6502.org

Hello guys.

New in the project. There was an Atari 2600 Jr. that was not working. I took his processor, which is a UM6507. It has a very similar architecture if you are not mistaken. But the point is: it worked! It just worked!

Does everything indicate that the whole problem was on my previous processor?

It is suggestive, but not definitive. The way a 28-pin version of the CPU slots into the breadboard is very different from a 40-pin version.

I'm really confused but I used the same breadboard and the same components. And everything is working fine now with the new processor.

The big disadvantage of the 28-pin 6507 is that you only get an 8KB address space, no interrupts, and some of the control signals from the 6502 are also missing.

I think to really answer the question, you should get a new WDC 65C02S. They're not expensive, and if that consistently works while your old NMOS chip consistently doesn't, then you'll know exactly where the fault lies. It might not be in the silicon, mind, but in surface deterioration of the pins.

Yes. I bought a new 65C02S to ensure what may have happened in my case. Unfortunately in my country (Brazil) I can not find practically anywhere. I had to import. This raises the time for dev. But as soon as I get news, I'll post it here.

Hello guys.

After some waiting time, I received today from China a new 6502 processor that I purchased to definitely test whether the problem I face is with my breadboard or not. Even because the 6507 processor I removed from an old Atari worked perfectly.

I will try the tests in the next few days and put the results here.

IS it a 6502 or a 65C02? If the latter, who made it?

65C02 UMC.

Is there any problems?

Do you really mean 65C02, or 6502C? As far as I can tell, UMC never made the CMOS version, only the NMOS version; the C suffix is a speed grade, indicating 4MHz capability.

UMC W65C02S8P-10

This is the reference.

That's a WDC part number, but paired with a UMC logo. I smell another counterfeit - an NMOS part re-marked as a relatively new CMOS one.

Anyone more experienced in these matters to confirm?

Hello Guys,

Now I had another problem. As I said earlier, I bought the processors with the following marking:

WBU
W65C02S8P-10
1701S

I put it in the same layout I had with my old 6502 processor. And now I'm having trouble with the reset signal. The signal comes from the NE555 and uses a NOT port to invert it through the 74ls04n IC. When I test this part of the circuit, it works correctly. But when I connect the RST to the 65C02, the signal is noisy. And from that nothing else responds correctly.

I have already tested the two 65C02 that have arrived, have remake the entire reset circuit, changed the 74ls04n, changed the NE555 and the behavior is always the same. If I use the 6507 processor, everything works.

PS: I tested clocked at 1 MHZ and 4 MHZ. And the behavior was the same.

Help me please.

Can you post a clear picture of the board so we can see the connections, particularly the ground, power, bypass, and clock connections? I have little doubt that the problem is parasitic inductance, and that it could be reduced a lot by working the connections differently. This will not change the schematic; but the schematic does not show these parasitic inductances. Again, note that what gets you in trouble is not clock rates, but edge rates, ie, now fast outputs swing from low ti high and vice-versa. A logic analyzer is of no help for troubleshooting this.

What would be the maximum acceptable time of edge rate?

The 4 that I was given, that all tested as re-marked NMOS parts, have the following on them:
UMC
W65C02S8P-10
0119S