Hey, everyone

I built Ben Eater's 6502 breadboard computer, but something strange is happening:
When I try to turn the computer on it just turns on the LCD backlight and I need to plug it in and out a lot of times so it can show the text on the screen.

Can this problem be fixed?

Best regards,
Krum

My best guess would be that the reset signal isn't doing what it needs to. Is there a reset button?

Power-on reset, done properly, is a bit more complicated that one might guess. But a reset button is simpler than that.

Other things might be the cause: the clock might not start up, for example. Or maybe the LCD needs some warm-up time and the software isn't giving it that time. (By warm-up, I mean it needs some time to build up an internal voltage.)

The LCD is fine I have used it before in other projects, but I have noticed that the reset switch doesn't work. I have checked the schematics and it seems like everything is OK.

I've attached how I wired the reset switch.

We might have to wait for some people with more experience. The problem is that mechanical switches are very indecisive: they bounce lots of times before settling. So the 6502 will see a whole series of reset pulses. What's important is that the final contact just before it lets go is long enough to work.

Do you have good power supply, and a bypass capacitor next to every chip?

Yes, I have bypass caps on every power line and I have a 2amp/ 5v power supply

We need some photos of your project, please. This is important -- can be very helpful. And tell us about the CPU you're using -- was it purchased new from a reputable dealer, or is its origin somewhat doubtful?

Also, do you have a voltmeter or DMM available? (What about other gadgets, such as a 'scope or logic probe?) I suggest you use the meter to measure between Gnd and pin 40 (Reset). Please verify that the voltage is normally 5V, and that it goes to 0 when you hold the button down.

Uhhh... to be clear, what is it you're plugging in and out? The LCD? The power cord for your PSU? Something else, perhaps?

Edit: is that resistor in your diagram really 220 ohm -- not 2200 ohm, perhaps?

-- 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

Right. It might be worth having a look at Garth Wilson's Reset Circuits page for some ideas on improvements you could make to the reset circuit. You probably don't have a DS1813 handy in your parts box, but you may have a 74xx14 or something else with a Schmitt trigger input. Even just a cap and resistor might help clean up the reset signal enough. I notice that Ben Eater's schematic uses a 1k resistor and 0.1 μF cap.

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Curt J. Sampson - github.com/0cjs

Unfortunately I don't have any other parts that I can use. The capacitor on the reset switch doesn't work, I tried it.

I also attached some pictures of my project:

The CPU is compleately new from Mouser electronics. The resistor is 1k I have typed 220 ohm by mistake. I have a voltmeter, checked the voltage - everything looks OK. I have to plug in/out the power plug.

Thanks for the photos!

When you say the capacitor didn't work, do you mean it didn't fix the problem?

So, you need to power-cycle the whole machine - reset isn't enough. Is never enough, or is usually not enough?

Have you any way to tell if it's just the LCD display which isn't coming up - can you read off activity on SYNC for example, or on address lines or data lines?

Is never enough, or is usually not enough? - never enough

Have you any way to tell if it's just the LCD display which isn't coming up - can you read off activity on SYNC for example, or on address lines or data lines? I've tried blinking some LEDs, but no luck.

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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?

But, Why does this problem occur? I built the computer exactly how Ben did. He didn't have this problem.

He didn't have any capacitors, any different parts etc.

Is there an easy way to fix the problem without adding any parts? Just rewire something?

It's usually a question of diagnosis. Quite often we might find, by careful review and inspection, that some wire is in the wrong place, or not quite making contact.

It's also possible, but less likely, that some component is defective - more likely, it is not quite connected, perhaps because of tarnish, or dirt.

What diagnostic equipment do you have? Multimeter? With frequency readout? Logic probe?

With patience and guidance you can learn a lot with just an LED and a logic gate or two - but while it's possible there is a quick fix, it might take some diligence.

It's a bit like software: rarely does a newly written program have no bugs. Learning to debug is an essential part of learning to program. It's like that with hardware too - unless you're lucky!

Edit: we should all have high confidence that this can be fixed, provided you stick with it!

What are the other chips you have in the circuit? If any of them require 5v, you might want to check the voltage at the power pin of each chip and the LCD individually. Depending on the breadboard and wires you're using, it could be dropping significant voltage, though it looks like you're using the breadboards that Ben Eater recommends