I received an email from CPU-builder Dieter Müller, describing his very impressive M02 project to build a custom CPU binary compatible with the 6502, using standard 74LS and similar parts. In short, a project quite similar to my own, except that he’s already done it. Take a look at the photos and hardware description on his site http://freenet-homepage.de/dieter.02/m02.htm. He even got the Commodore 64 kernel and BASIC running on the M02.

Here’s a job that sounds reasonable enough: connect an 8-wide dip switch to a pair of 7 segment LED displays, so that the 8-bit binary number on the dip switch is displayed as a 2-digit hex number on the LED displays. I didn’t think it would take me too long to configure a GAL to control which segments of the LED displays to turn on, and then wire the whole thing up. Boy, was I wrong.

My basic plan was to connect the dip switch to the GAL’s inputs, and the LED displays to the GAL’s outputs. Easy cheesy.

Problem 1. A dip switch doesn’t provide logical 0 or 1 values (0 or +5 volts) that can be connected right to the GAL’s inputs. It’s just 8 switches, each one either open or closed. A simple pull-up resistor circuit is needed to generate a 0 or +5 volt input from the switch position. When the switch is open, no current flows through the resistor, so there’s no voltage drop, and the output terminal is at +5 volts. When the switch is closed, the output terminal is tied directly to ground (0 volts).

One problem with this design is that when the switch is closed, current is constantly flowing from +5 through the resistor to ground. With a 440 Ohm resistor (the largest I had on hand), 11 milliamps (5 / 440) flows through the resistor. With all eight switches closed, that’s 88 mA wasted. My hack-tastic power supply can probably only supply a few hundred milliamps, so that’s a big fraction of the total available supply.

Problem 10. Decoding a 4-bit number is fairly simple: 4 inputs from the dip, the GAL decodes the data, 7 outputs to the LED display. An 8-bit number using the same approach would require 8 inputs and 14 outputs, but the GAL only has 10 outputs. Doh!

My solution is to share the same 7 outputs between two different LED displays, with an 8th and 9th output to enable one or the other display. Only one display can be illuminated at a time, but if you switch between the displays rapidly enough, it looks as if they’re both illuminated. I used a 1 MHz clock oscillator to switch back and forth between the two displays. The 7 data outputs from the GAL are connected to the positive terminal of the 7 LED segments on each display. The 8th data output is connected to the shared negative terminal of the LED segments in the first display, and the 9th data output is connected to the shared negative terminal of the second display.

To illuminate a display, its shared negative terminal is set to 0, and the appropriate data outputs are set to 1, generating a positive voltage across the LED segments. To deactivate a display, its shared negative terminal is set to a high-impedance (disconnected state), so none of the LED segments will light up regardless of what the data outputs are doing. It would also be possible to deactivate a display by setting its shared negative terminal to 1 (+5 volts), but this would put a reverse voltage across the LED segments. That’s best to avoid, since too large of a reverse voltage can damage a diode.

Problem 11. Writing the logic equations for the GAL to decode 8 input bits into 2*7 LED segments proved more difficult than I expected. My first attempt had way too many product terms per output, and was too complex to fit in the GAL. I did some hand optimization, but it was tedious and error-prone. Finally I found a website with an interactive Java applet for making Karnaugh maps, and used it to simplify the equations enough to fit the GAL. The Java app was nearly as tedious as hand optimization, though, and it took me a long time to finish the simplification.

Problem 100. You can’t just connect a chip’s output pin directly to an LED. Since the diode offers nearly zero resistance, it would be like shorting the power supply to ground if the output pin’s value were a logic 1. Instead, a resistor must be put in series with the LED, to limit the current. Larger resistances will limit the current more, but will also diminish the LED’s brightness. I used 220 Ohm resistors in series with each LED segment, which should result in a current of 23 mA (5 / 220). Most LED’s appear designed to hit their advertised brightness level at about 10-20 mA.

Problem 101. There are a lot of wires to connect! OK, this isn’t really a problem, but I was still surprised at how many wires I needed to cut, strip, and push into the protoboard for such a simple project. 8 from the power supply to the 440 Ohm resistors, 8 from the resistors to the dip switch, 8 from the dip switch to the GAL, 7 from the GAL to the 220 Ohm resistors, 14 from those resistors to the two 7 segment displays, plus the 2 shared negative terminal connections, the 1 MHz oscillator, and a handful of other miscellaneous wires. Ugh.

Problem 110. After I’d finished everything and turned on the power, it didn’t come even remotely close to working. LED segments light up in totally random ways. The same dip settings didn’t even always generate the same results. Changing the MSB of the input changed which display was illuminated. Even just touching the circuit with my hand sometimes caused things to change. I was totally confused. At first I thought maybe I was drawing too much power from my power supply, and the voltage levels were getting out of spec, causing erroneous behavior in the GAL. A lot of poking and probing with a multimeter suggested everything was OK in the voltage department. Then I hunted for wiring errors, but found none. I rechecked all my logic equations as well.

After a very long time, I discovered that the GAL had been misprogrammed somehow, and some of the outputs were using the clock input as if it were a data input. I have no idea how that happened– is the chip failing? Is my chip programmer faulty? Sun spots? I reprogrammed the GAL, and things started working much better. It was still broken, but at least now it was broken in a deterministic way, and it sometimes displayed recognizable digits on the LED displays.

Eventually I discovered several errors in my logic equations that I’d overlooked the first ten times. In one place I’d used an AND instead of an OR, and in another I’d omitted an entire term from the equation. It’s amazing how many times I looked at those equations without seeing the mistakes.

After fixing those, the whole thing was almost working, except for:

Problem 111. Strange things happen with long (and not so long) wires involving cross-talk and noise. I never did satisfactorily explain how the MSB of the data input seemed to be acting like a clock input. When I wired the clock input pin straight to ground, the GAL would still often appear to have been clocked (the active LED display would switch) when I changed the MSB. The problem went away when I used a shorter wire to connect the clock pin to ground, so I suspect it was some kind of cross-talk from the neighboring wire. When I later connected the clock pin to the 1 MHz oscillator, it seemed to work fine.
The disconcerting thing is that the clock wire was not especially long (3 inches) nor close to the MSB wire, except that they happened to connect to adjacent pins on the GAL. If it’s truly that easy to accidentally perform false clocking, then I’m going to have a lot of problems building the full machine.

Problem 1000. Everything was finally working, but I wanted to examine the circuit behavior in more detail. Earlier I mentioned the 220 Ohm resistors I put in series with the LEDs, which should have resulted in a 23 mA current through each LED. I measured the current, and it was only about 6 mA. I also measured the voltage at the GAL’s data output pins, and rather than +5 volts, it was only 3.2 volts. This suggests a problem.

I can’t explain why I saw 6 mA of current specifically, but after reading the GAL datasheet more carefully, I decided I was lucky (?) to get even that much. The chip’s maximum output current per pin at a high (logic 1) output voltage is only 3.2 mA. Exceeding that by nearly 2X explains why the output voltage was dragged down from 5 to 3.2 volts. It’s possible that could damage the chip. At the very least, it doesn’t seem like a good design. I’m still curious why I saw 6 mA and not 3.2 or 23, though. Maybe I reached the maximum amount of current that my power supply can deliver, or maybe the diode itself has some current limiting effect?

The math tells me a 1562 Ohm resistor in parallel with the LED would give me the maximum rated current of 3.2 mA per LED segment. Would that be enough current to get sufficient brightness, though?

Although I didn’t measure it, I think I’m running into similar problems with the shared negative terminal of the LEDs, which is also connected to a GAL output pin. The datasheet says the max current per pin at a low (logic 0) output voltage is 16 mA. If I’m sending 6 mA per LED segment through seven segments with a shared negative terminal, then that’s 42 mA (6 * 7), way more than the rated maximum.

Problem 1001. Now I have something that works, but feels iffy. I wouldn’t incorporate it into a finished computer design, due to concerns about greatly exceeding the GAL’s rated maximum output current. The trouble is, I’m not sure how to fix it without introducing more parts between the GAL and the LEDs to provide and regulate current. I’ve seen ready-made LED driver chips, but they’re always binary coded decimal rather than hex. Whatever the ultimate solution, it feels like an awful lot of work just to display a single byte on a pair of LED displays.

To put a positive spin on this experience, I learned more about the kinds of difficulties and pitfalls I’m likely to encounter when I begin to build the machine on the wire wrap board. It’s much better to face them here on the protoboard, where alternative solutions are easy to try, than on the more unforgiving wire wrap board.