1. The duty cycle is determined by the setting in the timer channel
register (TPMxCnVH:TPMxCnVL). The polarity of this PWM signal is determined by the setting in the
ELSnA control bit. 0% and 100% duty cycle cases are possible.
The output compare value in the TPM channel registers determines the pulse width (duty cycle) of the
PWM signal (Figure 15-15). The time between the modulus overflow and the output compare is the pulse
width. If ELSnA=0, the counter overflow forces the PWM signal high, and the output compare forces the
PWM signal low. If ELSnA=1, the counter overflow forces the PWM signal low, and the output compare
forces the PWM signal high.
Figure 15-15. PWM Period and Pulse Width (ELSnA=0)
When the channel value register is set to 0x0000, the duty cycle is 0%. 100% duty cycle can be achieved
by setting the timer-channel register (TPMxCnVH:TPMxCnVL) to a value greater than the modulus
setting. This implies that the modulus setting must be less than 0xFFFF in order to get 100% duty cycle.
Because the TPM may be used in an 8-bit MCU, the settings in the timer channel registers are buffered to
ensure coherent 16-bit updates and to avoid unexpected PWM pulse widths. Writes to any of the registers
TPMxCnVH and TPMxCnVL, actually write to buffer registers. In edge-aligned PWM mode, values are
transferred to the corresponding timer-channel registers according to the value of CLKSB:CLKSA bits, so:
• If (CLKSB:CLKSA = 0:0), the registers are updated when the second byte is written
• If (CLKSB:CLKSA not = 0:0), the registers are updated after the both bytes were written, and the
TPM counter changes from (TPMxMODH:TPMxMODL - 1) to (TPMxMODH:TPMxMODL). If
PERIOD
PULSE
WIDTH
OVERFLOW OVERFLOW OVERFLOW
OUTPUT
COMPARE
OUTPUT
COMPARE
OUTPUT
COMPARE
TPMxCHn