Interfacing AVR to MOS logic

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svofski
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Posted by svofski: Thu. Nov 12, 2009 - 01:07 PM
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I'm doing something atmega8-based that plugs to a Commodore 64 (heh) joystick port. This something causes me some unexpected trouble. C64 joystick port lines are shared with keyboard matrix lines, so screwing with them makes the keyboard malfunction. Normally keyboard is being scanned with a running 0 and joystick is being checked when keyboard is not active. Joystick is typical Atari, normally open, switches close to the ground. Joystick port 1 is connected to keyboard row lines, joystick port 2 is connected to keyboard column lines.

(for the really curious, the schematic part that shows control ports and keyboard connections is here: http://zimmers.net/anonftp/pub/c... )

My atmega8 has joystick lines connected directly to PORTC pins. The plan was to keep pullups off and lines tristated. When I'd need to imitate a switch closing, I'd write "1" to corresponding DDR bit and that will be it. Hard to imagine anything more simple than this.

But not all is well. When I plug my magic device in, C64 basically goes nuts. Namely, certain keyboard keys, probably related to one of the pins, get broken readings. This is possible, because some pullup or output could interfere. But this happens even when the entire code consists of, just to be really sure:

cli(); SFIOR |= PUD; while(1);

It keeps going on even when the device is in reset state. I checked all lines, I get at least 30MOhm to the ground on each pin.

Now I never really dealt with MOS logic before. Is there something very special about it that has to be taken care of? I don't even know where to start because I seem to have no control of the situation (except for plugged/unplugged ;) To add to curiosity, stick emulation works on some lines and on some it doesn't. Probably those that don't work are the same that are shared with keyboard lines that get affected. I'm perplexed. Any random ideas from your trick bags?

I realize this question is bordering with GE forum, but it's also very AVR-specific so I posted it here.

[update]
6526 CIA datasheet:
http://archive.6502.org/datashee...

Says that PA/PB are TTL/CMOS compatible... Even weirder now.

The Dark Boxes are coming.

Tags:

AVR Microcontrollers, megaAVR and tinyAVR, ATmega8
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ossi
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Posted by ossi: Thu. Nov 12, 2009 - 01:23 PM
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Have you observed the lines with an oscilloscope ?

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svofski
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Posted by svofski: Thu. Nov 12, 2009 - 01:32 PM
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Yeps, they look pretty normal to me, not particularly flat or anything. But I probably should collect a profile on each line, how it behaves with and without the device and with other joysticks too. It may be a little hard to do because they're being affected by the keyboard scanning too so the patterns I see are somewhat irregular, plugged or not.

[edit] after poking into some old hardware I can understand your avatar much better ;D

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j0n90
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Posted by j0n90: Thu. Nov 12, 2009 - 01:55 PM
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Try isolating the AVR outputs from the C64 with an series of opto isolators.

Also are you powering off the joystick port?

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ossi
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Posted by ossi: Thu. Nov 12, 2009 - 02:01 PM
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I sometimes connect a line under observation
to 2.5 Volts via a 10k resistor, so that
I can distinguish high/low from high-Z.
It sometimes helps.

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svofski
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Posted by svofski: Thu. Nov 12, 2009 - 02:05 PM
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Yes, I'm powering off the joystick port. Docs say 50mA tops, but it seems to be directly connected to the main +5V bus inside of the C64 without any tricks. I wouldn't exceed 50mA anyway. The power near atmega8 is filtered with a few tankish and a couple of smallish capacitors, as usual. You think I could filter it better? Add some L-C-L stuff? The AVR itself doesn't seem to object to power quality: it also works with the analog inputs and it handles them perfectly well at the same time.

The Dark Boxes are coming.

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j0n90
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Posted by j0n90: Thu. Nov 12, 2009 - 02:31 PM
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I think i would be inclined to directly emulate the joystick hardware either with an opto or relay and then work back trading off parts for compatability. Although the parts cost for a couple of mocd217's and a few 1k resistors is less than a quid. There shouldn't be any issue with tri stating the outpus without pullups etc but clearly something is going on.

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Jepael
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Posted by Jepael: Thu. Nov 12, 2009 - 02:38 PM
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So the joystick is connected to the matrix, but is not scanned as matrixed, since joystick can just pull line to GND or not.

Your thinking is correct how to drive the lines. I guess more oscilloscoping is needed.

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svofski
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Posted by svofski: Thu. Nov 12, 2009 - 04:57 PM
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Hi Giorgos! A joystick is a very dumb and simple device: it just closes switches, it can't monitor when something is being scanned or not. Yes, it interferes with keyboard scanning when it's being used and it's a known feature, but when all switches are open they are open and no interference must happen. So I hoped it will be with my circuit but alas, it's close but no cigar.

POTX/Y are comparator inputs with additional capability to be pulled to ground. They are used to discharge a capacitor to ground and then measure the time it takes to charge it back to about +5V through an analog paddle potentiometer. With those I interface perfectly well BTW, they behave.

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Jepael
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Posted by Jepael: Thu. Nov 12, 2009 - 06:01 PM
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The keyboard *could* be a part of matrix if the buttons would be rows and the common would be some column for example, but since the buttons are returned to GND instead of the matrix, so it is not a matrix.

However I can't stop thinking about the movie "Matrix" now.

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svofski
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Posted by svofski: Thu. Nov 12, 2009 - 08:39 PM
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I solved the problem with optics. It was a tiny bridge between two PCB traces. I checked each pin to ground individually, but never checked resistance between the pins. Heh heh. I talked to a friend about my woes and he told a story of a bridge between pcb traces he had in a board once that he could only find with a microscope. I came home and — d'oh, what's that glitter!?

So thank you everyone, it's solved now! It's a PS2 -> 1351 mouse adapter just in case anyone's interested :)

The Dark Boxes are coming.

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Jepael
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Posted by Jepael: Fri. Nov 13, 2009 - 10:00 AM
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svofski wrote:
I solved the problem with optics. It was a tiny bridge between two PCB traces. I checked each pin to ground individually, but never checked resistance between the pins.

It happens, and usually last thing to measure :)

Sometimes you just can't see or fix where the problem is, like if you have more than two PCB layers, and the short is in the middle layers or vias.

In those cases, you can try to blow it like a fuse, with very low voltage like 1V or less if components are mounted, and then increase the lab power current limit until current flows no more. If the PCB was already broken, it can either fix the problem, not fix the problem, or make things worse like blow a trace from a wrong place, so usually you have nothing to lose.

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svofski
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Posted by svofski: Fri. Nov 13, 2009 - 01:36 PM
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Quote:
Sometimes you just can't see or fix where the problem is, like if you have more than two PCB layers, and the short is in the middle layers or vias.

In those cases, you can try to blow it like a fuse, with very low voltage like 1V or less if components are mounted, and then increase the lab power current limit until current flows no more. If the PCB was already broken, it can either fix the problem, not fix the problem, or make things worse like blow a trace from a wrong place, so usually you have nothing to lose.


Wow, I would never think of that but it's a trick worth remembering. Thanks!

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sparrow2
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Posted by sparrow2: Fri. Nov 13, 2009 - 04:56 PM
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As I remember it the C64 can't see the difference between some of the joystick possitions and the keyboard. (The adc's is only used for the pads).

Jens

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