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RVCop64

RVCop64 is a bitstream for The Orange Cartridge that implements a RISC-V co-processor for the 6510/8502 processor in the C64/C128. The RISC-V processor is based on VexRiscv and implements the RV32IM architecture. Integration of peripherals such as external RAM, SD-card and USB is done though the LiteX framework.

The bitstream includes an EXROM containing BASIC extensions and a machine code monitor.

For machine code programmers, there are some extra I/O registers provided for communication between the 6510/8502 and the RISC-V co-processor.

For debugging via USB, serial port or JTAG, please see the debugging documentation.

Building

To build the bitstream, go to the hw directory and run the python script bitstream.py:

$ cd hw
$ python3 bitstream.py --platform orangecart

If all goes well, the bitstream will be created as build/gateware/orangecart.bit under the hw directory. Expect one warning about the use of IDDRXN and ODDRXN primitives on the same pin.

There are a number of arguments that can be passed to the bitstream.py script:

--platform {orangecart}

Specifies the hardware platform to target. This is a mandatory argument. Currently only the value orangecart is valid.

--sys-clk-freq SYS_CLK_FREQ

Changes the clock frequency of the RISC-V processor from the default of 64 MHz. The speed should be specified in Hz. If increasing the frequency, please watch out for warnings from nextpnr about failure to close timing. If decreasing the frequency, be aware that too low frequency can cause the external RAM to malfunction.

--uart {serial,usb_acm,jtag_uart,crossover}

Changes the connection of the RISC-V processor's built-in serial port. By default it is connected to a virtual UART which is accessed through the rvterm BASIC command. Specifying serial connects it to the physical serial port pins (J2) instead. Specifying usb_acm connects it to the USB port, as an ACM device. Specifying jtag_uart connects it to the JTAG, for use with litex_term jtag. Specifying crossover connects it to a virtual UART accessible through litex_server (please also enable one or more debug ports).

--uart2 {vuart,serial,usb_acm,jtag_uart,crossover}

Adds a second built-in serial port to the RISC-V processor. It offers the same set of connections as the primary serial port, but the two ports must not use the same connection. By default no secondary serial port is added; explicitly specify vuart to get a second serial port connected to the virtual UART connected to rvterm.

--usb {eptri,simplehostusb,debug}

Adds custom USB functionality. This argument can not be used together with --uart usb_acm. The value debug adds a USB bridge for litex_server. The values eptri and simplehostusb adds register interfaces for USB device and USB host respectively, which can be used by the software running on the RISC-V processor.

--jtag-debug

Adds a JTAG bridge for litex_server. This argument can not be used together with --uart jtag_uart.

--serial-debug

Adds a serial port (J2) bridge for litex_server. This argument can not be used together with --uart serial.

--serial-debug-baudrate SERIAL_DEBUG_BAUDRATE

Sets the baudrate for the serial port debug bridge enabled with --serial-debug. The default is 115200 bps.

--seed SEED

Specifies a random seed for nextpnr. In case routing fails, trying a different seed might help. This should normally not be needed.