bluesmoke328:
Could you explain in more detail exactly how the power configuration options you mention in the first post in this thread works?
LOL, yes, that description of the power rail configuration options in the first post is a bit “stream of consciousness” – sorry about that. I think I must have written that a bit too close to the time I had my head down in the nitty gritty of the board design.
In reality, the options are pretty simple. 99% of the time it boils down to whether you want to run Vcc to the chip at 5V or 3v3. In that case, there are two jumper settings to choose from.
So, let’s assume you are primarily powering the “Skinny Bob” board from a DC supply >= 6V applied to Vin, either by the barrel jack or to the Vin rail directly (e.g., via a header pin).
On the 2x3 male pin power header, the jumper setting for 5V looks like this:
o o o
[o o] o
5V Vcc 3v3
And 3v3 looks like this:
o o o
o [o o]
5V Vcc 3v3
That’s it. Move one jumper to choose 5V or 3v3 Vcc for the 1284p.
So what are the jumper pins on the top row used for?
These are for less usual situations when you want tie rails together for some reason, typically for when you would be using some other arrangement for power than 6V+ supplied to Vin. (I’ll elaborate a bit further in a follow-up post on some more "exotic" possible configurations, perhaps when I've drawn up a nice schematic.of the power section.)
And finally, what about switching to a USB power input from the Vin power input? E.g., what if we want to power the board by connecting a USB/TLL cable to the USB/TTL header, or an ISP programmer to the ICSP header? Do we have to change the power jumper block settings in that case?
Well, no. Just like a standard Arduino, the “Skinny Bob” will automatically switch to using a power supply available from USB on the 5V rail if there is no supply on Vin. It achieves this via a considerably simpler “auto-switching” mechanism (using a Schottky diode) than the FET/op-amp arrangement on the Arduinos, but ultimately the same effect.
So all pretty uncontroversial and unexciting so far. Which is as planned. As I say, I'll get back with a description of some more special case possible power configurations.