nano-z80

Z80 based SoC for the Tang Nano 20k FPGA Board which is designed to run CP/M. It reuses many IP-blocks from my nano6502 project which is very similar but is built around a 6502-core instead of a Z80.

Current features:

• 64 k RAM (currently implemented with block RAM)
• 8k ROM which can be switched out (also block RAM)
• SD card storage
• UART (on the built in USB-C connector)
• 80-column text mode HDMI video output, 640x480 60 Hz (80x30 characters)
• USB keyboard support (with nanoComp carrier board)
• Bidirectional GPIO on the header on the nanoComp carrier board
• Control of the LEDs on the Tang Nano 20k board
• Additional UART on carrier board UART header

Everything is clocked of the pixel clock, so the Z80-core is running at 25.175 MHz.

A port of David Given's CP/Mish gives a very nice CP/M 2.2 environment for this computer. See the cpmish page for more details. Here are some screenshots:

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 ./nano-z80.fs

Prepare the SD card

Write the nano-z80.img file into the SD-card using dd or your preferred SD-card image writer.

Peripherals and IO model

In order to maximize the amount of available IO ports, a simple banked IO model is used.
The IO select register (port 0x7f) performs banking of the IO ports and can be set to the following values:
0x00: LED control selected
0x01: GPIO selected 0x02: USB selected
0x03: SD card control selected.
0x04: Video control selected.
0x05: UART selected.

In addition to the banked port, the following ports are always available:
0x70: UART A TX data
0x71: UART A TX ready
0x72: UART A RX data
0x73: UART A RX available
0x74: Keyboard data available
0x75: Keyboard data
0x76: Video TTY data write
0x77: Video TTY busy 0x7e: ROM disable

LED ports (0x7f == 0x00)

0x00: LEDs - byte 0-6 connected to the on board LEDs
0x01: WS2812 Red - On board RGB led is automatically updated on write
0x02: WS2812 Green - On board RGB led is automatically updated on write
0x03: WS2812 Blue - On board RGB led is automatically updated on write

GPIO ports (0x7f == 0x01)

0x00: Data register 1 (GPIO 0-7)
0x01: Data register 2 (GPIO 8-12)
0x02: Direction register 1 (GPIO 0-8), 0=Input, 1=Output
0x03: Direction register 2 (GPIO 8-12), 0=Input, 1=Output

USB host registers (0x7f == 0x02)

0x00: New key available - clears on read
0x01: Keypress ASCII data
0x02: Key modifier
0x03: Mouse button
0x04: Mouse dX
0x05: Mouse dY
0x06: Gamepad direction {4'b0000, game_d, game_u, game_r, game_l}
0x07: Gamepad buttons {2'b00, game_sta, game_sel, game_y, game_x, game_b, game_a}
0x08: New USB report available - clears on read
0x09: Device type - 0: no device, 1: keyboard, 2: mouse, 3: gamepad
0x0a: USB error code

SD-card ports (0x7f == 0x03)

0x00: SD card sector address (LSB)
0x01: SD card sector address
0x02: SD card sector address
0x03: SD card sector address (MSB)
0x04: SD card busy
0x05: SD card read strobe (write any value to initiate a sector read)
0x06: SD card write strobe (write any value to initiate a sector write)
0x07: Sector data page register (0-3), selects which 128 bytes of the sector are availabe on 0xfe80-0xfeff
0x08: SD card status (debug only)
0x09: SD card type (debug only)
0x80 - 0xff: 128 byte data page, paged by the page register so that all 512 bytes can be accessed

Video/TTY ports (0x7f == 0x04)

0x00: Active line - selects which line in memory that is available at 0xfe80
0x01: Cursor X position
0x02: Cursor Y position
0x03: Cursor visible
0x04: Scroll up strobe
0x05: Scroll down strobe
0x06: TTY write character
0x07: Busy flag - no registers can be changed when this is high
0x08: Clear to end-of-line strobe
0x09: Clear screen strobe 0x0a: Clear to end-of-screen strobe
0x0b: Delete line strobe
0x0c: Insert line stribe
0x0d: TTY enabled
0x0e: Scrolling enabled
0x10: Foreground Red
0x11: Foreground Green
0x12: Foreground Blue
0x13: Background Red
0x14: Background Green
0x15: Background Blue
0x80 - 0xff: Active line data, for direct access

UART ports (0x7f == 0x05)

0x00: TX data UART A - write to initiate transmission
0x01: TX ready UART A - UART is ready to accept a new TX byte
0x02: RX data UART A
0x03: RX data available UART A - high if a new byte is available in RX data
0x04: TX data UART B - write to initiate transmission
0x05: TX ready UART B - UART is ready to accept a new TX byte
0x06: RX data UART B
0x07: RX data available UART B - high if a new byte is available in RX data
0x08: Baudrate UART B - 0: 4800, 1: 9600, 2: 19200, 3: 38400, 4: 57600, 5: 115200

Known bugs

• Direct writing / reading to the video memory is glitchy due to some timing issue in the FPGA.

Credits

Some parts in this project are reused from other projects:
The Z80 Core used is TV80
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

The monitor in ROM is based on the bsx monitor.