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A ZipCPU based demonstration for the TinyFPGA BX board
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README.md

Tiny FPGA

This design is currently a work in progress. The goal is to place a ZipCPU onto a TinyFPGA-BX board.

Vision

My goals for this project are briefly to demonstrate the ZipCPU with a small supporting set of peripherals:

  1. SPI Dual, with support for

    • Dual flash I/O mode (34 clocks per read),

    • Successive dual flash reads (18 clocks per read)

    • GPIO override--so the design doesn't need the flash write capability, but yet can still write

    All of this functionality currently works and has been formally proven

  2. USB UART port (pair?)

  3. GPIO, to provide access to the external ports

  4. Some other TBD peripheral--to be added to the design using AutoFPGA

Planned internal peripherals include:

  1. Block RAM

  2. Timer

  3. Watchdog interrupt

  4. (Bus-based) Interrupt controller

Further, the project will be built using yosys, and simulated via Verilator--especially if I can figure out how to simulate the USB port with Verilator.

Status

20180810 - Both the cputest and Hello world work in the simulator. There are just a couple of known issues with the design:

  • When using AutoFPGA, the linker script that it produces is messed up. There's just not enough RAM to use that script. boardflash.ld should work, just don't overload your memory requests.

  • The flash bring-up may be inappropriate for the TinyFPGA board.

  • I haven't tested the flash driver necessary for programming the flash (yet)

  • There's no support for USB serial in either the design or the simulation ... yet.

Well, that and the obvious that it currently only works in simulation and hasn't seen the actual hardware yet. Given that the current design only uses 4427 elements (so far), there should be plenty of room for adding full USB support back in. (I'm told that's only about 1k LUTs or so.)

License

This project is released under the terms of the GPL v3.

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