Hardly seems killer.
I'd shut down the USB interface, use the UART instead.
The chips come with boot code in ROM that speaks to the Arduino IDE via the UART,
if you use that?you don't need to worry about loading the special USB bootloader.?
And you have to worry about that nasty 12mhz stuff on the USB wires being inside your radio.?
This USB to UART dongle is configurable for 3.3v or 5v IO:
? ? https://www.banggood.com/CJMCU-CP2102-USB-To-TTLSerial-Module-UART-STC-Downloader-p-970993.html?cur_warehouse=CN
Alternately, on the blue pill I got, the +5v USB pin goes to a feedthru, then over the the 5v pin on the header.
Cut the trace immediately after the feedthru, then add your diode from feedthru to the header 5v pin.
Jerry
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On Mon, May 21, 2018 at 11:59 am, W3JDR wrote:
While laying out my substitute Raduino PCB, I think I might have discovered a fatal flaw in trying to use the "Blue Pill" as a substitute for the Arduino Nano. It seems that the Blue Pill board has no reverse isolation diode or any other reverse current isolation between the USB power source and the external 5V power source. There is a direct connection between the USB connector +5V and the 3.3V regulator on the Pill board. This means you can't power the board from an external 5V source while the USB connector is in use. In fact, trying to do so will probably fry either the PC's USB or the external 5V supply.
This is a major bummer. It seems that virtually every cheap ARM-based alternative board I've evaluated has some sort of flaw that makes it difficult or impossible to use as a Nano footprint-compatible substitute.
Anyone have any insights/ideas besides using a $20+ Teensy or a larger and more expensive solution? I might have to revert to the TI Nucleo; that was about $11, but it had a Cortex M4 core with FPU? & 256K Flash as? bonuses.
