Hello, I'm developing a new device and it's interesting to me to be able to introduce wait states into the Z80 from the expansion port.
Did anyone try to set the WAIT signal from an external device? Would it interfere with the WAIT states added by the gate array? is the READY signal in the gate array Open Drain or there is a chance of short circuit if the external device tries to set the opposite value?
Regards,
Abalore
I answer myself, in the great new redrawn 464 schematics from
@SerErris it can be read:
READY (O.C.600R PULL UP)
Which I interprete as "open collector with 600 ohm pull-up resistor), so all my concerns have vanished now.
Good work with the schematics!
Yep, that is how it is supposed to work. However consider that this is still only 600ohm. That will mean a pretty decent amount of current coming out of it. So your device should be as well open collector, so that the GA does not toast it every 1ms. Or you may want to put a resistor in front to make reduce the current. I am not sure how much current the Z80 would need to check the signal.
Quote from: SerErris on 13:00, 16 November 23Yep, that is how it is supposed to work. However consider that this is still only 600ohm. That will mean a pretty decent amount of current coming out of it. So your device should be as well open collector, so that the GA does not toast it every 1ms. Or you may want to put a resistor in front to make reduce the current. I am not sure how much current the Z80 would need to check the signal.
My device is a CPLD that sets the signal either to 0 or to Z (high impedance), the current should be around 8mA. Not a big deal for the pin. I don't know the current required to actually pull down the signal in the Z80, let's see in practice. The worst thing it can happen is to need to put a MOSFET in the middle with some current limiting resistor.
Yes, that makes sense as well. High-Z is an option as you do not care about the Ready signal, you only want to stop the CPU.
If you want to control the bus longer then just for a few cycles, the #BUSRQ would be an option too.
Redrawn 464 schematics? Where can I find these?
Of cause in the Wiki :-)
https://www.cpcwiki.eu/index.php/Schematics
https://www.cpcwiki.eu/index.php/File:464SchematicRedraw_white.pdf