HCS08 datasheet(244/316 Pages) NXP

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Timer/PWM Module (S08TPMV3)

MC9S08SG8 MCU Series Data Sheet, Rev. 8

244

Freescale Semiconductor

16.2.1.1

EXTCLK — External Clock Source

Control bits in the timer status and control register allow the user to select nothing (timer disable), the

bus-rate clock (the normal default source), a crystal-related clock, or an external clock as the clock which

drives the TPM prescaler and subsequently the 16-bit TPM counter. The external clock source is

synchronized in the TPM. The bus clock clocks the synchronizer; the frequency of the external source must

be no more than one-fourth the frequency of the bus-rate clock, to meet Nyquist criteria and allowing for

jitter.

The external clock signal shares the same pin as a channel I/O pin, so the channel pin will not be usable

for channel I/O function when selected as the external clock source. It is the user’s responsibility to avoid

such settings. If this pin is used as an external clock source (CLKSB:CLKSA = 1:1), the channel can still

be used in output compare mode as a software timer (ELSnB:ELSnA = 0:0).

16.2.1.2

TPMxCHn — TPM Channel n I/O Pin(s)

Each TPM channel is associated with an I/O pin on the MCU. The function of this pin depends on the

channel configuration. The TPM pins share with general purpose I/O pins, where each pin has a port data

whenever a port pin is acting as an input.

The TPM channel does not control the I/O pin when (ELSnB:ELSnA = 0:0) or when (CLKSB:CLKSA =

0:0) so it normally reverts to general purpose I/O control. When CPWMS = 1 (and ELSnB:ELSnA not =

0:0), all channels within the TPM are configured for center-aligned PWM and the TPMxCHn pins are all

controlled by the TPM system. When CPWMS=0, the MSnB:MSnA control bits determine whether the

channel is configured for input capture, output compare, or edge-aligned PWM.

When a channel is configured for input capture (CPWMS=0, MSnB:MSnA = 0:0 and ELSnB:ELSnA not

= 0:0), the TPMxCHn pin is forced to act as an edge-sensitive input to the TPM. ELSnB:ELSnA control

bits determine what polarity edge or edges will trigger input-capture events. A synchronizer based on the

bus clock is used to synchronize input edges to the bus clock. This implies the minimum pulse width—that

can be reliably detected—on an input capture pin is four bus clock periods (with ideal clock pulses as near

as two bus clocks can be detected). TPM uses this pin as an input capture input to override the port data

and data direction controls for the same pin.

When a channel is configured for output compare (CPWMS=0, MSnB:MSnA = 0:1 and ELSnB:ELSnA

not = 0:0), the associated data direction control is overridden, the TPMxCHn pin is considered an output

controlled by the TPM, and the ELSnB:ELSnA control bits determine how the pin is controlled. The

remaining three combinations of ELSnB:ELSnA determine whether the TPMxCHn pin is toggled, cleared,

or set each time the 16-bit channel value register matches the timer counter.

When the output compare toggle mode is initially selected, the previous value on the pin is driven out until

the next output compare event—then the pin is toggled.

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