Tom's Life Bits

Read in the Sync Channel Blog about how to get a second UART on the HUZZAH. The author recommends pin 15 as Serial Rx and pin 13 as serial tx using the espsoftserial library on github at
https://github.com/scottwday/EspSoftSerial

There is a tiny gps example of using espsoftserial on the Sync Channel github at
https://gist.github.com/SyncChannel/7a905511be7e33990d91

The library said it will support 4 channels at 19,200. I need one hardware channel aone software serial channel, both running at 56k.

This post is about the Adafruit Feather line of microcontroller development boards. It will serve as a guide to cross-Feather compatibility when designing both hardware add-on circuits and software to work with all of the Feather variants.

Left to right: Feather 32U4 Adalogger, my GPS FeatherWing, Feather M0, and Feather HUZZAH.

Serial communication on the Feather 32U4 is pretty simple, and your Arduino projects can probably transfer straight over. You have the hardware UART on pins D0 and D1. Also, you can use SoftwareSerial in the IDE to turn other digital pins into serial pins. On the M0, things are a little different. SoftwareSerial is not (currently) supported. However, you have several SERCOMs available that can act as hardware UARTs. They are somewhat documented in this post on the Arduino forum. Great! Right? …

Unfortunately the SERCOM pins don't line up well with the usable pins in SoftwareSerial for the Feather 32U4 (basically a Leonardo in the IDE). For example, SERCOM1 uses pin D12 as RX, but that is not a possible RX pin in SoftwareSerial on the 32U4.

Serial communication is once again different on the HUZZAH. You do have the hardware UART that lines up with pins D0 and D1 on the M0/32U4, but you should really (really) leave that one alone. It's connected to the serial bridge, and you don't want USB traffic interfering with your serial-connected device or vice versa. There is a second hardware UART in the ESP8266. It's documents on the github ESP8266/Arduino page, but you can see just the relevant part in this post on the ESP8266 forum. That UART would work fine with a 32U4-compatible pinout, but doesn't work with the SERCOM pinout on the M0.

So what to do if you need a second non-USB UART? My recommendation is to use pin D10 on the 32U4 and M0 (pin #15 on HUZZAH) as a second serial RX. That will work with the M0 SERCOM and 32U4 SoftwareSerial, and there are some SoftwareSerial-like libraries out there for the ESP8266 (1, 2, probably more…) that should work for your project. For the TX line, I recommend implementing a jumper in your circuit to select between pins D11 and D12 (pins #12 and #13 on HUZZAH). Select the pin D11 jumper to use the SERCOM on the M0, and select the other jumper for 32U4 and HUZZAH.

For a second UART, I recommend using pin D10 (#15 on HUZZAH) as serial RX and implementing a jumper to choose between pin D12 for the M0 and D11 (#13 on HUZZAH) for TX. Your sketches will be different for all three in order to set up the necessary configurations or libraries.