A little help with a CNROM multicart

A place that you can discuss reproduction of classic titles or "licensed-for-reproduction" homebrew for personal use.

Moderators: B00daW, Moderators

Forum rules
1. NO BLATANT PIRACY. This includes reproducing homebrew less than 10 years old, with the exception of free software.
2. No advertising your reproductions, with the exception of free software.
3. Be nice. See RFC 1855 if you aren't sure what this means.
Post Reply
User avatar
Fisher
Posts: 1249
Joined: Sat Jul 04, 2015 9:58 am
Location: -29.794229 -55.795374

A little help with a CNROM multicart

Post by Fisher »

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
Posts: 11432
Joined: Sun Apr 13, 2008 11:12 am

Re: A little help with a CNROM multicart

Post by lidnariq »

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
User avatar
Fisher
Posts: 1249
Joined: Sat Jul 04, 2015 9:58 am
Location: -29.794229 -55.795374

Re: A little help with a CNROM multicart

Post by Fisher »

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.
User avatar
koitsu
Posts: 4201
Joined: Sun Sep 19, 2004 9:28 pm
Location: A world gone mad

Re: A little help with a CNROM multicart

Post by koitsu »

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.
lidnariq
Posts: 11432
Joined: Sun Apr 13, 2008 11:12 am

Re: A little help with a CNROM multicart

Post by lidnariq »

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?
User avatar
Fisher
Posts: 1249
Joined: Sat Jul 04, 2015 9:58 am
Location: -29.794229 -55.795374

Re: A little help with a CNROM multicart

Post by Fisher »

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 6004 times
Here's some pics of the mess:
Front
Front
Back
Back
lidnariq
Posts: 11432
Joined: Sun Apr 13, 2008 11:12 am

Re: A little help with a CNROM multicart

Post by lidnariq »

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.
User avatar
Fisher
Posts: 1249
Joined: Sat Jul 04, 2015 9:58 am
Location: -29.794229 -55.795374

Re: A little help with a CNROM multicart

Post by Fisher »

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!
Post Reply