MC68HC908MR8 Datasheet, PDF (205/372 Pages) Motorola, Inc – Microcontrollers

Timer Interface A (TIMA)
Functional Description
has occurred by enabling input capture interrupts or by polling the status
flag bit.
The free-running counter contents are transferred to the TIMA channel
status and control registers TACHxH and TACHxL (see 11.10.5 TIMA
Channel Registers) on each proper signal transition regardless of
whether the TIMA channel flag (CH0F–CH1F in the TASC0–TASC1
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 two bus cycles after 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
11.10.5 TIMA 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.
NOTE: Reset does not affect the contents of the input capture channel register
(TACHxH–TACHxL).
11.4.3 Output Compare
With the output compare function, the TIMA can generate a periodic
pulse with a programmable polarity, duration, and frequency. When the
MC68HC908MR8 — Rev 4.1
Freescale Semiconductor
Timer Interface A (TIMA)
Technical Data
205