There are several varieties of C-64 motherboards out there, of which I've only used two. On both of these, there was enough room to have the completed circuit sit on top of the keyboard connector in the computer and still close the C-64's case. Look at your C-64, and figure out how you'll physically lay out the circuit board if you want to do this. I haven't had the opportunity to take apart a 64C, but from the looks of the case I don't think there's enough room inside to hold the converter.
The converter draws about 125mA while operating. The two keyboards I've used also draw a similar amount of current -- not surprising, considering both the keyboard and converter circuits use 8748s which draw about 100mA by themselves. Anyway, figure about 250mA total. Although this isn't a lot to most power supplies, Commodore 64 power supplies are notoriously weak, and this added load may lead them to their grave. This is normally the case only when you already have extra load on the power supply in the form of something like a REU, printer interface, etc.; if the converter is the
extra load on the C-64 you should have no problem whatsoever.
A program called "Matrix Display" is included in this distribution. All it does is show what the C-64's 8x8 keyboard matrix looks like. A 1 indicates that the corresponding key is being pressed. To determine the corresponding key, look on the next page. The matrix display program is useful for insuring that all of your keys are working exactly as they should be.
@Troubleshooting:
No matter how much you try, you just might finish building this circuit only to find that it doesn't do a darn thing. Before you call your favorite attorney, here are a few things to check (most of these assume you know a little bit about electronics... if you don't, see if you can find someone who does): First, make sure that there's 5V between Vcc and Gnd of the 8748 and DS1277. Next, make sure there's an 11MHz square wave at pins 2 & 3 of the 8748. Now, insure that when you first apply power to the 8748, pin 4 (Reset) is pulled low for a split second, and then returns high. If all of these checks are OK, you can be reasonably sure that the 8748 is beginning to run its program unless you have a bum 8748. The next thing you want to do is to insure that your keyboard is doing what it's supposed to. The keyboard clock and data lines (pins 1 and 7 on the 8748) should pulse whenever you hit or release a key. If you hold a key down, it will (after a short delay) repeatedly start pulsing the clock and data lines over and over again. If this happens, make sure pins 1,2, and 3 of the DS1277 pulse whenever a key is hit or released; these are the pins that the 8748 uses to talk to the DS1277. Unlike the clock and data lines from the keyboard, these pins should only pulse once regardless of how long a key is held down. If all of this is occurring, you should try running the "Matrix Display" program. If it shows
keys being pressed, you most likely have a wiring error between the 20 pin keyboard connector and the DS1277.
A few real "gotchas" to watch out for: First, if pin 26 (Vdd) of the 8748 is not tied to +5V, the 8748 may work sporadically. Second, if pins 22 and 21 (Bat & Vee) of the DS1277 are not both tied to ground, the DS1277 may work sporadically. All this means is that your converter may work for 30 seconds and then suddently stop dead in its tracks. If you turn your computer off and then on again, you may get another 30 seconds of work done. Check the above connections if you have symptoms like these.
Finally, if after a good amount of debugging on your part you still can't get the thing working, send it to me along with $10 and I'll get it working for you. Of course, this assumes that I can tell that you know the difference between a soldering iron and a blow torch, and that you didn't fry any of the chips in the meanwhile (...in which case I'll charge you for a new chip). If it turns out that your circuit doesn't work due to an error in my schematic, I'll cheerfully return your $10.
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