This repository contains drivers to build a minimal bare-metal software for the Carbon1 RISC-V tape-out.

Carbon1 is an open-source RISC-V MCU written in SpinalHDL and taped out with Cadence for IHP's 130nm SG13S semiconductor process. The design uses a VexRiscv RV32IMC CPU with standard low-speed peripherals (UART, GPIO, I2C, SPI) and system (MTIMER, PLIC) components. Moreover, it has a 512 Byte on-chip RAM, which provides space for the stack and data segments, and fetches instructions from an external SPI NOR device through the SPI controller.

The chip was developed with the Elements SDK, an open-source hard-/software co-design tool to develop, debug and maintain all stacks of an open-source System on Chip (SoC). Additionally, the project can generate all necessary files for the ASIC flow and allows the reproduction of the chip.

• I2C: Used tri-state IOs with pull-ups instead of open-drain. The potocol looks well with an oscilloscopei, but it is active high and therefore not I2C conform.

• VexRiscv RV32IMC
• SPI Controller XIP
• On-Chip RAM
• UART TX+RX
• UART IRQ on RX
• PLIC
• GPIO
• MTIMER

X2 has an ARM 20-Pin JTAG interface pinout; therefore, all compatible JTAG debugger can be used.

Use a jumper to connect pin 5 with 7 (UART CTS/RTS) to bypass the flow control when the UART-USB converter only has a TX and RX pin.

Download a pre-build risc-v gcc.

wget https://github.com/stnolting/riscv-gcc-prebuilt/releases/download/rv32i-2.0.0/riscv32-unknown-elf.gcc-10.2.0.rv32i.ilp32.newlib.tar.gz
mkdir riscv32-unknown-elf
tar -xvf riscv32-unknown-elf.gcc-10.2.0.rv32i.ilp32.newlib.tar.gz -C riscv32-unknown-elf/
rm riscv32-unknown-elf.gcc-10.2.0.rv32i.ilp32.newlib.tar.gz

Install OpenOCD with VexRiscv support (when debugging is required).

git clone [email protected]:aesc-silicon/elements-openocd.git
cd elements-openocd
./bootstrap && ./configure && make -j `nproc`
cd ../

A makefile exists in the root directory to build the kernel image.

make

driver/ and include/ contain driver for all IPs.

carbon/ contains the start-up code and main loop.

Carbon1 has a read-only SPI XIP controller and does not support programming the external memory.

Connect a Bus Pirate with all four SPI XIP pins on pin header X6 and execute the corresponding make rule.

make flash_buspirate

flashrom first reads the entire SPI flash content. Unfortunately, the SPI NOR has a size of 64MB and the flash procedure takes 20 minutes.

This project included support from other people whom I would like to thank:

* Steffen Reith (Hochschule RheinMain)
* Matthias Harter (Hochschule RheinMain)
* Andreas Grothe (Hochschule RheinMain)
* Elvira Liandres (Fraunhofer IIS)

This tape-out was funded by Hochschule RheinMain and PHYTEC Messtechnik GmbH sponsored the circuit boards.

Copyright (c) 2022 Daniel Schultz. Released under the license.