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


Test DFU flags
--------------
The DFU bootloader flags of the programmed Pro-Micro can be read by a script:
    read-dfu-bootld-flags.sh

This script runs naturally also be used for other boards, but might output different
results other than expected.