nano-z80

Z80 based SoC for the Tang Nano 20k FPGA Board which is designed to run CP/M and FUZIX. 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:

• 8192 k RAM, banked in four 16k banks
• 8k ROM which can be switched out (in block RAM)
• SD card storage
• Two UARTs (on the built in USB-C connector, one on the carrier board) with RX interrupts
• 80-column text mode HDMI video output, 640x480 60 Hz (80x30 characters), with 4 switchable buffers
• 160x120 8 bit color grapics mode, double buffered with 24 bit palette
• 320x200 8 bit color graphics mode with 24 bit palette
• USB keyboard support with interrupt (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
• Programmable interrupt controller with 8 irq channels
• Programmable timer for generating timer interrupts

Everything is clocked of the pixel clock, so the Z80-core is running at 25.175 MHz but is a bit waitstated by the SDRAM which runs at 100.7 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.

The FUZIX port provides four separate video terminals (TTY1-TTY4, switchable with F1-F4), access to both serial ports (TTY5 and TTY6), 4096 kB of addressable memory, and a FUZIX disk image on a separate location on the SD card so that it can be installed alongside CP/Mish.

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.

Write the FUZIX image file to the SD-card, with an offset of 0x100000 blocks. If you use dd, the following command can be used:

dd if=./FUZIX-disk.img of=/dev/sdX bs=512 seek=$((0x00100000))                         

Booting

In the monitor, press B to start CP/Mish from the SD-card.
In order to start FUZIX, run fuzix.com which is included on the A-drive in CP/Mish.

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
0x06: Interrupt controller selected
0x07: Timer selected

In addition to the banked ports, the following ports are always available:
0x60: MMU bank 0 LSB (8 bits) (0x0000-0x3FFF)
0x61: MMU bank 0 MSB (1 bits) (0x0000-0x3FFF)
0x62: MMU bank 1 LSB (8 bits) (0x4000-0x7FFF)
0x63: MMU bank 1 MSB (1 bits) (0x4000-0x7FFF)
0x64: MMU bank 2 LSB (8 bits) (0x8000-0xBFFF)
0x65: MMU bank 2 MSB (1 bits) (0x8000-0xBFFF)
0x66: MMU bank 3 LSB (8 bits) (0xC000-0xFFFF)
0x67: MMU bank 3 MSB (1 bits) (0xC000-0xFFFF)
0x68: MMU fixed top (0xF000-0xFFFF)
0x69: MMU page (0-127, switches all four bank registers)
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 0x78: Timer seconds counter
0x79: Interrupt flags
0x7a: UART B TX data
0x7b: UART B TX ready
0x7c: UART B RX data
0x7d: UART B RX available
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)
0x0a: SD data byte interface, auto increments when accessed and reset to 0 on SD card read/write
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
0x16: Ink (0-15, corresponds to Fuzix terminal colors), only used in video mode 0x04
0x17: Paper (0-15, corresponds to Fuzix terminal colors), only used in video mode 0x04
0x18: Inverse, only used in video mode 0x04
0x20: Video mode - 0x00 Text mode, 0x01 160x120x8, 0x03 320x200x8, 0x04 Multi-color text mode, (0x02 is text mode with access to 320x200 buffer)
0x21: Pixel Y (page 2 starts at line 120 for 160x120)
0x22: Pixel X LSB
0x23: Pixel X MSB (not used in 160x120)
0x24: Pixel data (auto-increments to next pixel on write)
0x25: Video page (0 or 1), only used in 160x120 mode
0x26: Palette color (0-255)
0x27: Palette Red
0x28: Palette Green
0x29: Palette Blue
0x2A: Vblank, output only, for synchronization
0x30: Active virtual buffer (0-3)
0x31: Visible virtual buffer (0-3)
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

Interrupt controller ports (0x7f == 0x06)

0x00: Interrupt enable bitmask, active high
0x01: Interrupt flags, active high
0x10: IRQ0 IM 2 vector
0x11: IRQ1 IM 2 vector
0x12: IRQ2 IM 2 vector
0x13: IRQ3 IM 2 vector
0x14: IRQ4 IM 2 vector
0x15: IRQ5 IM 2 vector
0x16: IRQ6 IM 2 vector
0x17: IRQ7 IM 2 vector

IRQ0: Timer
IRQ1: Keyboard
IRQ2: UART A (USB-C connector)
IRR3: UART B (Carrier board)

Timer ports (0x7f == 0x07)

0x00: Timer start (0 = stopped, 1 = running)
0x01: Timer compare value in ms, LSB
0x02: Timer compare value is ms, MSB
0x03: Timer current value in ms, LSB (read only)
0x04: Timer current value in ms, MSB (read only)

Known bugs

• Direct writing / reading to the video memory in text mode 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.