mcf51ac256a Freescale Semiconductor, Inc, mcf51ac256a Datasheet - Page 229

• Manufacturer Part Number: mcf51ac256a
• Description: Mcf51ac Flexis
• Manufacturer: Freescale Semiconductor, Inc

The CLKS[1:0] bits may be read or written at any time and disabling the FTM counter (writing 00 to the
CLKS[1:0]) does not affect the FTM counter value or other registers.
The system clock and system clock divided by 2 require no synchronization. The system clock divided by
2 is provided for TPM compatibility.
The fixed system clock requires no synchronization in FTM module. In fact, a synchronization circuit is
used at chip level to synchronize the crystal-related source clock to the system clock.
The external clock source may be connected to any FTM channel pin. This clock source always has to pass
through a synchronizer to assure that counter transitions are properly aligned to system clock transitions.
The system clock drives the synchronizer; therefore, to meet Nyquist criteria even with jitter, the frequency
of the external clock source must not exceed 1/4 of the system clock frequency. With ideal clocks, the
external clock can be as fast as system clock divided by four.
When the external clock source is shared with an FTM channel pin, this pin should not be used for other
channel timing functions. For example, it would be ambiguous to configure channel 0 for input capture
when the FTM channel 0 pin was also being used as the external clock source. It is the user’s responsibility
to avoid such settings.
The FTM channel can still be used. When ELSnB:ELSnA are set to 0:0 the associated FTM channel
physical output is disabled however, compare and match events will continue to set the appropriate flags.
11.4.2
The selected counter clock source passes through a prescaler which is a 7-bit counter. The value of the
prescaler is selected by the PS[2:0] bits
counter and FTM counter.
11.4.3
The FTM has a 16-bit counter which is used to produce the channels output. The FTM counter clock is the
clock generated using the prescaler counter
The FTM counter can be an up or up-down counter according to the CPWMS bit.
Freescale Semiconductor
Prescaler
Counter
TPM EPWM
PS[2:0] = 001
FTMxCNTINH:L = 0x0000
FTMxMODH:L = 0x0003
selected input clock
MCF51AC256 ColdFire Integrated Microcontroller Reference Manual, Rev. 5
prescaler counter
FTM counter
Figure 11-26. Example of the prescaler counter
0
1
0
1
(Table
1
0
(Section 11.4.2,
2
1
11-5).
0
3
1
Figure 11-26
0
0
1
“Prescaler).
0
1
1
0
shows an example of the prescaler
2
1
0
3
1
0
0
1
FlexTimer Module (FTMV1)
0
1
11-29