1
\$\begingroup\$

I have a little PCM2704 based circuit assembled from a kit. The chip takes digital input for volume up/down but I would like to replace those with a knob.

Regardless whether the knob is attached to a potentiometer or a rotary encoder, I still need a microcontroller to translate its output to the signals that PCM2704 is expecting.

The chip itself is powered directly from its USB port by 5v, however, it requires <4v for its digital inputs. It provides 3.3v through its SSPND digital output pin (which is always on for my purposes), this is what the kit design uses to feed through the buttons and back into its input pins to control volume.

Now I'm wondering whether I can use the power from the SSPND pin to actually power something like a PIC12F675-I? I know digital outputs can't handle a lot of current but then again.. the PIC chip doesn't draw that much either. From looking at the PCM2704's spec sheet, the output current for that pin(or any digital output) is unspecified.

\$\endgroup\$
  • \$\begingroup\$ I don't recommend it, but you might be able to make it work. Just get a cheap 3.3V linear regulator. You could also buffer the output with an op amp, if you have some of those laying around. \$\endgroup\$ – Hearth May 12 '18 at 18:29
  • \$\begingroup\$ The datasheet clearly states that digital outputs handle -2 mA at 2.4 V level. That's it. \$\endgroup\$ – Ale..chenski May 12 '18 at 18:36
  • \$\begingroup\$ @AliChen I didn't see the max current given anywhere - you've quoted the test conditions for \$I_{OL}\$. Still wouldn't recommend it; why not just use the 5V from the USB and clip the output with a zener? \$\endgroup\$ – awjlogan May 12 '18 at 20:53
  • 1
    \$\begingroup\$ 3.3V regulators are dirt cheap, and they're built for the purpose. \$\endgroup\$ – Dampmaskin May 12 '18 at 21:18
  • 2
    \$\begingroup\$ @awjlogan, if test condition at -2 mA gives you only 2.4 V, then higher loads will not make it higher. So +-2 mA is the max current to provide specified voltage levels. For this reason there is no such parameter as "max current on digital pin", it is "current that provides valid logic levels" \$\endgroup\$ – Ale..chenski May 12 '18 at 22:41
0
\$\begingroup\$

Yes you can. PIC12F675-I with ADC disabled can work down to 2V.

Using a low clock speed and carefully designing you can stay under the 2mA pointed in the comments.

You don't need a voltage regulator. A decoupling capacitor and a resistor to limit the current are enough.

\$\endgroup\$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.