adding boot from spi_slave

[davideschiavone]

Very similar to booting from JTAG - it was also implemented in PULPino back in the days (https://github.com/pulp-platform/pulpino/blob/master/tb/tb.sv#L251)

This will be implemented after (#617)

and will require to change both the testbench and the bootrom code and HW

@JuanSapriza - shall we also add boot from uart? I prefer not as the bootrom code may become too long

[JuanSapriza]

In my mind this is a small conditional in the boot rom that checks the fourth possible boot state. If it's "boot from RAM" it will jump directly to the address where it would have copied the data using the SPI FLASH -- i.e. it only by-passes the action of copying data using the SPI FLASH, but all the rest remains the same.

The only difference in the programming procedure is that you need to write in the RAM yourself using the SPI slave and then reset the chip. Also need to use the fourth state of the boot_sel pins.

I think we need to modify the bootrom, the testbench (optional), but not the hardware. Am i right?

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Regarding boot from UART, i think that would indeed require HW that moves the data from the UART into memory to boot.