helping with a circuit to get automatic reset
native USB port: ah, you mean with full USB-OTG functionality native on the MCU? yyeahhhh it surprised the heck out of me to learn that the STML4 series... doesn't have USB at all!
2) [mosfet] 800mA which i'm more comfortable with than the 0.5W spec of the FDM340P. i guess though that the DPACK version of the MC33269 _is_ a bit overkill... i'll swap that for the SO223.
i'm going to have to connect I2C to pins 20 and 21, there's really nothing spare
Hmm. How about replacing the ch340g with a cheap usb-capable micro, ala the 16u2 used on real Uno/Mega2560 (but cheaper.) I don't know what's cheap in China, but the STM32F070 might be a candidate. It would be nice if it were capable of "crystal-less" operation, of course.
I'd be happy with just "USB device." Perhaps the USB circuitry doesn't fit into the "low power" regime that that 32L series is supposed to occupy.
The big 5V regulator is ok. Good, even - people tend to plug the external supply jack into 12V wall warts (could be 16+V if "unregulated"), and they need whatever extra power dissipation can fit on the board.
The MOSFET is a power switch, so it actally has very low dissipation. The FDN340 says Rdson is down around 0.07 ohm, so even at 1A it's dissipating less than 100mW...That's fine.
Most of the existing boards parallel them with some other pins; I think they're more to provide a "standard location for I2C" than "extra pins."
I really don't like the part libraries that the Arduino folk used.
I don't understand the round SMTs,
and the silkscreen layer is very sparse - I tend to like lots of silkscreen. I'm not sure if there's an easy way to do a wholescale change, though...
QuoteI'm not sure if there's an easy way to do a wholescale changeyes there is... you can set "layer".
I'm not sure if there's an easy way to do a wholescale change
the 072 (which is my favourite "crystal-less" MCU) is like... $1
add Layer 49 to the silkscreen
yes there is... you can set "layer".I mean "replace all the 0805RND packages from their RCL library with "My favorite 0805" package from my "myrcl" library, or the equivalent.I suppose.
But you also eliminate a crystal and a bunch of other components associated with the auto-reset function - those are probably worth another $0.20 or so?
Yeah, I usually finish things off by running "silk-new.ulp", which copies all the relevant objects to a new layer, where they can be shifted around/deleted/added/duplicated at will. But in this case, I'm objecting that the RCL packages don't have any outlines or symbols on them to copy...
If you want some help doing a "final" layout cleanup, let me know. I seem to enjoy that sort of thing :-(
if there was any other tiny processor with USB that was $0.50 i'd use it, definitely.
is the layout cleanup automatable?
I'd be shocked if there isn't some Chinese product that would work. STC, perhaps?
Not that I know of. It's a lot of pushing tracks around "a little bit", ripping up and re-routing a small subset of them at a time, and stuff like that, trying to make things look "even" and pretty without changing the electrical at all. Some people hate it...
This looks great! Especially as the Otto will probably be delayed forever. Put me down for two boards.
I've been frustrated with the cost of 32 bit boards as well, there are the STM32 "pills", some Uno sized boards, but nothing suitable in the Mega format. I would really like something that is a drop-in replacement for Due so it can be used with shields for 3D printers.
I don't have a lot of time (moving house), but if you have a prototype design I can convert it to KiCad.
Wow - A lot changed since the first time I looked at this! There was less to clean up, too.
Updated versions in [/url]https://drive.google.com/drive/u/0/folders/0B6dMB5dovDUZYzhjY2M1NmEtZmI2NC00MDZkLTliODMtOWYzNjQyZmViNWEy[/url]
A couple design comments:Using an SMT reset switch might be advantageous, and cheaper to assemble.Does your micro-SD socket allow traces underneath the socket? Some don't...
I'd say ditch the Arduino "dog ear styling" and go with plain rounded corners, like we did for "Freeduino", but that's just me being style-less :-)