82C54 Timer Card

[Alan Cox]

The other board I've been testing and writing code for is an 82C54 timer.

Those who followed my 8085 hacking will remember that I couldn't get the Z80 companion chips to behave sanely with an 8085. That left the awkward problem of what (except the TMS9918A) to use as a time source for Fuzix on the 80C85.

It's a fairly simple board with lots of room on it. The usual 74HCT688 and 74HCT32 do the decoding. The device occupies four addresses (I stick it at 0x3C) and

has three counters. Counter 0 counts CPU clocks and has a jumper for the output as I have cunning plans for a single step card. Counter 1 and counter 2 are fed from a 1.8432MHz crystal (because I had some spare). Counter 1 feeds counter 2 and counter 2 feeds a 74HCT03 and the IRQ line.

You set counter 1 up as a 16bit divider and feed that into counter 2 in mode 0. It then counts down pulses and when the count hits zero the output goes high,

interrupt goes low and your CPU gets a kick up the backside.

The other half of the board is unrelated. There was lots of room on it so I stuck a max232 on it. I've not tested that half yet because I've run out of 1uF capacitors.

With all this done 80C85 Fuzix is working rather nicely providing I keep the clock speed down. At 8MHz the 512/512K card seems to get flaky. I see similar behaviour with a 4MHz 6502 so I am guessing the shorter address set up times than Z80 are the cause ? I have a cunning plan for that - when I redid the 80C85 CPU card to fix the interrupt problem in Ben's original I added a simple S100 style banking MMU so it can use the simpler and cheaper flat memory cards. That one is waiting a 74HCT10 - I have a 74HCT109 in my drawer for no apparent reason but no HCT10. I have a feeling the two are connected!

Next up to test when the other components arrive is a Z80PIO and I2C card with connectors to take a classic Arduino Uno shield. It didn't seem fair that only they got all the cool add ons.

Alan

[Sergey Kiselev]

You shouldn't need that 74*32 to qualify RD/WR unless it is some kind of buggy 8254.