MC68HC08AS32 Datasheet, PDF (210/394 Pages) Motorola, Inc – M68HC08 Family of 8-bit microcontroller units (MCUs)

Timer Interface (TIM)
The result obtained by an input capture will be one more than the value
of the free-running counter on the rising edge of the internal bus clock
preceding the external transition. This delay is required for internal
synchronization.
The free-running counter contents are transferred to the TIM channel
registers (TCHxH–TCHxL) (see 16.9.5 TIM Channel Registers) on
each proper signal transition regardless of whether the TIM channel flag
(CH0F–CH5F in TSC0–TSC5 registers) is set or clear. When the status
flag is set, a CPU interrupt is generated if enabled. The value of the
count latched or “captured” is the time of the event. Because this value
is stored in the input capture register when the actual event occurs, user
software can respond to this event at a later time and determine the
actual time of the event. However, this must be done prior to another
input capture on the same pin; otherwise, the previous time value will be
lost.
By recording the times for successive edges on an incoming signal,
software can determine the period and/or pulse width of the signal. To
measure a period, two successive edges of the same polarity are
captured. To measure a pulse width, two alternate polarity edges are
captured. Software should track the overflows at the 16-bit module
counter to extend its range.
Another use for the input capture function is to establish a time
reference. In this case, an input capture function is used in conjunction
with an output compare function. For example, to activate an output
signal a specified number of clock cycles after detecting an input event
(edge), use the input capture function to record the time at which the
edge occurred. A number corresponding to the desired delay is added
to this captured value and stored to an output compare register (see
16.9.5 TIM Channel Registers). Because both input captures and
output compares are referenced to the same 16-bit modulo counter, the
delay can be controlled to the resolution of the counter independent of
software latencies.
Reset does not affect the contents of the input capture channel
(TCHxH–TCHxL) registers.
Advance Information
210
Timer Interface (TIM)
MC68HC08AS32 — Rev. 3.0
MOTOROLA