Interrupt (IRQ) on a C1P

[Thomas]

In one of my C1P, I had an Interrupt (IRQ) circuit implemented, that was pretty simple but useful for many projects that required background processing. If someone likes to check this out and have fun with interrupt routines, here the technical details.
Components needed:
2x 1nF
1x 47kOhm
1x 22Ohm
1x 2N3904 NPN or similar

This circuit will convert a square wave signal into a 12 to 20 usec pulse to ground. Such a pulse is required for a correct IRQ detection by the CPU.
For example, 3.84 kHz can be found on U60 pin 13 (C9) on C1P / SB600 boards at the 74LS163 video counters. C9 will provide a 260 usec interrupt interval.

IRQ Circuit.png (149.63 KiB) Viewed 10080 times

Because video counters are used, interrupts occur synchro to the video signal generation. In this case (3.84 kHz), the interrupt will take place every 4 display lines at the beginning of the horizontal retrace.

Other IRQ intervals can be easily chosen from the various counter outputs.
Attached a sample program showing background screen effects while in Basic.

IRQ.BAS

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Thomas