27.4.1 Clock Domains
The TPM module supports two clock domains.
The bus clock domain is used by the register interface and for synchronizing interrupts.
The TPM counter clock domain is used to clock the counter and prescaler along with the
output compare and input capture logic. The TPM counter clock is considered
asynchronous to the bus clock, can be a higher or lower frequency than the bus clock and
can remain operational in Stop mode. Multiple TPM instances are all clocked by the
same TPM counter clock in support of the external timebase feature.
27.4.1.1 Counter Clock Mode
The CMOD[1:0] bits in the SC register either disable the TPM counter or select one of
two possible clock modes for the TPM counter. After any reset, CMOD[1:0] = 0:0 so the
TPM counter is disabled.
The CMOD[1:0] bits may be read or written at any time. Disabling the TPM counter by
writing zero to the CMOD[1:0] bits does not affect the TPM counter value or other
registers, but must be acknowledged by the TPM counter clock domain before they read
as zero.
The external clock input passes through a synchronizer clocked by the TPM counter
clock to assure that counter transitions are properly aligned to counter clock transitions.
Therefore, to meet Nyquist criteria considering also jitter, the frequency of the external
clock source must be less than half of the counter clock frequency.
27.4.2 Prescaler
The selected counter clock source passes through a prescaler that is a 7-bit counter. The
value of the prescaler is selected by the PS[2:0] bits. The following figure shows an
example of the prescaler counter and TPM counter.
Functional Description
KL02 Sub-Family Reference Manual, Rev. 2.1, July 2013
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Freescale Semiconductor, Inc.
