Re: Powering ATtiny Tutorial PCB from ISP 5V #asm_tutorial


On 11/6/21 10:13 AM, Mont Pierce KM6WT wrote:
On Tue, Nov 2, 2021 at 10:45 AM, Mont Pierce KM6WT wrote (click here):
on further testing I found that the low noise on the signals is only present when I power the board from 12V source.  If I pull the 12V power, and instead power the board from the 5V line on the ISP programmer, the noise is gone.
I think the noise may be from a ground loop
Actually, powering the ATtiny PCB from the ISP port is NOT a good idea...
Let me confirm...not a good idea :)

Looking over the schematic (click here, see bottom right), I could not logically figure out how the 3v3 regulator was being powered....

It appears to me:  After trying this on three different boards, it appears on my first board there was enough voltage leaking back through the 7805 to power the 78L33, and thus power the onboard 3v3 Si5351.  Relying on voltage leak back through the 7805 is not reliable.  I don't think it's designed for this, and the amount of current leaked back varies from component to component.

Diz:  what about powering the 78L33 from the 5V rail instead of from 12V rail ?

Not needed if the above is observed


Just a thought...

Interestingly, Hans powers the QCX's Si5351a from 5V through two diodes to drop the voltage down from 5V to 3.6V (just within tolerance). 


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