How much current can I safely draw from the digital I/O power rails? (Peak / continuous.)
There is nothing in the specifications about these power rails beyond their nominal voltages of 3.3 V and 5 V.
PS. I assume the ‘input impedance’ of 400 ohm under section ‘output’ on page 33 of specification v23-1028 is the ‘output impedance’ of the digital output pins. A much more reasonable input impedance of > 10 Mohm is given earlier on the same page under ‘input’.
Thanks for the note. You’re right, the input impedance is 10 MOhm and output impedance is 400 Ohm.
So the Digital I/O is not a current limited system. As you’ve guessed, the Logic Analyser has a 400 Ohm resistor in series with the output which has a linear effect on the current. The amount of current drawn will be determined by how much of a voltage drop that you can have across your system.
As such, these high impedance pins are well suited to digital logic. This is so that there is no ambiguity when determining the logic of your circuit, as it minimises current variations through the system so that it does not vary the outputs of your logic gates.
Thanks! That is what I expected from the I/O pins in the output mode. But, is there a similar resistor also in the two power-rail pins (as there are two pins that are labelled to be the 5 V and the 3.3 V power rails, and not mixed I/O pins)? I am mostly looking at the 5 V, as it would allow noticeably more power at same current draw.
I probably should give this a bit more context: I am looking to power a tiny isolated dc-dc converter, as my external logic runs on +/- 15 V (and a few logic level converters, but they are a rounding error in terms of power budget). And, unless I get a tiny SMD converter, all the “jellybean” converters have maximum power output of 1 W. I am of course not going to draw anywhere near that from a logic pin (or even the logic power rail), but such a 1 W converter has both:
Massive nominal surge power draw on power-up (I have previously managed to make such a converter ran on ~50 ohm output impedance, so about a factor of 10 below its specified surge current draw, but still well outside what a typical logic pin can supply, but easily within reach of almost all power rails).
Constant load, for a stable output, the common converters need about 10% load. So, about 20–25 mA from the 5 V power rail.
I could, of course, power it of the Moku:Go PSU on the M2 model (or a small battery, or even the tiniest of USB power supplies), but if at all possible, I would want to have my logic board not have a separate power connector (to avoid the failure mode of losing power to the isolated dc-dc converter whilst connected to the Moku digital I/O pins).