I have a really basic question. I have to design an 65C02-based personal computer for a school project. I have to draw my own footprint for the 65C02. My question is what type of pin is the IRQB (Interrupt) pin? I know what the purpose of the pin is. I have no idea if it is Input or Output... or Bidirectional. The datasheets have no mentions on what each pin is as far as type.

  • 3
    You should add a link to the particular data sheet you're using so we can tell you if you're reading it correctly. (It would be a bit of a weird data sheet if it didn't explain enough about the pin to determine if it's an input or ouput, unless it's a summary or overview rather than a proper data sheet.)
    – cjs
    Commented Jul 28, 2019 at 15:03
  • I think I'm using a summary! It is only 12 pages long... Commented Jul 29, 2019 at 7:44

3 Answers 3


The interrupt pin is an input.


The W65C02 data sheet does explain it (p. 9):

3.4 Interrupt Request (IRQB)

The Interrupt Request (IRQB) input signal is used to request that an interrupt sequence be initiated. The program counter (PC) and Processor Status Register (P) are pushed onto the stack and the IRQB disable (I) flag is set to a "1" disabling further interrupts before jumping to the interrupt handler. These values are used to return the processor to its original state prior to the IRQB interrupt. The IRQB low level should be held until the interrupt handler clears the interrupt request source. When Return from Interrupt (RTI) is executed the (I) flag is restored and a new interrupt can be handled. If the (I) flag is cleared in an interrupt handler, nested interrupts can occur. The Wait-for- Interrupt (WAI) instruction may be used to reduce power and synchronize with, as an example timer interrupt requests.

So it not only tells you that this is an input signal, it also explains how long it should be held at low level.

The General Timing Diagram (Figure 6-3, p. 26) has some more timing information for IRQB.

  • Thank you! I haven't seen that particular datasheet, there are multiple versions. I will follow that datasheet in the future as it is more detailed. Commented Jul 29, 2019 at 7:22

An interrupt pin of a CPU will always be an Input.

After all, it's whole purpose is to tell the CPU about an interrupt pending. Wouldn't work if it's an output, would it?

  • Some microcontrollers use bidirectional pins for interrupts or even, for some Motorola HC11 variants, the reset line(!?). If a Computer Operating Properly timer (watchdog) fires, it will pull the reset line low, thus allowing it to reset other devices at the same time. I don't care for the design, but it does indicate that such pins not need be purely inputs.
    – supercat
    Commented Jul 29, 2019 at 4:10
  • Yeah, I've seen situations like those. That's why I was so confused. Thank you! Commented Jul 29, 2019 at 7:23
  • 1
    @supercat Well, you already put the distinction in your comment: They are microcontollers, not micro processors. Their pins aren't CPU pins but system busses. A Timer is already an external component pulling the reset line low that happens to be on this controller/SoC. Not so a single CPU, like the 6502. While it often doesn't matter to distinguish between a microprocessor and more integrated chips, this is one were it does.
    – Raffzahn
    Commented Jul 29, 2019 at 8:46
  • @Raffzahn: The 68HC11 in question was often used as a microprocessor, with code stored in an external ROM.or RAM (as it was in the application where I was in fact using it), and the /RESET input was in fact being used as a /RESET pin, but it's driven low for IIRC two cycles if a watchdog timeout occurs.
    – supercat
    Commented Jul 30, 2019 at 1:11
  • @supercat Haven't argued about. I'm well aware of the that chip. Build one a (somewhat) successful device base on two of them. Still, no matter if an external ROM is used or not, it'll continues to be a Controller/SoC, not just a microprocessor.
    – Raffzahn
    Commented Jul 30, 2019 at 7:30

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