MC68HC908GR16 Datasheet, PDF (235/276 Pages) Motorola, Inc – Microcontrollers

I/O Registers
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 18-3 shows how ELSxB and ELSxA work. Reset clears
the ELSxB and ELSxA bits.
Table 18-3. Mode, Edge, and Level Selection
MSxB:MSxA ELSxB:ELSxA
Mode
Configuration
X0
00
Output
Pin under port control; initial output level high
X1
00
preset
Pin under port control; initial output level low
00
01
Capture on rising edge only
00
10
Input
capture
Capture on falling edge only
00
11
Capture on rising or falling edge
01
01
Toggle output on compare
01
10
Output compare
or PWM
Clear output on compare
01
11
Set output on compare
1X
01
Buffered output Toggle output on compare
1X
10
compare
Clear output on compare
or buffered
1X
11
PWM
Set output on compare
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.
MC68HC908GR16 Data Sheet, Rev. 5.0
Freescale Semiconductor
235