Rabbit Rabbit 4000, User Manual

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220 Rabbit 4000 Microprocessor User’s Manual
The capture counter can be synchronized with Timer B outputs to load parallel port output
registers. This makes it possible to generate an output signal precisely synchronized with
an input signal. Usually it will be desired to synchronize one of the input capture counters
with the Timer B counter. The count offset can be measured by outputting a pulse at a pre-
cise time using Timer B to set the output time and capturing the output pulse with an input
capture channel. Once the phase relationship is known between the counters it is then pos-
sible to output pulses a precise time delay after an input pulse is captured, provided that
the time delay is great enough for the interrupt routine to processes the capture event and
set up the output pulse synchronized by Timer B. The minimum time delay needed is prob-
ably less than 10 µs if the software is done carefully and the clock speed is reasonably high.
21.1.2 Input-Count Mode
In the input-count mode the channel simply increments the counter each time the start
condition occurs. The count is enabled by the first Timer A8 clock after the mode is
selected, and the stop condition is generated when the count matches the value written into
the counter MSB and LSB registers. This allows an interrupt to be generated and the
counter halted when a particular count is reached. The stop condition will never occur if
no value is written into the registers.
21.1.3 Block Diagram
Capture
Count
Timer
A8
Counter
Interrupt
Generation
Trigger
Edge
Detection
Parallel
Port Pins
perclk
Interrupt
Request
Input Capture Channel x
ICCR
TAT8R
ICS x R
ICT x R
ICCR
ICT x R
ICCSR
ICM x R ICL x R
CLK
Enable
Stop
Start