HCS08 datasheet(279/374 Pages) NXP

279 / 374 page

Timer/PWM Module (S08TPMV3)

MC9S08JM60 Series Data Sheet, Rev. 5

Freescale Semiconductor

279

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).

Reset clears the TPM counter registers. Writing any value to TPMxCNTH or TPMxCNTL also clears the

TPM counter (TPMxCNTH:TPMxCNTL) and resets the coherency mechanism, regardless of the data

involved in the write.

4–3

CLKS[B:A]

Clock source selects. As shown in Table 16-3, 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 system clock. When there is no PLL, 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 is present) is synchronized to the bus clock by an on-chip synchronization circuit. When a PLL 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 16-4. 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 16-3. TPM-Clock-Source Selection

CLKSB:CLKSA

TPM Clock Source to Prescaler Input

No clock selected (TPM counter disable)

Bus rate clock

Fixed system clock

External source

Table 16-4. Prescale Factor Selection

PS2:PS1:PS0

TPM Clock Source Divided-by

000

001

010

011

100

101

110

111

128

Table 16-2. TPMxSC Field Descriptions (continued)

Field

Description

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