Flashing an AT91SAM9G20-EK from bare metal

Since I just had cause to do this and it was harder than it needed to be due to bitrot in the public documentation I could find I thought I’d write up how to get a modern bootloader onto older Atmel boards. These instructions are written for the AT91SAM9G20-EK though they should also apply to other Atmel boards of a similar generation.

These instructions are for booting from NAND since it’s the default thing for the board, for this J34 should be fitted to enable the chip select and J33 disconnected to disable the dataflash. If there is something broken programmed into flash then booting while holding down BP4 should cause the second stage bootloader to trash itself and ensure the ROM bootloader puts itself into recovery mode, or just removing both J33 and J34 during power on will also ensure no second stage bootloader is found.

There is a ROM bootloader but it just loads a small region from the boot media and jumps into it which isn’t enough for u-boot so there is a second stage bootloader called AT91Bootstrap. Download sources for current versions from github. If it (or a more sensibly written equivalent) is not yet merged upstream you’ll need to apply this patch to get it to build with a modern compiler, or you could use an old toolchain (which you’ll need in the next step anyway):

diff --git a/board/at91sam9g20ek/board.mk b/board/at91sam9g20ek/board.mk
index 45f59b1822a6..b8251ca2fbad 100644
--- a/board/at91sam9g20ek/board.mk
+++ b/board/at91sam9g20ek/board.mk
@@ -1,7 +1,7 @@
-       -mcpu=arm926ej-s
+       -mcpu=arm926ej-s -mfloat-abi=soft
-       -mcpu=arm926ej-s
+       -mcpu=arm926ej-s -mfloat-abi=soft

Once that’s done you can build with:

make at91sam9g20eknf_uboot_defconfig
make CROSS_COMPILE=arm-linux-gnueabihf-

producing binaries/at91sam9g20ek-nandflashboot-uboot-${VERSION}.bin. This configuration will look for u-boot at 0x40000 in the flash so we need a u-boot binary. Unfortunately modern compilers seem to produce binaries that fail with no output. This is normally a sign that they need the ABI specifying more clearly as above but I got fed up trying to spot what was missing so I used an old CodeSourcery 2013.05 release instead, hopefully future versions of u-boot will be able to build for this target with older toolchains. Grab a recent release (I used 2015.01) and build with:

cd ${UBOOT}
make at91sam9g20ek_nandflash_defconfig
make CROSS_COMPILE=arm-linux-gnueabihf-

to get u-boot.bin.

These can then be flashed using the Atmel flashing tool SAM-BA. Start it and connect to the target (there is a Linux version, though it appears to rely on old versions of TCL/TK so if you get trouble starting it the easiest thing is to use the sacrificial Windows laptop you’ve obtained in order to run the “entertaining” flashing tools companies sometimes provide without risking a real system, or in my case your shiny new laptop that you’ve not yet installed Linux on). Start it then:

  1. Connect SAM-BA to the device following the dialog on start.
  2. Make sure you’ve selected “NandFlash” in the memory type tabs in the center of the window.
  3. Run the “Enable NandFlash” script.
  4. Run the “Erase All” script.
  5. Run the “Send Boot File” script and provide the at91bootstrap binary.
  6. Set “Send File Name” to be the u-boot binary you built earlier and “Address” to be 0x40000.
  7. Click “Send File”
  8. Press the reset button

which should result in at91bootstrap output followed by u-boot output on the serial console. A similar process works for the AT91SAM9263, there the jumper you need is J19 (sadly u-boot does not flash pictures of cute animals or forested shorelines on the screen as the default “Basic LCD Project 1.4” firmware does, I’m not sure this “full operating system” thing is really delivering improved functionality).

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