I'm new to micro design, so I was wondering if someone could give me some pointers about this schematic:
http://socialkonstrukt.com/hunley-1.GIF
The PT0-7 will be to a parallel receipt printer, DSP0-7 will go to this LCD display http://www.allelectronics.com/make-a-store/item/LCD-212/2-X-12-LCD-WITH-LED-BACKLIGHT/1.html through a MAX233, and KB0-7 is to an ascii keyboard. The enable/strobe pins for these are all on one 6522 port together, so the printer and display can be strobed together. I know I'm wasting a lot of address space with the decoder setup, but that's OK with me for now.

Would this work? Is there some glaring or subtle design mistake I should fix before ordering the parts? Do I need to buffer the i/o? Am I missing something obvious?

The processor's phase-1 input must be used for the clock circuit, whether you use the internal oscillator or not. Phase 2 on the 6502 is an output, not an input.

You'll need a RST circuit, even if it's just an RC. (The CMOS 6502 has a schmitt-trigger input there, and timing is not critical like it was on the NMOS 6502.) Take the RST line to the 6522's also. Don't leave their RST lines floating.

If you use a WDC 65c02, pin 1 is a VP (vector pull) output, so you won't want to ground that. If it's another brand, just leave it grounded.

The RAM's W\ line will have to go to the processor's R/W\ output.

You need to gate the RAM select with phase 2, because the address lines are not guaranteed to be valid before the R/W\ line is, let alone with enough set-up time for the RAM. Without gating RAM select with phase 2, you could inadvertently write to wrong addresses and corrupt your data. Don't gate everything with phase 2 though, as the 6522's need the chip-select lines to be valid and stable before phase 2 rises. The address decoding for much more than this can be done with a single 74xx00, and allow faster running too. See http://www.6502.org/users/garth/project ... chematic=2 . (Replace the inverter with a section of the 74xx00.)

You need the 3.3K pull-up resistor on the IRQ line. If you use WDC's 65c22's which have totem-pole IRQ\ outputs, you'll have to AND them together and feed the output of the AND gate to the IRQ\ input of the processor.

Don't leave the processor's NMI, RDY, and S.O. inputs floating. Tie them high.

The MAX233 won't be of any value feeding the LCD which, according to the data sheet, only has a 4- and 8-bit parallel interface with TTL levels, not RS-232 levels.

When you get ready to write the software for the LCD, contact me, as there's a trick to getting a dependable reset on it.

If you wanted to, you could make the same 8 data lines to feed the LCD and the printer and the keyboard, then use the other port of the same 6522 for the handshake and select lines.

This was very hastily written. I won't feel bad at all if someone points out things I missed!

I think I understand what you mean here. I have the canned oscillator going straight into phase-1 now.

I'm using a MAX706 here. I'm ignoring the watchdog timer. Do you think that will be OK?

It's a Rockwell, I think.

fixed.

Thank you very much for this circuit. It's much more elegant. I'm also upping my RAM up to 16k like your bench computer. It's only a few pennies more.

Fixed. Stupid mistake. Thanks!

More noobishness on my part. Thanks

Yeah, I don't know what I was thinking.

I'm curious to know now

You're right, that would be much more clever. I'll ponder this while I'm painting my kitchen this weekend.

Thank you very much. These were most helpful.
EDIT: Here's the revised schematic: http://socialkonstrukt.com/hunley-6502-rev2.GIF. Does anyone else see anything wrong with the design? I would really appreciate the help. Also, does anyone know where I can source a parallel ascii keyboard?
EDIT2: pretend that the 2865 eeprom is a 32k device.

I'm sure it will be fine, but it might be easier to do something like in http://www.6502.org/users/garth/project ... chematic=1 .


I get an error 404 (page not found) on that one.

For a keyboard with the matrix interface brought out, there are probably a lot of vintage computers you could get one from.

I'll through in a few items:

I'm not sure you can tie the outputs of the 74138 together to create the /CS for the RAM. You can use A15 as the /CS for the RAM. That gives you 32k of RAM space.

Also, you cannot connect the /CS and /OE together on the RAM. The /OE will need to have an inverted CPU R/W as it input. Otherwise, during a write cycle, both /OE and /WR would be active.

I'll check over your revised schematic, once its posted correctly.

Daryl

SORRY!
http://socialkonstrukt.com/hunley-2.gif
That should work. Thanks guys.

Yes you can as /OE is justified internally with /CE and //WE. It is even specifically allowed in most static RAM data sheets.

You can, if you wish, hold /OE permamently low to no ill effect in most cases.

Lee.

Garth, you said you have some experience with this type of lcd. If I'm running at 1mhz, would it be better to hang it off a VIA, or to put it straight on the bus. I ask because I'm digging through the datasheets, and it seems like the programming will be greatly simplified if it's directly on the bus. If so, do you have any tips for doing so?

_________________
http://WilsonMinesCo.com/ lots of 6502 resources
The "second front page" is http://wilsonminesco.com/links.html .
What's an additional VIA among friends, anyhow?

What do you do about the busy flag? just do your own timing to figure out when it should be free again?

I stand corrected. I should have read the datasheet before I spoke.

Thanks for keeping me honest, Lee!

Daryl

Another question: Why, in Garth's 74x00 address decoder, does the RAM need to be gated with phase2, but the ROM doesn't?

_________________
http://WilsonMinesCo.com/ lots of 6502 resources
The "second front page" is http://wilsonminesco.com/links.html .
What's an additional VIA among friends, anyhow?

Garth, could you shed some light on my previous question. Specifically, if you never read from the display, how do you ascertain whether its busy or ready to accept input?

_________________
http://WilsonMinesCo.com/ lots of 6502 resources
The "second front page" is http://wilsonminesco.com/links.html .
What's an additional VIA among friends, anyhow?