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thseiler Update README
added note to README about 
- problem with gcc > 4.5 and a hint for the "newer-gcc-support-experimental" branch
- problem with newer SD cards due to wrong initialization command, currently unfixed
Latest commit af33cbf Apr 24, 2017



This is a proof-of-concept dual bootloader for Atmel AVR atmega chips.
It combines a serial bootloader and a mmc/sd card based one in the 
same 2kb of flash memory that is normally used by the arduino serial 
loader alone.

You might ask Why? Well, I have quite a few arduino "in the field". 
I can simply drag a .hex file to an SD card, insert it in an SD shield,
and field-upgrade my boards without additional hardware. At the same time, 
I can connect the serial port and start developping with the normal Arduino 

* Uses just 2kb of flash, so it fits into the normal boot block of most atmegas
* Serial bootloader is arduino compatible (avrdude / stk500v1)
* MMC bootloader can read directly a *.hex file from an SD/MMC card
* Supported are FAT16 formatted cards up to 2GB in size (no SDHC yet)
* All of ladyada's fixes to the original bootloader are included

How does the MMC/SD stuff work?
The MMC/SD part of the bootloader is disabled by default. This is actually
a safety feature, as people do not generally expect their bootloader to
output random signals on the SPI lines during boot-up.

2boots will therefore look at the end of the EEPROM. Only if it finds
a name there, 2boots will configure the SPI bus and try to init the
SD card.

So if you want to enable the MMC part of 2boots, you will have to give
your board a name. For this, simply load the included Sketch 
"NameBoardSketch.pde", optionally edit the name, and then run the sketch.
The sketch simply writes that name to the end of the EEPROM, in inverse order.
IMPORTANT: The name you choose must be a MS DOS compatible name. This means:
* The name can not be longer than 8 characters.
* The name must contain only UPPER CASE letters. Not lower case allowed, as
  MS DOS converts them to upper case before saving.
The default name in NameBoardSketch.pde is a simple "ARDUINO".

Now, prepare an SD card with the first partition: FAT16 and less than 2GB
in size. Copy the hex file directly into the root folder of the SD Card
and give it the same name as your board: i.e. ARDUINO.HEX.
IMPORTANT: Again, use only upper case letters, even in the extension.

To run the upgrade, simply insert the SD card with the .hex file into
your Arduino, power on, and press the reset button, after a while, the new
sketch should start. Power down, and remove the SD card to prevent a reflash
on next power up. Thats it.

If you have several boards, you may of course chose to write different names
to their eeproms. In that way you can carry the firmware for many different boards
on a single SD card.

How do I build it?
You need a build environment on a linux box that can compile for avr, so
please install avr-gcc and libc-avr and avrdude. 

Then, you should be able to just run
$ make

This should build 2boots for all boards and configurations. Once its done,
you can find the different .hex files in the ./build subfolder.

Important: If you modified the code, you should double-check that the "text"
segments are smaller than 2048 bytes. Otherwise, 2boots wont fit into the
bootblock of the AVR, and strange bugs will result.

Note: There are issues with newer gcc versions (> 4.5), you can try the new makefile in
the "newer-gcc-support-experimental" branch, this uses the link time optimisation
framework of newer gcc versions, but it is experimental and not very well tested.

Where can I find the .hex file in the Arduino IDE ?
It used to be in the applet folder. This changed with arduino-0018
Have a look at this thread:

Which .hex the right one for my board?
The .hex files are named after the following sheme:
where PIN denotes the MMC/SD Chip select line to use.

A few examples:
* 2boots-arduino-atmega328p-16000000L-PD4.hex is for:
  - Arduino ETHERNET
  - Arduino with atemga328 (i.e. Uno) plus Arduino EthShield SD
* 2boots-arduino-atmega168-1600000L-PD4.hex for
  - Arduino with atmega168 (i.e.) plus Arduino EthShield SD
* 2boots-arduno-atmega328p-8000000L-PD4.hex:
  - Arduino Pro 3.3V (w atmega328p) plus Arduino EthShield SD

How to flash the bootloader
* Connect your bare Arduino board (no Shields, no SD cards, nothing),
  to an ISP programmer of your choice.
* Edit the ./ script and adapt the variables ISPTOOL, ISPPORT, 
  and ISPSPEED to match your ISP programmer.
  (default values are for a AVRISP mkII USB programmer)
* Start flashing the bootloader, by issuing this command in a terminal:

# sudo ./ build/2boots-arduino-atmega328p-16000000L-PD4.hex

Some Notes on the Hardware
The MMC/SD card needs to be connected in the usual way to the SPI bus.
An additional pin is needed to be used as ChipSelect signal.
On Arduino Ethernet and Arduino EthShield SD, PD4 is used,
so this is also what 2boots uses by default. 

You can easily change this pin in the Makefile:
Look for a line similar to this:
board-arduino: MMC_CSS = nommc PD4

This is a space separated list that tells make which versions to build for
this board. "nommc" is a serial only version. And "PORTD4" will create a version which uses
PD4 as ChipSelect line.

Newer SD cards are not working well, the problem is in the initialization code.
SD card should be initialized by a command 41 instead of command 1. This logic
is currently missing in the code, but its on the TODO list.

Custom Boards
2boots was designed to allow you to easily add variants for your custom
boards. All you need to do is to create a new board-whatever.c/.h file.
In this file, you implement the main() function with the desired control flow.
I.e. you could add a blinking led, remove the serial loader, or only run the
loader if a certain button was held during reset...

Important: If you write your own code, check that the resulting binary is less than
2048 bytes in size, or strange bugs may result.

Something didn't work?
I'm interested to add support for as many configurations as possible.
So if 2boots does not work for you out of the box, and you had to change
something, please let me know...
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