SNES 1Chip PAL 60Hz mod, higher clock speed?

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dino112
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SNES 1Chip PAL 60Hz mod, higher clock speed?

Post by dino112 »

Hi,

I modded SNES 1Chip PAL to 60Hz.

It was a permanent mod (S-CPUN pin 111 detached and connected to GND, and a crystal switch).

The mod went fine, I'm using FXPAK Pro (SD2SNES), but I'm getting slightly higher clock speed.

FXPAK Pro reads SNES master clock at 21479050Hz while I'm expecting 21477...Hz, so the clock is running around 2000Hz higher.

I didn't notice any glitches so far, should I be worried about a bit higher clock speed or not?

I installed this crystal https://hr.mouser.com/ProductDetail/Cit ... qLtQ%3D%3D

I assume clock speed could be related to crystal Load Capacitance which is 18pF for the crystal I used. I couldn't find information what load capacitance is expected in a SNES crystal?

As well I got this crystal https://hr.mouser.com/ProductDetail/ECS ... viiA%3D%3D which Load Capacitance is SERIES so wasn't sure if this one would be a better fit for a more precise clock? I believe the first crystal has parallel capacitance and this one series, so maybe this one isn't safe to be used?

Thanks!
dino112
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Joined: Sun Nov 27, 2022 3:06 pm

Re: SNES 1Chip PAL 60Hz mod, higher clock speed?

Post by dino112 »

I've investigated this a bit. I'm missing a TC1 (trimmer capacitor) next to the crystal. It seems this one isn't installed in PAL SNES, only NTSC. I believe it is required to precisely configure crystal frequency due to NTSC signal requirements.

I'm using RGB output so NTSC isn't important to me, but still I'm thinking to install TC1 (20pF) and tweak it to adjust proper frequency?
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Re: SNES 1Chip PAL 60Hz mod, higher clock speed?

Post by Unseen »

Use it to your advantage - get into speedrunning, find something very long to run that uses RTA timing and claim the top spot because your SNES happens to be 0.0093% faster than nominal.

If it were a clock, it would deviate by 8 seconds per day. That's not that good for a quartz-controlled clock, but if your SNES works fine and you don't need the composite or S-Video signals I don't see any reason to worry about it.
dino112
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Joined: Sun Nov 27, 2022 3:06 pm

Re: SNES 1Chip PAL 60Hz mod, higher clock speed?

Post by dino112 »

Unseen wrote:Use it to your advantage - get into speedrunning, find something very long to run that uses RTA timing and claim the top spot because your SNES happens to be 0.0093% faster than nominal.

If it were a clock, it would deviate by 8 seconds per day. That's not that good for a quartz-controlled clock, but if your SNES works fine and you don't need the composite or S-Video signals I don't see any reason to worry about it.
Thanks! Haha, good point. I was more worried is there any strain/damage on the cpu/components due to be frequency being slightly off. As well I noticed that frequency gradually goes up. e.g. it goes for 10Hz up in a couple of minutes after console was turned on. I assume I shouldn't be worried about this as well? :D
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Re: SNES 1Chip PAL 60Hz mod, higher clock speed?

Post by Unseen »

dino112 wrote:As well I noticed that frequency gradually goes up. e.g. it goes for 10Hz up in a couple of minutes after console was turned on. I assume I shouldn't be worried about this as well? :D
Many components change their values slightly depending on environmental factors like temperature and atmospheric pressure - some of them are even optimized to do that so they can be used as sensors. In this case you are probably seeing the frequency change caused by the temperature change when the console heats up after it has been turned on.

Devices that require a high frequency accuracy (e.g. measurement instruments) usually solve this by encapsulating the crystal and some of the associated electronics in a thermally insulated block that is heated to a stable temperature, but that is way beyond the requirements of a mere game console.
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