ultrageranium
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Hello!

I'm trying to make a hot switchable clock mod for the Gameboy Classic but it does not work as expected.

I am using one of these LTC1799 based PCB (http://www.circuitbenders.co.uk/forsale/LTC/LTCPCB.html). The output feeds into the onboard original crystal (bottom pin which I think is the pin out of the crystal). It works as expected, I can change the clock rate and I used the onboard PCB trim pot to prevent overclocking crashes.

However when I add a switch to this setup (basically adding the switch between the LTC1799 PCB out and the crystal pin out, so that it either feeds nothing or the output of the oscillator), it only works if I first turn off the GB, then use the switch, then power it back. Hot switching, that is while the GB is on, leads to a crash.

Am I overlooking something?

Thanks!

thursdaycustoms
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Cleveland, OH

Looking at the PCB it has a high limiting resistor of 3k. The CGB needs a resistor of 6-8k depending on what your running to prevent it from crashing(6k tends to be good for games, 8k is good for LSDJ). Try turning the trim pot to the slowest point and then try hot switching. You will still get occasional crashes when hot switching but it should work most of the time.

ultrageranium
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I have an old Nanoloop cart to make the tests.

Using the trim pot I reduced the overclocking and I could hot switch once, then after that it crashed. I lowered the pot further to the point where using the LTC is only working as underclock feed. Same result, works one time, maybe two, then crashes.

What if there is a switch not on the output, but between the B 220K pot that I use and a fixed value resitor that would somehow be close enough to make the LTC PCB produces the original GB frequency?

Would that work?

Also, I am not entirely sure why a lot of frequency mod tutorials keep the original crystal in place. Isn't it more clean to just remove it entirely and feed the GB with a circuit that does all the freq tricks desired?

Thanks!

thursdaycustoms
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Cleveland, OH

It's easier to leave the crystal in place. You would need a different device entirely if you were to remove the crystal in order to be able to revert to normal clock speed. Kitsch-bent has the "Quint" but it's a biggish external device.
If it works once and then crashes it sounds like you may have a bad switch. The Game Boy is picky about how the frequency is switched. I would recommend picking up a better switch if possible.

error
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I know this thread is a bit old, but I did manage to combine a variable, half, and normal speed clock with one switch, hot switchable. I started a thread on here last year when I was struggling with the wiring, but it turned out that the only problem was an invisible solder bridge on my Ltc module.

It required removing the original crystal, and using a dp3t on-on-on make before break toggle selecting between the ltc and kitch's hot switchable 1/1 to 1/2 clock, "easy-clk" I think it's called.

ultrageranium
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Hi there,

another necrobumb smile

I never managed to finish this mod as I had to deal with other things first, and will not have time before another half year I think. I did try several switches and noticed as thursdaycustom hinted, improvements with others, but still had random crashes every now and then.

In the last attempt I had moved the switch elsewhere to select between the pot and a small and small trim resistor to make the  LTC1799 generate variable or close to original freq. It started to get a bit overkill though. I'll have to restart from scratch.

@error: with your setup can you continuously switch without any crashes?

error
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Yep, it can keep switching, just not rapidly back and forth. With a reasonable little time between changes (for the cpu to settle to the new speed, I presume) it can keep switching without powering down or crashing.

ultrageranium
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Thanks!

I will try your method then. Follow-up in 6 months... wink