Add firmware build instructions

firmware/README.md

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+# Firmware
+
+The firmware is written in AVR assembly language.  It can be built for the ATtiny214, ATtiny414, and ATtiny814.
+
+## Build
+
+Building requires:
+
+- ASAVR (part of the [ASxxxx](https://shop-pdp.net/ashtml/) cross-assemblers package)
+- [SRecord](http://srecord.sourceforge.net/) version 1.64 or later
+- GNU [Make](https://www.gnu.org/software/make/)
+- A Unix-like operating system (e.g. Linux, macOS)
+
+Run `make` to produce Intel Hex files for the flash, fuses, and EEPROM:
+
+    make MCU=t414
+
+If `MCU=` is not given, the default target is `t414` (ATtiny414).  Valid targets are `MCU=t214`, `MCU=t414`, and `MCU=t814`.  It may be helpful to read the [`Makefile`](./Makefile) and the [GitHub workflow](../.github/workflows/main.yml).
+
+## Flash
+
+MXLock has a dedicated header for connecting to an [Atmel-ICE](https://www.microchip.com/en-us/development-tool/ATATMEL-ICE).  It also has pins on its 2.54mm header that can be used to connect to a different UPDI programmer.  In both cases, [AVRDUDE 7.2 or later](https://github.com/avrdudes/avrdude) is used.
+
+### Atmel-ICE
+
+Connect the Atmel-ICE to the dedicated header on the MXLock board.  Also connect a 5VDC power supply to the 5V and GND pins on the 2.54mm header.  Install the Atmel-ICE drivers and then run `make program`, specifying `MCU=` if needed.
+
+### SerialUPDI
+
+A SerialUPDI programmer can also be used.  One can be made from an FTDI [TTL-232R-5V](https://www.mouser.com/ProductDetail/895-TTL-232R-5V) cable and a 1KΩ resistor.  Make the following connections to the MXLock board:
+
+- FTDI RX → UPDI
+- FTDI TX → 1KΩ resistor → UPDI
+- FTDI GND → GND
+
+A 5VDC power supply must also be connected to the MXLock board.  The 5V pin on the FTDI cable may be used as the power supply.
+
+Since macOS and most Linux distributions ship with built-in FTDI drivers, drivers are not normally required.  Run `make program` and use `ISPFLAGS=` to set the SerialUPDI options for AVRDUDE:
+
+    make program MCU=t414 ISPFLAGS='-c serialupdi -P /dev/cu.usbserial-FTABCDEF'
+
+The above command will flash an ATTiny414 using SerialUPDI on port `/dev/cu.usbserial-FTABCDEF`.
+
+## References
+
+- [ATtiny214/414/814 Datasheet](https://web.archive.org/web/20231029180615if_/https://ww1.microchip.com/downloads/en/DeviceDoc/40001912A.pdf)
+
+- [AVR Instruction Set Manual](https://web.archive.org/web/20211122051203if_/http://ww1.microchip.com/downloads/en/devicedoc/atmel-0856-avr-instruction-set-manual.pdf)