Having trouble interfacing LP CMOS to ZX40

[drapal]

I read the app note, and I think I'm going to need a transistor switch to do this, but I'll ask the question anyway. I'd like to be able to just find the right pull-up value if that's possible.

What I have is an A6821 (http://www.allegromicro.com/datafile/6821.pdf) that I'm trying to read the serial output from. The datasheet doesn't indicate what the source/sink current is for the output (only that is low power CMOS), and with nothing connected to it, the voltmeter indicates that it's working properly. However, when I hook this up to the ZX40, if the pin is set to zxInputPullup, the read alwas reaturns a 1 (and the VM verifies that), so the 6821 clearly can't sink enough current. If I change the pin to zxInputTristate, it always reads 0 (can't source enough tto change the ZX input).

I'm pretty sure that I could put a transistor switch in and get it to work, but the board real estate is asking for only a pull-up resistor if possible. Any suggestions?

BTW, everything on the logic side is running at the std. 5V supply. The "drive" side of the 6821 is driving some 12V relays.

[GTBecker]

> ... only a pull-up resistor if possible...

The Allegro spec appears to indicate a normal CMOS data output, with
high >2.8v and low <0.3v with a 5v supply @ 200uA, the equivalent of a
~47k load. You should need no pullup on the ZX input pin but one
shouldn't affect the data, either.

I suspect something else is at fault.


Tom

[dkinzer]

Which output from the chip are you trying to read? What instructions are you using to do so?

[drapal]

I've tried both D.4 and a bit on Port C (don't recall exactly which one). To determine what was read, I've always used debug.print CStrHex(Register.PortD) (or PortC when I tried the other pin).

[drapal]

Just for grins, I changed to use debug.print CStr(GetPin(D.4)) and it works. So what is wrong about using Register.PortD?

[dkinzer]