6 hours ago, TomXP411 said:
The problem is that I don't know if there's a way to directly read CA1/CA2. Can I read those pins with a PEEK? Otherwise, I need a separate ACK line, since ACK is currently tied to CA1, which is also the interrupt pin. (And I don't want the board triggering an interrupt when I do a read cycle.)
I'll leave this here, since it's still relevant to programming a software serial port.
(1) Either CA1 or CA2 or both can be used to trigger an IRQ, depending on the setting of the Interrupt Enable Register ... CA1, CA2, CB1 and CB2 IRQ enable are four of the seven interrupt enables in the VIA (the other three are two timer underflows and the serial shift register has completed 8 bits).
(2) As inputs, CA1 and CA2 can ONLY detect edge transitions, they cannot detect levels. The only really convenient way to detect both edge transitions without constantly resetting the CA&CB mode register is to tie them together and have one set to detect positive edges and the other set to detect negative edges. Then they will detect any edge transition since the last Read/Write of Port A. Having Centronics ACK tied to a GPIO makes more sense ... similar to the way that have external device Initialize under program control instead of tied to the CX16 system reset via the reset signal sent to the VIA makes more sense, but now unless Port B pin 6 and 7 are available to perform those functions, we are starting to run out of pins.
(3) By contrast, CA2 as /Interrupt detect, and then read the status to find out the cause of the interrupt would work fine ... CA1 really only has two states, for detecting positive or negative edge transitions and is ALWAYS reset after a Port A Read/Write, but CA2 can be used as an independent interrupt, so even if you are polling for the interrupt, you won't lose it because there was a Port A Read/Write in between the interrupt trigger and the poll.
(4) Both the Interrupt Flag Register and Interrupt Enable Register can be PEEKed or POKEd.
