
I/O Registers
MC68HC908GT16 MC68HC908GT8 MC68HC08GT16 Data Sheet, Rev. 5.0
Freescale Semiconductor
235
ELSxB and ELSxA — Edge/Level Select Bits
When channel x is an input capture channel, these read/write bits control the active edge-sensing logic
on channel x.
When channel x is an output compare channel, ELSxB and ELSxA control the channel x output
behavior when an output compare occurs.
When ELSxB and ELSxA are both clear, channel x is not connected to port D, and pin PTDx/TCHx is
available as a general-purpose I/O pin.
Table 18-2
shows how ELSxB and ELSxA work.
Reset clears the ELSxB and ELSxA bits.
NOTE
Before enabling a TIM channel register for input capture operation, make
sure that the PTD/TCHx pin is stable for at least two bus clocks.
TOVx — Toggle On Overflow Bit
When channel x is an output compare channel, this read/write bit controls the behavior of the channel
x output when the TIM counter overflows. When channel x is an input capture channel, TOVx has no
effect.
Reset clears the TOVx bit.
1 = Channel x pin toggles on TIM counter overflow.
0 = Channel x pin does not toggle on TIM counter overflow.
NOTE
When TOVx is set, a TIM counter overflow takes precedence over a
channel x output compare if both occur at the same time.
CHxMAX — Channel x Maximum Duty Cycle Bit
When the TOVx bit is at 1, setting the CHxMAX bit forces the duty cycle of buffered and unbuffered
PWM signals to 100%. As
Figure 18-12
shows, the CHxMAX bit takes effect in the cycle after it is set
or cleared. The output stays at the 100% duty cycle level until the cycle after CHxMAX is cleared.
Table 18-2. Mode, Edge, and Level Selection
MSxB
MSxA
ELSxB
ELSxA
Mode
Configuration
X
0
0
0
Output preset
Pin under port control; initial output level high
X
1
0
0
Pin under port control; initial output level low
0
0
0
1
Input capture
Capture on rising edge only
0
0
1
0
Capture on falling edge only
0
0
1
1
Capture on rising or falling edge
0
1
0
0
Output compare or PWM
Software compare only
0
1
0
1
Toggle output on compare
0
1
1
0
Clear output on compare
0
1
1
1
Set output on compare
1
X
0
1
Buffered output compare or
buffered PWM
Toggle output on compare
1
X
1
0
Clear output on compare
1
X
1
1
Set output on compare