Jameco Electronics 3000, 2000 manual Rabbit Capabilities, Precisely Timed Output Pulses

4.RABBIT CAPABILITIES

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.

		Timer Output
A	B
	C	Parallel Port Output
Latency		Parallel Port Output
	Interrupt
	routine sets
	Setup Register	Timer Output

Figure 4-1. Timed Output Pulses

The timer output in Figure 4-1is 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.

User’s Manual 49