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Capacitance for the CIC chip
https://forums.nesdev.com/viewtopic.php?f=12&t=12068
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Author:  getafixx [ Wed Nov 26, 2014 4:47 pm ]
Post subject:  Capacitance for the CIC chip

Just a quick issue I've noticed a few times on my own dev boards. Sometimes after programming the SuperCIC onto my PIC12F629 chips and adding it to my board it doesn't work. Programmed and verified properly, but then fails to boot up. Replace the chip with another one, and it works. But if I reprogram that same chip it will work in another board.

At first I figured it was my programmer, but then I bought a new one (SuperPro 600p) because I was tired of the crappy GQ brand stuff, and it does the same thing. Chips program and verify correctly, but sometimes fail to boot on the system.

I'm using a 22uf cap in close proximity to it (but not dedicated to it, as it is close to another chip as well), as it is on a Nintendo branded cart. Is it possible that 22uf is too much for the PIC, or not enough? Or should I have a smaller dedicated cap on it right off the VCC and GND pins on the PIC?

Author:  Markfrizb [ Wed Nov 26, 2014 10:26 pm ]
Post subject:  Re: Capacitance for the CIC chip

You should have a .1uF right off the PIC's power legs. The 22uF (as I understand it) is just bulk capacitance and the ic's should use the smaller caps. I think .1uF is typical.

I've never had a pic behave like that.... They've always worked.

Author:  qwertymodo [ Wed Nov 26, 2014 11:08 pm ]
Post subject:  Re: Capacitance for the CIC chip

I'm with Markfrizb on this, but to expand a bit on the capacitor sizes, large caps are meant to handle large power fluctuations, such as hot-plugging a new device (not so much with the SNES, but USB hosts for example need something like 100uF per port to avoid brown-out conditions when plugging in a new device because the initial moment of plugging in draws a large spike of current which has the effect of dropping the voltage on the power line, and a capacitor helps to provide that momentary surge of current). Individual chips need smaller caps (100nF is probably the most commonly used) on their Vcc lines to filter high-frequency, low amplitude noise on the power line. It's a good rule of thumb to have a 100nF cap physically close to every Vcc pin (for devices with multiple Vcc pins). I would guess that adding a cap would help with weird issues like this.

Author:  getafixx [ Thu Nov 27, 2014 12:07 pm ]
Post subject:  Re: Capacitance for the CIC chip

Good to know. Thanks guys!

Author:  qwertymodo [ Thu Nov 27, 2014 1:35 pm ]
Post subject:  Re: Capacitance for the CIC chip

Also, another note is that the decoupling caps only need to go near the Vcc pins, you don't need one near the ground pin as well (the capacitor connects between Vcc and Gnd, but it goes near Vcc). I hope that makes sense.

Author:  getafixx [ Thu Nov 27, 2014 5:05 pm ]
Post subject:  Re: Capacitance for the CIC chip

Totally. I've already got caps near VCC on my rom and decoder chips, but i didn't have one on my CIC. I'll make the change and hopefully that fixes my CIC issue.

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