Update README.md

DFU instructions.

README.md

@@ -4,13 +4,12 @@ This is the firmware for the TNC3 version 2.1.1 hardware.
 
 Use Eclipse with CDT and the GNU MCU Eclipse plugins.
 
-
-
 # Debugging
 
 Logging is enabled in debug builds and is output via ITM (SWO).  The
 firmware is distributed with an openocd stlink config file that enables
-ITM output to a named pipe -- `swv`.
+ITM output to a named pipe -- `swv`.  You must create this pipe in the
+top level directory.
 
 To read from this pipe, open a terminal and run:
 
@@ -19,3 +18,32 @@ To read from this pipe, open a terminal and run:
 If you change the MCU's core clock, you need to adjust the timing in the
 `stlink-tnc3.cfg` config file.
 
+# Installing firmware
+
+Firmware can be installed via the on-board ST/LINK port or via USB DFU.
+
+## USB DFU
+
+Download the STM32CubeProgrammer from STMicroelectronics.
+
+https://www.st.com/en/development-tools/stm32cubeprog.html
+
+This programmer will work on Linux, OS X, and Windows.
+
+Download the ELF file from the release (or that you have built from source).
+
+Plug the TNC into a USB port.  You should see a USB serial port enumerated.
+
+Put the TNC into DFU mode by pressing the DFU button on the side.  The TNC
+will only enter DFU mode when plugged into a USB port.
+
+***There is no visible indication on the TNC that it is in DFU mode***
+
+You should see the serial port device go away and a new DFU device appear.
+
+Run the STM32CubeProgrammer from the command-line.
+
+    ./STM32_Programmer_CLI -c port=USB1 -d firmware-1_0_0.elf -v -g 0x8000000
+
+When that is complete, the DFU device will disappear and the serial port
+device will re-appear.