modify first stage bootloader

Hi everyone,
does it is possible to modify the first stage bootloader in order to use a custom one (send/get data only via serial port) ?
Must JTAG be using for this or is there another way?
I want to use FreeRTOS for A9 part and baremetal for M4 part (I want to test real-time capability and learn low level code)?

Thanks for your information.
Have a nice day.

 

I would suggest using u-boot and use it as a loader for A9/FreeRTOS. This way DRAM is setup, clocks are configured, etc. Just write to the UART register, for debugging.

 

Should I have to keep the SD card to hold u-boot or is there a way to put it on the internal flash?

 

There is no internal flash on Neo. You must use the SD card to load uboot.

 

Hi what about the 96 ko of internal ROM?

I wish use it to initialize the M4 part (copy code from SD to OCRAM, release reset,...) and then enable the A9 to boot on the SD in order to speed up the execution of M4 program.

So can we boot automatically from this memory?

 

We never did it. The only supported boot is from SD card. Then, from U-Boot, you can boot from anywhere you want.
Please note U-Boot takes also care of initializing a lot of registers.

 

The internal ROM can't be modified that is why is called ROM.

A suggestion, to partially speed it up, why don't you include the M4 image inside uboot, providing its small enough. So that when uboot is built it includes the image and then get uboot to copy it OCRAM/DDR. You would have to roll your own code to do this inside uboot. You can also remove some of the peripheral initialisation code in uboot if you don't require them.