Always wanted to do digital circuitry, and the current course I am taking in university has a lot to do with it (I am doing a CompSci major). I understand a lot of the hardware (if that's the right word) about microprocessors and various digital logic devices, but nothing about electrical engineering. However I would like to do some real world projects.

I began by desoldered a 6502 variant from the original Nintendo (Only thing with a 6502 I can part with, it's broken). The instruction set is the same, and you get a few bonus built in features.

-I wired VCC to 5v, and Grn to ground.
-I wired all the data bus pins to grn for a no-op instruction

-Wired VCC/Grnd to a ossilator, and wired the clock output, to the 6502's input.

-I grounded NMI and IRQ, and put the first 4 address lines through leds.

Reset is my biggest issue. I don't have a button on me right now, but would manually switching the wire between high, to low, back to high work?

I tried doing this, but if RESET is 5v, all lines are active, and if grounded all lines are low.

Just a note, the NES 6502 divides clockspeed by 12, so this may also be an issue, since I have a 5.37mhz oscillator, it's running at around 448khz.

I began by desoldered a 6502 variant from the original Nintendo (Only thing with a 6502 I can part with, it's broken). The instruction set is the same, and you get a few bonus built in features.

Welcome to the wacky world of 6502s and to the forum.


Actually, all zeros on the data bus results in a BRK instruction, not NOP. The opcode for NOP is $EA, or %11101010 in binary. So, to get NOP, wire the data bus as follows:

D0 - low
D1 - high
D2 - low
D3 - high
D4 - low
D5 - high
D6 - high
D7 - high


Presumably you used a TTL can oscillator. It's also possible to use a crystal, or even an R-C circuit.


IRQ and NMI should both be high. IRQ is level sensitive and if continuously held low, causes the MPU to get stuck in an IRQ loop. Also, IRQ takes the MPU through the vector at $FFFE-$FFFF, which has to point to valid code. NMI causes a jump through $FFFA-$FFFB.


After a fashion. However, you should have a short delay upon the transition from low to high. A simple R-C circuit will suffice.


See above comments.


It could be. NMOS MPUs have a minimum clock speed below which they won't function.

All good answers from BDD, but I think he's mostly thinking about the CMOS 6502. I don't think the NMOS one had a Schmitt-trigger input for RST\, so it needed a clean edge with a quick rise time. I can't find anything about it in the Rockwell or Synertek data sheets right now though. I do know that the NMOS ones had a die-heating problem with leaving the RST line down more than a tenth of a second or so. 50ms was what a lot of RST circuits were made to deliver. You will need more than just a switch, since the switch will produce a lot of bouncing instead of a clean RST signal. If the last low time is at least a few clock cycles long, you might be in luck, if the rise time is fast enough. Use a CMOS one and you won't have to worry about it.

The NMOS 6502 also required an external oscillator, unlike the CMOS one which could use an RC hung off the clock pins.

The NMOS 6502 was not guaranteed to work below 100kHz. The CMOS ones can be stopped without losing data from the registers, meaning you can even single-clock it with a button (with a debouncing circuit of course) so you can probe all the lines at each clock cycle if you want.

The NMOS 6502 had several bugs and quirks that got fixed in the CMOS one. The latter has more instructions and addressing modes too, so for anyone just starting out, I would definitely recommend going straight for the CMOS one. The exception is if someone wants to start programming on a Commodore 64 for example, since it used a 6502 variation that was never available in CMOS.

I believe the NES CPU lacks decimal mode, however, so software performing BCD arithmetic won't run.

Should 500khz clock be ok? I read it's due to DRAM data inside the CPU degrading if the chip is not clocked fast enough.

Put LEDs on the last 6 Address lines and I am seeing activity!!!!

You need to invest in a logic probe. You can get a reasonably good one at a reasonable price.

BTW, now that you seem to have some activity with your Frankenstein circuit, you should try to create a simple computer out of it. There is no shortage of examples around here, some that are very simple and some that aren't.

Going to need a eprom programmer though =\. Expensive.