PCM2902 difficulties - diyAudio
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Old 13th January 2006, 07:21 PM   #1
jmar is offline jmar  United States
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Default PCM2902 difficulties

I breadboarded one of these some months ago and the end result was the ugliest job I've ever done.

I just crossed my fingers and plugged it in and it worked great.

Now, I take the time to do it right, on a PCB that I drew etc etc and can't get the damn thing to work.

The only thing that I've discovered is the possibility of the oscillator not working and the Vdd is very low yet I have a good 5volts at Vbus.

I have the 5volt buss connected to pin 3 and using an external regulator at 3.8volts for Vcc.

Should I be able to measure the oscillator freq (12mhz) at either of the crystal pins? (Depending on the internal configuration--I've seen yes or no).

Would I be better off with a ceramic resonator?

Right now using a 12mhz crystal with a couple of 33pf (tried 22's also).


Appreciate any feedback on this......very frustrating.
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Old 13th January 2006, 07:33 PM   #2
jmar is offline jmar  United States
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OK HOLD YOUR HORSES.
I think I may have found it.

Question:

Why is it after you post a question--you see your mistake?

I'll get back to you and feel free to harrass me.
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Old 13th January 2006, 07:44 PM   #3
jmar is offline jmar  United States
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Go ahead.....make fun.

I had one of the Xtal pins grounded with my meticulous PCB.

Thank God for X-acto knives.


works great.
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Old 13th January 2006, 08:08 PM   #4
hifi is offline hifi  Sweden
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Quote:
I had one of the Xtal pins grounded
Been there, done that ,)
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Old 13th January 2006, 08:09 PM   #5
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At least you didn't put it throough your hand, like someone we all did.

No, it wasn't me.........but you are close.

WRT the rest........NO! don't use a ceramic resonator. Maybe for a CPU. Trying to measure frequency on either pin will most likely give you a wrong reading, assuming is still oscillates. The output pin........

Jocko
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Old 13th January 2006, 09:02 PM   #6
jmar is offline jmar  United States
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You know, something I still don't grasp about this chip is the issue of an external power supply.

I'm using a completely seperate power supply with an LM317 for about 3.8 volts to Vcc for improved performance. (?)

Is that due to the internal regulator being inadequate? Too noisy?

So why couldn't I add yet another regulator for the digital supply and
disconnect Vbus completely? I don't like the idea of the USB cable supplying ANY of the power requirements.

Is it part of the USB handshaking to see that Vbus is connected to the chip at pin 3?
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Old 13th January 2006, 09:16 PM   #7
ble0t is offline ble0t  United States
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Yes, there needs to be a voltage on the VBUS pin to signal to the device that it is indeed connected to a USB port so it can handshake. But, it really doesn't have to be the actual VBUS...you just need a voltage there. For optimum performance of the chip, using an external power supply or one that is derived from VBUS would be better than using VBUS itself (aka use some filtering and a linear reg on VBUS and drop it to 3.3V).
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Old 13th January 2006, 10:07 PM   #8
jmar is offline jmar  United States
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And the digital supply can take up to 5v right?

Is there any merit also to an increase on the analog from 3.3 to
3.8ish?

I don't know what the parameter was but supposedly a bit better
performance with the increase.

I love the sound of this chip myself....for now I'm just using some 10 mike electrolytics right into my amp.
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