Datasheet

Chapter 16 Timer/PWM Module (S08TPMV3)
MC9S08SH8 MCU Series Data Sheet, Rev. 3
Freescale Semiconductor 249
16.3.2 TPM-Counter Registers (TPMxCNTH:TPMxCNTL)
The two read-only TPM counter registers contain the high and low bytes of the value in the TPM counter.
Reading either byte (TPMxCNTH or TPMxCNTL) latches the contents of both bytes into a buffer where
they remain latched until the other half is read. This allows coherent 16-bit reads in either big-endian or
little-endian order which makes this more friendly to various compiler implementations. The coherency
mechanism is automatically restarted by an MCU reset or any write to the timer status/control register
(TPMxSC).
4–3
CLKS[B:A]
Clock source selects. As shown in Table 16-4, this 2-bit field is used to disable the TPM system or select one of
three clock sources to drive the counter prescaler. The fixed system clock source is only meaningful in systems
with a PLL-based or FLL-based system clock. When there is no PLL or FLL, the fixed-system clock source is the
same as the bus rate clock. The external source is synchronized to the bus clock by TPM module, and the fixed
system clock source (when a PLL or FLL is present) is synchronized to the bus clock by an on-chip
synchronization circuit. When a PLL or FLL is present but not enabled, the fixed-system clock source is the same
as the bus-rate clock.
2–0
PS[2:0]
Prescale factor select. This 3-bit field selects one of 8 division factors for the TPM clock input as shown in
Table 16-5. This prescaler is located after any clock source synchronization or clock source selection so it affects
the clock source selected to drive the TPM system. The new prescale factor will affect the clock source on the
next system clock cycle after the new value is updated into the register bits.
Table 16-4. TPM-Clock-Source Selection
CLKSB:CLKSA TPM Clock Source to Prescaler Input
00 No clock selected (TPM counter disable)
01 Bus rate clock
10 Fixed system clock
11 External source
Table 16-5. Prescale Factor Selection
PS2:PS1:PS0 TPM Clock Source Divided-by
000 1
001 2
010 4
011 8
100 16
101 32
110 64
111 128
Table 16-3. TPMxSC Field Descriptions (continued)
Field Description