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Bitbanging USB On Low Power ARMs


With the Adafruit Trinket, the Digispark, and some very clever work with the smallest microcontroller Atmel offers, it looks like the ‘in’ thing to do for embedded software developers is to bitbang the USB protocol on hardware that shouldn’t support it. There are a lot of very small ARM chips out there without USB support, so it was only a matter of time before someone was able to bitbang USB on the ARM Cortex M0+.

The board above is based on an Energy Micro EFM32ZG, a very small 24-pin QFN device with up to 32 kB of Flash and 17 GPIOs. As with all the bitbanged USB hacks, the differential data lines are attached directly to the microcontroller. A 24 MHz crystal is needed, but the team behind the project is working on using the internal RC oscillator instead.

The code is portable with minimal changes between other manufacturer’s Cortex M0+ chips, and with a little work, this could become a very, very cheap USB-programmable ARM dev board, something the community could certainly use.


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