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Is there a way to tell if something's being control by an analog or digital input without knowing and understanding all the rest of the circuitry involved?

I have a Wowwee robot (Elvis) that I'm trying to interface with an arduino, so I can "parasite" onto it and control its functions (very simply).

I've hooked in my multimeter to the output pins that head toward the motors (and their respective circuit boards). I get a signal of mostly 3.3v, but occasionally it dips down in the 2's and 1's. Now I know this looks like analog, as digital would just be 3.3v or 0v, but I'm wondering if it's still a digital signal just being PWM'd...?

I just got a logic sniffer/analyzer. I've been trying to interface it with the Elvis to see what the output's looking like on the pin(s), but I've been having other issues with the logic analyzer (and noisy, ungrounded pins). But I digress.

So is there a way to tell if this is an analog signal that's actually moving up an down the range between 0v and 3.3v, OR if it's still a digital signal that's just being pulsed fast enough to "effectively" be/look like an analog range?

Thanks!

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Like you already suspect a DMM won't give you necessarily the answer. If the only levels you read are only 3.3V (capital "V", since it's derived from a person's name) and 0V it's a safe bet that it's digital. A PWM signal is also digital, but will show a voltage between 3.3V and 0V on a DMM, since it includes a low-pass filter. The DMM won't tell an analog and PWM signal apart. You'll need an oscilloscope to actually see what the signal looks like.

If you can trace the signal back to the output of a microcontroller, without filtering, it's most likely a digital signal. While most microcontroller families are available with on-chip ADC, very few also have DACs on board.

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    \$\begingroup\$ Quite a few of newer Microchip's PICs do have DAC (e.g. 16F1826). However, they do not advertise that fact much because it is only 5-bit DAC they chose to implement. In practice those DACs can be used for e.g. contrast control and not much else... \$\endgroup\$ – Josip Medved Mar 24 '12 at 12:42

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