Datasheet

Chapter 16 Timer/PWM Module (S08TPMV3)
MC9S08SH8 MCU Series Data Sheet, Rev. 3
Freescale Semiconductor 255
The following sections describe the main counter and each of the timer operating modes (input capture,
output compare, edge-aligned PWM, and center-aligned PWM). Because details of pin operation and
interrupt activity depend upon the operating mode, these topics will be covered in the associated mode
explanation sections.
16.4.1 Counter
All timer functions are based on the main 16-bit counter (TPMxCNTH:TPMxCNTL). This section
discusses selection of the clock source, end-of-count overflow, up-counting vs. up/down counting, and
manual counter reset.
16.4.1.1 Counter Clock Source
The 2-bit field, CLKSB:CLKSA, in the timer status and control register (TPMxSC) selects one of three
possible clock sources or OFF (which effectively disables the TPM). See Table 16-4. After any MCU reset,
CLKSB:CLKSA=0:0 so no clock source is selected, and the TPM is in a very low power state. These
control bits may be read or written at any time and disabling the timer (writing 00 to the CLKSB:CLKSA
field) does not affect the values in the counter or other timer registers.
The bus rate clock is the main system bus clock for the MCU. This clock source requires no
synchronization because it is the clock that is used for all internal MCU activities including operation of
the CPU and buses.
In MCUs that have no PLL and FLL or the PLL and FLL are not engaged, the fixed system clock source
is the same as the bus-rate-clock source, and it does not go through a synchronizer. When a PLL or FLL
is present and engaged, a synchronizer is required between the crystal divided-by two clock source and the
timer counter so counter transitions will be properly aligned to bus-clock transitions. A synchronizer will
be used at chip level to synchronize the crystal-related source clock to the bus clock.
The external clock source may be connected to any TPM channel pin. This clock source always has to pass
through a synchronizer to assure that counter transitions are properly aligned to bus clock transitions. The
bus-rate clock drives the synchronizer; therefore, to meet Nyquist criteria even with jitter, the frequency of
the external clock source must not be faster than the bus rate divided-by four. With ideal clocks the external
clock can be as fast as bus clock divided by four.
When the external clock source shares the TPM channel pin, this pin should not be used for other channel
timing functions. For example, it would be ambiguous to configure channel 0 for input capture when the
TPM channel 0 pin was also being used as the timer external clock source. (It is the user’s responsibility
Table 16-8. TPM Clock Source Selection
CLKSB:CLKSA TPM Clock Source to Prescaler Input
00 No clock selected (TPM counter disabled)
01 Bus rate clock
10 Fixed system clock
11 External source