M68HC08 datasheet(97/172 Pages) FREESCALE

97 / 172 page

I/O Registers

MC68HC908JL3E Family Data Sheet, Rev. 3

Freescale Semiconductor

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 an I/O port, and pin TCHx is

available as a general-purpose I/O pin. Table 8-3 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 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 logic one, setting the CHxMAX bit forces the duty cycle of buffered and

unbuffered PWM signals to 100%. As Figure 8-8 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 8-3. Mode, Edge, and Level Selection

MSxB

MSxA

ELSxB

ELSxA

Mode

Configuration

Output

Preset

Pin under Port Control;

Initial Output Level High

Pin under Port Control;

Initial Output Level Low

Input

Capture

Capture on Rising Edge Only

Capture on Falling Edge Only

Capture on Rising or Falling Edge

Output

Compare

or PWM

Toggle Output on Compare

Clear Output on Compare

Set Output on Compare

Buffered

Output

Compare or

Buffered

PWM

Toggle Output on Compare

Clear Output on Compare

Set Output on Compare

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