A little help with a CNROM multicart

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Fisher
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A little help with a CNROM multicart

Post by Fisher » Thu Nov 03, 2016 2:16 pm

Hi.
I'm doing a Gyruss / Q*Bert multicart.
First, looking at Bootgod's site reveals them to have the same mirroring configuration, wich is not true.
However, soldering the mirror pad on the H position fixes Gyruss, and Q*Bert don't seems to care.
Would this means that it's safe to leave it on this position?
I couldn't pass the 2nd stage yet, the controls on this game are weird!

The second problem I have is with the game switching circuit.
I built one with a 74hc74, a diode 1n4148 and a 1nf (102) capacitor, using M2.
Works fine, but I need to hold reset down for ~7 seconds.
Can I do something to make it stantaneous ?
Sorry to don't post pics/schematics, but I'm only with my cell phone now :-(

lidnariq
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Re: A little help with a CNROM multicart

Post by lidnariq » Thu Nov 03, 2016 2:59 pm

The 74hc74 has no input current; the circuit as you named it has no good way to drain the capacitor.

Using either different technology (such as 74ls, with its tendency to source current from its inputs) or adding a resistor to drain the capacitor when M2 has gone Hi-Z would work.

(Right now I'm surprised that even 7 seconds is enough. There aren't any obvious sources of leakage current anywhere.)

See also: https://forums.nesdev.com/viewtopic.php?p=99137#p99137

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Fisher
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Re: A little help with a CNROM multicart

Post by Fisher » Thu Nov 03, 2016 4:14 pm

Tried the example and no luck.
Maybe I should add the resistor between the diode and the IC?
The nasty rat's nest I built may be draining current to the IC...
I'll post pictures as soon as I can.

The Game genie can't boot the cartridge either.
If it booted, I could test Q*Bert to the end and see if it plays fine with wrong mirroring.
Does anyone knows if this game has an end?

Or, I could try the mirroring circuit I used on Cosmo carrier 》Holy Diver mod.

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koitsu
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Re: A little help with a CNROM multicart

Post by koitsu » Thu Nov 03, 2016 4:38 pm

Fisher wrote:If it booted, I could test Q*Bert to the end and see if it plays fine with wrong mirroring. Does anyone knows if this game has an end?
It does. There's 36 total games (9 levels, 4 rounds per level). The final round, if you beat it, should consist of Qbert jumping onto his disc and floating off the screen, then a display that says "CONGRATULATIONS", followed by some Qbert + enemy animations, then back to level 1. Here's a video.

I might suggest instead of playing the game all the way through on real hardware, try an emulator like FCEUX, where under Debug -> Name Table Viewer, you can force the mirroring type (should work on mapper 3 games) to see if there's any ill effects. Maybe someone has a Game Genie code for forcing level 9 or so, or maybe a save state.

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Re: A little help with a CNROM multicart

Post by lidnariq » Thu Nov 03, 2016 4:42 pm

In a real 2A03, M2 is Hi-Z when in reset, and oscillates between strong drive high and low when operating. It's remotely conceivable that your clone is different? But that would be surprising.

You should be able to put an LED and ≈1kΩ resistor on the 74'74's outputs to verify what's happening.

You are using it as a clock source for a reset-based multicart?

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Fisher
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Re: A little help with a CNROM multicart

Post by Fisher » Thu Nov 03, 2016 5:36 pm

Nice idea Koitsu!
I'll try that tomorrow!

Exact Lidnariq !
I'm using a circuit based on this one:
reset based multicart.png
reset based multicart.png (5.17 KiB) Viewed 4024 times
Here's some pics of the mess:
Front
Front
Back
Back

lidnariq
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Re: A little help with a CNROM multicart

Post by lidnariq » Fri Nov 04, 2016 5:52 am

You cannot use his 74LS-using circuit as-is with a 74HC part. 74(letterless/S/F/LS/ALS) are all current-mode technologies—they signal a logical 0 with "current is flowing" and a logical 1 with "current is not flowing".

Hence why you have to add the resistor.

Since what you need is ↑CLOCK rather than /RESET, you could try flipping the entire circuit:

Code: Select all

       Vcc
        +-+
        | |
        R C
        | |
M2 -|<|-+-+-- 74HC input
This will be high when M2 is high or HiZ, and low when M2 is oscillating. i.e. the rising edge will occur when oscillation stops, rather than when it resumes.

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Fisher
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Re: A little help with a CNROM multicart

Post by Fisher » Fri Nov 04, 2016 12:16 pm

Thanks Lidariq!
The last circuit works far better.
Still glitches a little, but seems that the definitive solution should be an LS part.

Taking a closer look at bootgod's pictures of these games, both have the solder pad at the H position.
So I am leaving like this.

Just out of curiosity, I've done another multicart and the IC date code is 735F.
Are the first 2 digits the year?
If it is, I'm just trying to guess how much time this thing was thrashed! Wow!

Thanks guys! You're great!

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