RISC-V Atom Bootloader

The RISC-V Atom bootloader is a stage-0 bootloader. It is loaded in the BootROM and executable as the first thing after reset. The source code for Atom bootloader is located in RVATOM/sw/bootloader subdirectory. The bootloader can be built manually using the following make command.

$ cd sw/bootloader
$ make soctarget=hydrogensoc

During run-time, the bootloader uses RAM to implement stack. it includes code for platform initialization, loading executable and control transfer. Bootloader provides various bootmodes to the user allowing the user to control the boot process.

Following is a list of actions that are performed step-by-step during the boot process:

  1. Upon reset the control is transferred to reset vector, which by default points to start of BootROM.

  2. CPU starts executing the stage-0 bootloader.

    1. Bootloader performs common initialization.

    2. Bootloader then performs any platform specific initializations.

    3. Bootloader reads the bootmode pins and decides the bootmode.

    4. Actions are performed according to bootmode, which includes loading the executable into RAM from correct source.

    5. Finally, the control is transferred to User program.

Bootmodes

RISC-V Atom bootloader mainly supports 4 boot modes:

Pinval

Bootmode

Function

0b00

FLASHBoot

Loads a binary image into RAM from FLASH memory using SPI

0b01

Jump to RAM

Jumps to RAM, without memory initialization

0b10

UARTBoot

Loads a binary image into RAM from UART stream using XMODEM protocol

0b11

Infinite Loop

Executes an infinite loop

FLASHBoot

In this bootmode, user must write the binary image for the program directly in FLASH memory at a predefined offset value. During boot, this binary image will be copied as it is to the RAM and will be executed. FLASHBoot requires the SPI IP to communicate with FLASH memory and will give an boot-panic error if its missing.

UARTBoot

In this mode, the bootloader obtains the program binary image from UART using the XMODEM protocol. It supports XMODEM CRC16 with default packet size of 128 bytes.

Transferring files over UART

RISC-V Atom project provides the xmsend.py python script that can be used to transmit binary files over UART. xmsend can be used as follows.

$ xmsend.py -b 115200 -p /dev/ttyUSB0 firmware.bin

Tip

Make sure that no other process (like screen) is using the serial port before invoking the xmsend script.