Add ProMicro bootloader DFU/README.md

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+DFU-Bootloader for Sparkfun Pro-Micro or Arduino Micro Boards
+=============================================================
+
+This Bootloader is optimized for Sparkfun Pro-Micro or Arduino Micro Boards.
+The Bootloader will be entered by default for 5 seconds before the application
+code is executed. This time frame is signaled by a slow blicking green LED on
+the Pro-Micro board. So the application code can be reflashed via USB, although
+the application itself might not support any software update or possibly blocks
+the USB interface because of bugs. The reprogram of the CPU can be done with
+the dfu-programmer program.
+The bootloader will start after this 5 seconds timeout the application code,
+if a reset vector to the application is programmed after a power-on or
+external reset. This 5 second delay before the application code is started
+by the bootloader can be omitted for other kind of resets like USB reset,
+watchdog reset or low voltage (brown-out) or JTAG reset, if the FUSE_HWBE is
+*not* programmed.
+Once a DFU program interacts this the bootloader during this first 5 seconds,
+the timeout is stopped and the application needs to be started by DFU bootloader
+command manually or using another reset.
+To use the CPU pin of the yellow LED as a SPI slave select input signal, the
+yellow LED is disabled and not used by the bootloader. If this pin is also need
+as input signal, naturally the application code shall never program this pin as
+output. Note that this possibly requires changes in the Arduino core code for
+ProMicro or Micro boards.
+
+
+Bootloader compilation and programming
+--------------------------------------
+The Makefile is patched already to generate a DFU bootloader for ProMicro
+Boards. Generating the hex files requires a make, avrdude, an AVR gcc compiler
+installed and an ISP programmer available. The bootloader will be generated
+wiht a simple make:
+
+    cd .../lufa/Bootloaders/DFU
+    make
+
+Initial the bootloader BootloaderDFU.hex file must be programmed via an ISP
+programmer. Here an example for the USBasp ISP programmer:
+
+    avrdude -c usbasp -p m32u4 -U flash:r:flash-backup.hex:i -B100
+    avrdude -c usbasp -p m32u4 -U flash:w:BootloaderDFU.hex:i -B100
+
+The option "-B100" is only required, if the internal oscilator is enabled and/or
+the CPU clock prescaller is enabled.
+
+
+AVR Fuse settings
+-----------------
+Naturally the fuse settings is mostly depending on the application code. But at
+least the in the hfuse some fuse bits are recommended:
+- SPIEN: SPI ISP enabled as fallback
+- BOOTRESET: Move the reset boot vector into the bootloader, because ProMicro
+             uses normally a power-on reset.
+- BOOTSZ1, BOOTSZ0: define the bootloader of 4kByte size.
+
+
+A fuses calculator can be found for the Atmega32U4 here:
+https://www.engbedded.com/fusecalc/
+
+Here fuses example for a ProMicro with 3.3V supply with debugging disabled:
+  lfuse: 0xff : CKDIV8, CKSEL0                    (ext. 8 MHz Quarz)
+  hfuse: 0xd8 : SPIEN, BOOTSZ1, BOOTSZ0, BOOTRESET(4kByte Boot & Reset to Boot)
+  efuse: 0xXe : BODLEVEL0                         (Brown-out 2.0V)
+
+    avrdude -c usbasp -p m32u4 -B100 -U efuse:w:0xce:m -U hfuse:w:0xd8:m -U lfuse:w:0xff:m
+
+
+Here fuses example for a ProMicro with 3.3V supply with debugging enabled and
+always starting the application over the bootloader:
+  lfuse: 0xff : CKDIV8, CKSEL0                    (ext. 8 MHz Quarz)
+  hfuse: 0x18 : SPIEN, BOOTSZ1, BOOTSZ0, BOOTRESET
+                JTAGEN, OCDEN                     (4kByte Boot & Reset to Boot)
+  efuse: 0xX6 : BODLEVEL0, HWBE                   (Brown-out 2.0V)
+
+    avrdude -c usbasp -p m32u4 -B100 -U efuse:w:0xc6:m -U hfuse:w:0x18:m -U lfuse:w:0xff:m
+
+