user39438: I've been trying to add a MIDI INput to an ATtiny-based synthesizer, powered by a CR2032 battery (3v) and running softwareSerial.
Currently I'm using an H11L1 optocoupler, which works fine at 5v, but I am missing notes at 3V; I'm not sure whether the problem is the optocoupler, or that the ATtiny can't establish proper communication on that voltage; any pointers will be much appreciated :) 

user6802: The H11L1's minimum supply voltage is 3 V.
There are low-power optocouplers like the TLP2361, but event those do not go below 2.7 V. (And those limits are actually designed for 3.3 V ± 10 % or 3.0 V ± 10 %.)
To get a high-speed optocoupler for lower voltages, you have to build it yourself from discrete parts, e.g.:

(R1 is sized for the PC817-A's minimum CTR of 50 %; higher CTRs should use a lower value for R1. Q1 can be any random small-signal transistor. Adjust R2 for about 1 mA, depending on VCC.)

39438: <p>I've been trying to add a MIDI INput to an ATtiny-based synthesizer, powered by a CR2032 battery (3v) and running softwareSerial.</p>

<p>Currently I'm using an H11L1 optocoupler, which works fine at 5v, but I am missing notes at 3V; I'm not sure whether the problem is the optocoupler, or that the ATtiny can't establish proper communication on that voltage; any pointers will be much appreciated :) </p>
 6802: <p>The H11L1's minimum supply voltage is 3&nbsp;V.
There are low-power optocouplers like the TLP2361, but event those do not go below 2.7&nbsp;V. (And those limits are actually designed for 3.3&nbsp;V&nbsp;±&nbsp;10&nbsp;% or 3.0&nbsp;V&nbsp;±&nbsp;10&nbsp;%.)</p>

<p>To get a high-speed optocoupler for lower voltages, you have to build it yourself from discrete parts, e.g.:<br>
<img src="https://i.stack.imgur.com/4Awtc.png" alt="MIDI input with PC817"></p>

<p>(R1 is sized for the PC817-A's minimum CTR of 50 %; higher CTRs should use a lower value for R1. Q1 can be any random small-signal transistor. Adjust R2 for about 1 mA, depending on V<sub>CC</sub>.)</p>
