Minimum required to test a standalone PPU video output

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ArchieNash
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Minimum required to test a standalone PPU video output

Post by ArchieNash »

Hello!
I have a pretty beat up RP2C02E-0 and I want to test if it still works. I've looked up the pinout and hooked up most inputs (bar address and data lines) and I've got a 21.47727 MHz clock running to it, however the chip seems to heat up more than I'd expect and obviously no video out on pin 21 (including video circuitry). I can't put this chip in a working NES (use sockets Nintendo) so I'm trying to test it on a breadboard. What would be the least I have to hook up to get some sort of video out (or even just test the chip is working somehow)? I'd preferably not want to hook up the address lines yet and not use any additional ICs.
(btw I'm hoping to use this PPU in a homebrew 6502 computer)

Thanks!
lidnariq
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Re: Minimum required to test a standalone PPU video output

Post by lidnariq »

Minimum viable test is 21.5MHz clock and tying the /RST input to +5V.
ArchieNash
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Re: Minimum required to test a standalone PPU video output

Post by ArchieNash »

I've tried that and it still doesn't seem to work. I'm starting to suspect that the chip is dead. However, PPU address and data lines are flashing, suggesting functionality. I've narrowed it down to three things:
Faulty clock signal
Faulty video buffer
(Faulty chip)
However it's hard to say which
ArchieNash
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Re: Minimum required to test a standalone PPU video output

Post by ArchieNash »

It seems that pin 21 is constantly pulled high through an internal resistor and is not generating any friendly signals.
I'm gonna look into getting a functional chip or clone off of eBay (or maybe a more suitable vdc like the tms9918) and call that chip junk. If I end up with another ppu I'll put it in that same breadboard and see if does anything different, to test whether the old one could have worked.
lidnariq
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Re: Minimum required to test a standalone PPU video output

Post by lidnariq »

Output stage of the PPU should be one of several voltages via one of several different resistors (depending on what it's drawing at that moment). Unloaded voltages anywhere between 0.7V and 2.7V are reasonable.
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Quietust
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Re: Minimum required to test a standalone PPU video output

Post by Quietust »

ArchieNash wrote: Sat Jun 03, 2023 4:47 pm and call that chip junk
In case it isn't already clear, a non-working RP2C02E-0 is not something we would consider to be "junk" - at the very least, you can mail it to someone who can take it apart (potentially using extreme heat and/or strong acids) so the circuitry inside it can be photographed and analyzed (so that we can learn exactly how it's different from the newer revisions).

Granted, an RP2C02E-0 has already been depackaged (back in March), but there's no harm in checking more of them.
Quietust, QMT Productions
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ArchieNash
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Re: Minimum required to test a standalone PPU video output

Post by ArchieNash »

lidnariq wrote: Sat Jun 03, 2023 8:11 pm Unloaded voltages anywhere between 0.7V and 2.7V are reasonable.
It does seem that the voltage is around about 2.3V on the VOUT pin but it doesn't look like any sort of composite signal (This is measured through an arduino and a serial plotter and compared to a toploader composite cable). I could take apart the toploader and compare what I should be looking for out of the chip.
Quietust wrote: Sat Jun 03, 2023 8:38 pm In case it isn't already clear, a non-working RP2C02E-0 is not something we would consider to be "junk"
I won't throw the chip away cuz there's no guarantee there's a problem with it but I will look to see anyone who wants it.
ArchieNash
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Re: Minimum required to test a standalone PPU video output

Post by ArchieNash »

Hmm now it'll occasionally show an image (grey) however it won't hold it for long and is very prone to interference. This maybe because nothing else on the chip is hooked up yet.
The chip is probably working but I think it'll be quite difficult to work with.
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krzysiobal
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Re: Minimum required to test a standalone PPU video output

Post by krzysiobal »

Why dont you just probe the pin 21 with scope? If you get a signal that has known pattern: sync + back porch + then video data, then it is at least half-alive. If pin 21 output constant voltage then it is dead for sure. You don't even need 21 MHz crystal for that, any crystal will work. Even if PPU has disabled rendering, it outputs sync + back porch.

I'm not sure if you need the 2.2k pullup on pin 21 as in first stage video amp, but you probably should add it to be 100% you are not missing anything.

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ArchieNash
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Re: Minimum required to test a standalone PPU video output

Post by ArchieNash »

krzysiobal wrote: Sun Jun 04, 2023 8:32 am Why dont you just probe the pin 21 with scope?
I don't have a (good enough) scope to measure pin 21 clearly but using an arduino it looks as if the pin is dancing randomly below 1V occasionally making a video signal for about a second before the tv cuts to no signal. It also seems to work better if I push the chip down into the breadboard, implying dodgy connections, especially since both the GND and VOUT pins were ripped right out of the chip. It's now sitting in a socket with small legs that barely go deep enough into the breadboard.
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krzysiobal
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Re: Minimum required to test a standalone PPU video output

Post by krzysiobal »

ArchieNash wrote: Sun Jun 04, 2023 9:24 am
krzysiobal wrote: Sun Jun 04, 2023 8:32 am Why dont you just probe the pin 21 with scope?
I don't have a (good enough) scope to measure pin 21 clearly but using an arduino it looks as if the pin is dancing randomly below 1V occasionally making a video signal for about a second before the tv cuts to no signal. It also seems to work better if I push the chip down into the breadboard, implying dodgy connections, especially since both the GND and VOUT pins were ripped right out of the chip. It's now sitting in a socket with small legs that barely go deep enough into the breadboard.
So instead of using 21MHZ crystal, you can feed square input clock generated by arduino of any arbitrary frequency (lets say 1MHz) and you can sample it witch slower rate using arduino.
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krzysiobal
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Re: Minimum required to test a standalone PPU video output

Post by krzysiobal »

Here as you can see I clock PPU with 50 Hz and it does not have any problems with outputting video lines every 35 seconds.
Only +5V, GND, clk and reset are wired.
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Fiskbit
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Re: Minimum required to test a standalone PPU video output

Post by Fiskbit »

ArchieNash wrote: Sun Jun 04, 2023 6:10 am Hmm now it'll occasionally show an image (grey) however it won't hold it for long and is very prone to interference. This maybe because nothing else on the chip is hooked up yet.
If you've left the EXT pins (14-17) floating, that could be responsible for the interference. These should be grounded.
ArchieNash
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Re: Minimum required to test a standalone PPU video output

Post by ArchieNash »

I can't reproduce anything that looks like a composite signal. Tomorrow night I'll work on it again for a final shot at it. The chip gets stuck in either a high state (~2.3V) or a low state (~0V) suggesting that it might be a faulty clock...?
Any clock that I throw at it though (555, Arduino, 1MHz can, 21.5MHz full speed) doesn't seem to change it's output. I'll try to troubleshoot the pin tomorrow then I'm out of ideas.
ArchieNash
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Re: Minimum required to test a standalone PPU video output

Post by ArchieNash »

The clock line seems to be tied high for some reason. It just eats whatever clock is thrown at it. And of course, no composite signal (maybe because of that). I'm pretty stumped on this one, can't think of anything more to do.
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