Datasheet

Chapter 16 Timer/PWM Module (S08TPMV3)
MC9S08SH8 MCU Series Data Sheet, Rev. 3
Freescale Semiconductor 259
The output compare value in the TPM channel registers (times 2) determines the pulse width (duty cycle)
of the CPWM signal (Figure 16-16). If ELSnA=0, a compare occurred while counting up forces the
CPWM output signal low and a compare occurred while counting down forces the output high. The
counter counts up until it reaches the modulo setting in TPMxMODH:TPMxMODL, then counts down
until it reaches zero. This sets the period equal to two times TPMxMODH:TPMxMODL.
Figure 16-16. CPWM Period and Pulse Width (ELSnA=0)
Center-aligned PWM outputs typically produce less noise than edge-aligned PWMs because fewer I/O pin
transitions are lined up at the same system clock edge. This type of PWM is also required for some types
of motor drives.
Input capture, output compare, and edge-aligned PWM functions do not make sense when the counter is
operating in up/down counting mode so this implies that all active channels within a TPM must be used in
CPWM mode when CPWMS=1.
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
TPMxMODH, TPMxMODL, TPMxCnVH, and TPMxCnVL, actually write to buffer registers.
In center-aligned PWM mode, the TPMxCnVH:L registers are updated with the value of their write buffer
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
the TPM counter is a free-running counter, the update is made when the TPM counter changes from
0xFFFE to 0xFFFF.
When TPMxCNTH:TPMxCNTL=TPMxMODH:TPMxMODL, the TPM can optionally generate a TOF
interrupt (at the end of this count).
Writing to TPMxSC cancels any values written to TPMxMODH and/or TPMxMODL and resets the
coherency mechanism for the modulo registers. Writing to TPMxCnSC cancels any values written to the
channel value registers and resets the coherency mechanism for TPMxCnVH:TPMxCnVL.
PERIOD
PULSE WIDTH
COUNT=
COUNT= 0
COUNT=
OUTPUT
COMPARE
(COUNT DOWN)
OUTPUT
COMPARE
(COUNT UP)
TPMxCHn
2 x TPMxMODH:TPMxMODL
2 x TPMxCnVH:TPMxCnVL
TPMxMODH:TPMxMODLTPMxMODH:TPMxMODL