Information

The external clock passes through a synchronizer clocked by the system clock to assure
that counter transitions are properly aligned to system clock transitions.Therefore, to
meet Nyquist criteria considering also jitter, the frequency of the external clock source
must not exceed 1/4 of the system clock frequency.
35.4.2 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 00 0 0
1
1 1
2 2
3 3
1
1
1 1 11 1 1
1
selected input clock
prescaler counter
FTM counting is up.
PS[2:0] = 001
CNTIN = 0x0000
MOD = 0x0003
Figure 35-126. Example of the prescaler counter
35.4.3 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.
The FTM counter has these modes of operation:
• Up counting
• Up-down counting
• Quadrature Decoder mode
35.4.3.1 Up counting
Up counting is selected when (QUADEN = 0) and (CPWMS = 0).
CNTIN defines the starting value of the count and MOD defines the final value of the
count, see the following figure. The value of CNTIN is loaded into the FTM counter, and
the counter increments until the value of MOD is reached, at which point the counter is
reloaded with the value of CNTIN.
Chapter 35 FlexTimer Module (FTM)
K20 Sub-Family Reference Manual, Rev. 2, Feb 2012
Freescale Semiconductor, Inc. 743