IROM boot loader from UART

This contains the code necessary to boot an Arty A7 by loading the firmware from the UART. This produces a cpu0_irom.vhx file that should be baked into the bitfile. On boot, this will listen on the UART and expects to read the file that would normally by in cpu_iram.vhx.

It reads data until it encounters a blank line.

NOTE: This is intended for testing and for software development on the CHERIoT Ibex, it is absolutely not a good idea to use this in production!

Building

This is built with a simple Makefile that expects to be passed the locations of various tools and the RTOS directory as part of the make invocation:

$ make CHERIOT_LLVM_ROOT=path/to/llvm/bin/ CHERIOT_RTOS_SDK=/path/to/cheriot-rtos/sdk/

This command will produce the cpu0_irom.vhx file. Copy this into cheriot-safe/build/firmware and then run the build_arty_a7 script to build the FPGA image.

Loading firmware

Make sure that you have an IRAM vhx file that ends with a blank line. By default, vhx files do not and so you will need to add one. Then:

1. Connect a serial terminal to the UART on the device.
2. Wait for the 'Ready to load firmware' message.
3. Instruct your serial terminal to paste the vhx file. In minicom, this is meta-z, y, then select the file.
4. Validate that the first word, last word, and length match.

If you do it correctly, you will see something like this in the UART output:

Ready to load firmware, hold BTN0 to ignore UART input.                                
Starting loading.  First word was: 40812A15
..............................................................................
Finished loading.  Last word was: 020000B8
Number of words loaded to IRAM: 00004D92
Loaded firmware, jumping to IRAM.

All of the loader's output will be in red, to make it easier to distinguish it from other output.

If you start loading the wrong thing and get error messages, hold button 0 (in the corner on the same side as the reset button) as long as the UART RX LED remains lit. This will cause the loader to ignore everything until you release the button. You can then try loading again.

The UART will then usually be reinitialised by the firmware that you've loaded.

The loader runs the UART at 115,200 Baud, eight data bits, one stop bit, no flow control.

Modifying

The assembly stub runs with Feel free to submit patches to improve this.