venomix666 / nano6502

6502 SoC for the Tang Nano 20k FPGA Board

nano6502

nano6502 is a 6502 SoC for the Tang Nano 20k FPGA Board.

Current features:

• 64 k RAM (currently implemented with block RAM)
• 8k ROM which can be switched out (also block RAM)
• UART (on built in USB-C connector for now)
• SD card storage
• 80-column text mode HDMI video output, 640x480 60 Hz
• USB keyboard support (with nanoComp carrier board)

Everything is clocked of the pixel clock, so the 65C02 core is running at 25.175 MHz which gives a rather speedy user experience.

A port of CP/M-65 is just about the only software that exists for this SoC right now. It currently uses the UART for input as the USB keyboard support is not yet implemented but is otherwise fully functional with a SCREEN driver and 15x1 MB partitions on the SD-card.

Gettings started

Set up PLL

In order to set up the external PLL on the Tang Nano 20K for generation of the 25.175 MHz video clock and the 12 MHz USB clock, do the following:

• Open a serial terminal connection to the board
• Press Ctrl+x, Ctrl+c, enter
• Enter the command: pll_clk O0=25175K -s
• Enter the command: pll_clk O1=12M -s
• Enter the command: reboot

Program the FPGA

If you don't want to synthesize the project yourself, you can download the bitstream file and program it to the FPGA configuration flash memory using openFPGAloader:

openFPGAloader -b tangnano20k -f ./nano6502.fs

Peripherals and IO model

In order to maximize the amount of available RAM, a simple banked IO model is used.
The IO select register (address 0x0000) performs banking of the IO page (0xfe00-0xfeff) and can be set to the following values:
0x00: ROM on IO page.
0x01: UART on IO page.
0x02: LED control on IO page.
0x03: SD card control on IO page.
0x04: Video control IO page.
0x05: Timer IO page.

UART registers

0xfe00: TX data - write to initiate transmission
0xfe01: TX ready - UART is ready to accept a new TX byte
0xfe02: RX data
0xfe03: RX data available - high if a new byte is available in RX data

LED registers

0xfe00: LEDs - byte 0-6 connected to the on board LEDs
0xfe01: WS2812 Red - On board RGB led is automatically updated on write
0xfe02: WS2812 Green - On board RGB led is automatically updated on write
0xfe03: WS2812 Blue - On board RGB led is automatically updated on write

SD-card registers

0xfe00: SD card sector address (LSB)
0xfe01: SD card sector address
0xfe02: SD card sector address
0xfe03: SD card sector address (MSB)
0xfe04: SD card busy
0xfe05: SD card read strobe (write any value to initiate a sector read)
0xfe06: SD card write strobe (write any value to initiate a sector write)
0xfe07: Sector data page register (0-3), selects which 128 bytes of the sector are availabe on 0xfe80-0xfeff
0xfe08: SD card status (debug only)
0xfe09: SD card type (debug only)
0xfe80 - 0xfeff: 128 byte data page, paged by the page register so that all 512 bytes can be accessed

Video/TTY registers

0xfe00: Active line - selects which line in memory that is available at 0xfe80
0xfe01: Cursor X position
0xfe02: Cursor Y position
0xfe03: Cursor visible
0xfe04: Scroll up strobe
0xfe05: Scroll down strobe
0xfe06: TTY write character
0xfe07: Busy flag - no registers can be changed when this is high
0xfe08: Clear to end-of-line strobe
0xfe09: Clear screen strobe
0xfe0a: TTY enabled
0xfe0b: Scrolling enabled
0xfe10: Foreground Red
0xfe11: Foreground Green
0xfe12: Foreground Blue
0xfe13: Background Red
0xfe14: Background Green
0xfe15: Background Blue
0xfe80 - 0xfeff: Active line data, for direct access

Timer registers

0xfe00: Timer idle - 1 when idle, 0 when busy
0xfe01: Timer start strobe
0xfe02: Timer time in centiseconds LSB
0xfe03: Timer time in centiseconds MSB
0xfe04: Timer reset strobe

Known bugs

• Direct writing / reading to the video memory is glitchy due to some timing issue in the FPGA.
• USB keyboard sometimes double triggers on letters, to be fixed...

Credits

Some parts in this project are reused from other projects:
The 6502 Core used is Arlet's 6502 core with 65C02 instruction extension
The low-level SD-card state-machine is reused from MiSTeryNano
The USB HID host core was made by nand2mario
The font used is from this romfont repository