Si5351A clock generator - any good for a booster?

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tzok
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Si5351A clock generator - any good for a booster?

Post by tzok » Mon Jun 03, 2019 8:48 pm

I have recently found a very interesting chip called Si5351A. It is a 3 channel clock generator (8kHz - 160MHz). All 3 clocks can be phase locked. Chip is programmable by the I2C bus. It is available as an breakout board from Adafruit, or from Aliexpress. Have anyone considered using it for an ST booster?

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exxos
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Re: Si5351A clock generator - any good for a booster?

Post by exxos » Mon Jun 03, 2019 9:13 pm

There are a lot of clock gens out there.. It depends what your thinking of exactly ? But first problem is how to create the I2C comms to talk to the chip ? If your thinking software controlled, then it would need firmware logic to program it to 8Mhz to get the machine to boot in the first place. Then need software to change its speed (and someone to write the software) . This wouldn't be a good idea for games as you would need to run a auto folder program to set the speed of the chip. Or go further to save the setting into flash , but then its just getting really complicated for no real reason.

Also I am strongly against PLL's. I used them in some early booster prototypes, the problem is you need a perfectly stable signal otherwise the thing glitches, And as nothing is stable on the Atari, this just is not a good combination. IMHO they are just more trouble than they are worth.

The RSO's such as LTC1799 I have used on my Falcon CT60 CTCM modules. You can set the speed just by a simple resistor. These are actually pretty good.. But I really decided against using them as they are still not totally perfect. Though theoretically they could be used in my SEC booster as it uses a external 50MHz OSC. People could experiment with overclocking, but don't think it's really worth the trouble.

In terms of clock distribution and division, on the remake board, we simply just used a LS161 synchronous binary counter. So we can feed in 32MHz and get 16,8,4,2 MHz output all in phase.
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project23
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Re: Si5351A clock generator - any good for a booster?

Post by project23 » Mon Jun 03, 2019 9:40 pm

FWIW This chip is heavily used in amateur radio. I have one here on a breakout board and its easily programmable with an arduino nano or the like.

It's pretty stable, but i doubt it'll be as stable as a crystal oscillator.

I think (i could be wrong) i've seen Stephen use one of these in his terriblefire videos, testing the TF53x series. For this kind of use, where clock needs to be varied for testing, I can see it being pretty useful. But i'm not sure fitting it all onto/around an accelerator is worth it outside of this use.

John

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