Hi folks, newbie here learning 6502 assembly via the path of Ben Eater's 6502 computer project.

My 65c02 is being logic probed using Ben's method of an Arduino Mega 2560. I am setting the reset vector as per part 1 of his video series but no matter what I do, the vector always shows as going to FFFE and FFFF rather than FFFC and FFFD - as if IRQB is low.

I've checked NMI and IRQB - both are pegged high during testing.
Grounds are good. Voltage is 5.09V
The address bits change on all 16 bits. None of the bits are "stuck".

If I let the clock run (slowly of course) and ground the NMI pin, the vector goes to FFFA and FFFB as expected.
Here's the kicker though. If I let the clock run and ground the IRQB pin, nothing changes. It just keeps iterating through memory as it reads the NOP (EA) instruction I have hard wired to the data pins.

It's as if IRQB is internally shorted to ground in the 65c02. I've probed this and there are no shorts to ground between IRQB and VSS.

Here's the output of the serial monitor after reset:


Thoughts? Could it just be a dud 65c02?

Ok this is weird. I just un-seated and re-seated the CPU in the breadboard, re-powered it and I got this sequence just now:



FFFC followed by FFFF?

Hello, and welcome.

I'm not familiar with Ben's videos, but I or someone else here might be able to help anyway. Just be prepared to supply lots of detail about what you're doing. Can you supply some photos of your project, please? These can be quite helpful, and you can simply attach them to your post (no need for a third-party image sharing site).

What brand of 65C02 are you using? If you're running the chip very slowly then Rockwell and other brands may present a problem, but WDC will be OK.

-- Jeff

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In 1988 my 65C02 got six new registers and 44 new full-speed instructions!
https://laughtonelectronics.com/Arcana/ ... mmary.html

I appear to have found the issue. If I press down on the IRQB pin really hard, I can make it give a consistent FFFC and FFFD, but otherwise it's either randomly FFFC but nearly always FFFE and FFFF.

I think I have a broken 65c02. Rare? This is a WDC chip.

Could well be a bad connection, but it's far more likely to be due to a low-quality breadboard than a flaky chip.

-- Jeff

_________________
In 1988 my 65C02 got six new registers and 44 new full-speed instructions!
https://laughtonelectronics.com/Arcana/ ... mmary.html

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http://WilsonMinesCo.com/ lots of 6502 resources
The "second front page" is http://wilsonminesco.com/links.html .
What's an additional VIA among friends, anyhow?

Thanks - I checked that in the tutorials page here. It seems that a correctly functioning 65c02 should never go to FFFE after either a a power-on or reset signal, regardless of the level of the IRQB pin.

It really seems to suggest that maybe the Arduino is mis-reading A1, but I'm monitoring the pin with an oscilloscope and sure enough, when the Arduino says the pin is high, the oscilloscope sees A1 go high.

It's not even as if there is bleed over from the neigbouring pins because the serial monitor shows examples of A0 and A2 at a different level to A1.

I think the best test here is to wire up the ROM in the upper 32KB as per Ben's guide and flash it with values in 0xFFFC and 0xFFFD (technically, 0x7FFC and 0x7FFD in the ROM itself). It doesn't really matter what the values are - as long as regardless of what the Arduino says the address is, the returned hex value is what I programmed into those addresses.

I can also play this out by putting different values in FFFE and FFFF so that I can validate if indeed the processor is really borked and reading from FFFE and FFFF after reset.

Ultimately it's not like this is a problem because the goal is to learn the 6502. I'm certainly getting a crash course in that.

Post some photos of your project if you can. In situations like this they can prove helpful.

-- Jeff

_________________
In 1988 my 65C02 got six new registers and 44 new full-speed instructions!
https://laughtonelectronics.com/Arcana/ ... mmary.html

Problem solved folks. It was the fact that I had removed the power rail filter capacitors. It caused both the 65c02 and also the Arduino clock pulse detection to wig out. I didn't consider the two issues related, but alas they were symptoms of the same cause.

The system is running now with the ROM and is behaving consistently and as expected.

Geez imaging doing this in the '70s. Then again they probably wouldn't have done something so idiotic as forget to filter the power supply.

It's funny; Ben doesn't use many decoupling caps in his videos, but everything always seems to work, and he gets nice clean signals on his oscilloscope. He has a video where he talks about stray capacitance in breadboards and loopy jumper wires, but I don't think he ever has more than 2 - 4 decoupling caps on his power rails. After reading the primer and these forums I've started putting one at each end of the power rails on my breadboards, plus one per IC next to its VCC connection. That has helped a lot.

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"The key is not to let the hardware sense any fear." - Radical Brad

Ben uses 74LS chips. They have a fairly high constant current draw, with relatively low peaks when switching. They don't need a lot of decoupling. CMOS is different - it needs very little current while nothing is happening, but suddenly needs a lot when switching. Power distribution becomes a lot more important with CMOS.

The 74LS is also how he gets away with leaving unused inputs floating. TTL inputs will safely float high if you don't connect them. CMOS gets unhappy and can oscillate.

He cuts a lot of corners that I wouldn't, but those details would get in the way of the main point of the videos. And his designs do work.

This is a good watch if interested...

https://www.youtube.com/watch?v=P8MpZGjwgR0

-Gordon

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--
Gordon Henderson.
See my Ruby 6502 and 65816 SBC projects here: https://projects.drogon.net/ruby/

Checking this sort of detail is one of the reasons I asked for photos of your project.

Good points, John.

-- Jeff

_________________
In 1988 my 65C02 got six new registers and 44 new full-speed instructions!
https://laughtonelectronics.com/Arcana/ ... mmary.html

His followup video, Bypass Capacitors Visualised, is quite good as well.