The external clock passes through a synchronizer clocked by the system clock to ensure
that counter transitions are properly aligned to system clock transitions. Therefore, to
meet the Nyquist criteria and account for jitter, the frequency of the external clock source
must not exceed 1/4 of the system clock frequency.
12.4.2 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 FTM counter.
FTM counter
0
0
0
0
0
0
0
0
0
0
0
0
1
1
1
2
2
3
3
1
1
1
1
1
1
1
1
1
EPWM
selected input clock
prescaler counter
PS[2:0] = 001
CNTINH:L = 0x0000
MODH:L = 0x0003
Figure 12-187. Example of the prescaler counter
12.4.3 Counter
The FTM has a 16-bit counter that is used by the channels either for input or output
modes. The FTM counter clock is the selected clock divided by the prescaler (see
The FTM counter has these modes of operation:
)
• up-down counting (see
12.4.3.1 Up counting
Up counting is selected when (CPWMS = 0).
CNTINH:L defines the starting value of the count and MODH:L defines the final value
of the count; see the following figure. The value of CNTINH:L is loaded into the FTM
counter, and the counter increments until the value of MODH:L is reached, at which
point the counter is reloaded with CNTINH:L.
Chapter 12 FlexTimer Module (FTM)
MC9S08PT60 Reference Manual, Rev. 4, 08/2014
Freescale Semiconductor, Inc.
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