Jameco Electronics Rabbit 3000 - Manual (Page 58)

User’s Manual 49
4. R
ABBIT
C
APABILITIES
This chapter describes the various capabilities of the Rabbit that
may not be obvious from the technical description.
4.1 Precisely Timed Output Pulses
The Rabbit can output precise pulses under software control. The effect of interrupt latency
is avoided because the interrupt always prepares a future pulse edge that is clocked into
the output registers on the next clock. This is shown in Figure 4-1.
Figure 4-1. Timed Output Pulses
The timer output in Figure 4-1 is periodic. As long as the interrupt routine can be com-
pleted during one timer period, an arbitrary pattern of synchronous pulses can be output
from the parallel port.
The interrupt latency depends on the priority of the interrupt and the amount of time that
other interrupt routines of the same or higher priority inhibit interrupts. The first instruc-
tion of the interrupt routine will start executing within 30 clocks of the interrupt request
for the highest priority interrupt routine. This includes 19 clocks for the longest instruction
to complete execution and 10 clocks for the interrupt to execute. Pushing registers requires
10–12 clocks per 16-bit register. Popping registers requires 7–9 clocks. Return from inter-
rupt requires 7 clocks. If three registers are saved and restored, and 20 instructions averag-
ing 5 clocks are executed, an entire interrupt routine will require about 200 clocks, or 10
µs with a 20 MHz clock. Given this timing, the following capabilities become possible.
A B
Latency
Interrupt
routine sets
C
Timer Output
Parallel Port Output
Timer Output
Parallel Port Output
Setup Register