I have a chip I'm trying to connect to the IRQ line of a Motorola 6800-style bus. Typical 68xx-family devices have open-collector interrupt outputs, so multiple chips can connected to the IRQ input in a wired-or configuration.
However, the interrupt output of the chip I have (a TMS9918A Video Display Processor ) is an active-low logic output; it's driven both high and low
I figure I could connect the interrupt output to an inverter and then to the base of an NPN transistor, and then connect the collector to the IRQ line.
I have two questions though:
- What value should I use for the base resistor? I can never remember...
- Can I use a PNP transistor and save an inverter? What would the schematic for this look like?
Thanks,
-matt
However, the interrupt output of the chip I have (a TMS9918A Video Display Processor ) is an active-low logic output; it's driven both high and low
I figure I could connect the interrupt output to an inverter and then to the base of an NPN transistor, and then connect the collector to the IRQ line.
I have two questions though:
- What value should I use for the base resistor? I can never remember...
- Can I use a PNP transistor and save an inverter? What would the schematic for this look like?
Thanks,
-matt