DIY Stompboxes > Digital & DSP

FV-1 pin lifts floor of signal from uC (I think?). Need help understanding

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So I have something like the following. And if this simplified version doesn't explain the problem, then I can sketch out a more complete picture.

An FV-1 and ATTiny share ground, but have their own separate regulators.

The attiny outputs a square wave, but its low value is about 700mV:

When I disconnect it from the FV-1, it drops back down to 0V.

Anything obvious I'm misunderstanding that would explain what's happening? I expect the low value to always be around 0V.

Edit: and the high value of the signal isn't moving. Only the gnd.

> The attiny outputs . . . . . When I disconnect it from the FV-1,....

Disconnect what? The ATTiny? The 10k?

Why would it go to ZERO? Only because the internal transistor has gone to zero Ohms. It can't. It may go to milliVolts on its own, but with tiny transistors it may not go way low with 10k pulling up.

I'm missing 3 fingers this evening and can't Google or draw. Sketch the whole plan, both ways, and link the datasheets.

Sorry for not being clearer, and thanks for taking a look.

The first scope pic shows the signal at the "PROBE" net in the drawing.  The second scope pic shows the signal when R6 is removed.

i.e. the attiny in isolation is outputting a square wave between roughly 0V and 3V3. This is what I want the FV-1 to read.

When R6 is connected, however, the square wave goes between roughly 0.7V and 3V3. This is the undesirable behavior I'm trying to understand.

Datasheets: attiny85, FV-1.

I'll get to work on a complete schematic, but what's missing is very close to the FV-1 datasheet.

Here's a complete schematic. I removed one resistor and replaced it with a switch to more clearly illustrate.

When the U3 switch near the bottom is closed, the first scope image is the result. When it's open, the wave is its full intended 0V to 3V3.

(edit: replaced image w/ higher res)

And in case anyone ever comes up with similar symptoms in the future, it was a boneheaded mistake in the AVR code: I never set its pin as an output.

So while it was outputting the high/low values just fine it wasn't able to drive any sort of load.


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