Jameco-electronics Rabbit 3000 Bedienungsanleitung PDF herunterladen (Seite 169)

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160 Rabbit 3000 Microprocessor

Timer B can be used for various purposes. The 10-bit counter can be read to record the

time at which an event takes place. If the event creates an interrupt, the timer can be read

in the interrupt routine. The known time of execution of the interrupt routine can be sub-

tracted. The variable interrupt latency is then the uncertainty in the event time. This can be

as little 19 clocks if the interrupt is the highest priority interrupt. If the system clock is 20

MHz, the counter can count as fast as 10 MHz. The uncertainty in a pulse width measure-

ment can be nearly as low as 38 clocks (2 x 19), or about 2 µs for a 20 MHz system clock.

Timer B can be used to change a parallel port output register at a particular specified time

in the future. A pulse train with edges at arbitrary times can be generated with the restric-

tion that two adjacent edges cannot be too close to each other since an interrupt must be

serviced after each edge to set up the time for the next edge. This restriction limits the

minimum pulse width to about 5 µs, depending on the clock speed and interrupt priorities.

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