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 the built in USB-C connector, and the UART header on the carrier board)
• SD card storage
• 80-column text mode HDMI video output, 640x480 60 Hz
• USB keyboard support (with nanoComp carrier board)
• Bidirectional GPIO on the header on the nanoComp carrier board
• Sound through the speaker output from a 3-voice programmable sound generator

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, apart from the boot ROM and monitor. The on-board USB UART or a USB keyboard can be used for input and it has a SCREEN driver, a SERIAL driver for UART B and 15x1 MB partitions on the SD-card.

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

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

Write the nano6502.img file into the SD-card using dd or your preferred SD-card image writer. If you are updating the image and want to preserve the data on all drives except A, write the nano6502_sysonly.img instead.

Note: The image supplied with the release here may be outdated, please check the development build on the main CP/M-65 repository if you want the latest version.

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 or RAM 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.
0x06: USB HID page.
0x07: GPIO page.
0x08: Sound generator page.

The boot ROM normally resides at 0xe000 - 0xffff, but can be switched out by writing 0x01 to address 0x0002 in order to have RAM from 0x0000 - 0xfeff. The last page is always assigned to ROM so that the reset vector is correct.

0xfe00: TX data UART A - write to initiate transmission
0xfe01: TX ready UART A - UART is ready to accept a new TX byte
0xfe02: RX data UART A
0xfe03: RX data available UART A - high if a new byte is available in RX data
0xfe04: TX data UART B - write to initiate transmission
0xfe05: TX ready UART B - UART is ready to accept a new TX byte
0xfe06: RX data UART B
0xfe07: RX data available UART B - high if a new byte is available in RX data
0xfe08: Baudrate UART B - 0: 4800, 1: 9600, 2: 19200, 3: 38400, 4: 57600, 5: 115200

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

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

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

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

0xfe00: New key available - clears on read
0xfe01: Keypress ASCII data
0xfe02: Key modifier
0xfe03: Mouse button
0xfe04: Mouse dX
0xfe05: Mouse dY
0xfe06: Gamepad direction {4'b0000, game_d, game_u, game_r, game_l}
0xfe07: Gamepad buttons {2'b00, game_sta, game_sel, game_y, game_x, game_b, game_a}
0xfe08: New USB report available - clears on read
0xfe09: Device type - 0: no device, 1: keyboard, 2: mouse, 3: gamepad
0xfe0a: USB error code

0xfe00: Data register 1 (GPIO 0-7)
0xfe01: Data register 2 (GPIO 8-12)
0xfe02: Direction register 1 (GPIO 0-8), 0=Input, 1=Output
0xfe03: Direction register 2 (GPIO 8-12), 0=Input, 1=Output

0xfe00: Frequency Oscillator 1 LSB
0xfe01: Frequency Oscillator 1 MSB
0xfe02: Pulse wave duty cycle Oscillator 1 LSB
0xfe03: Pulse wave duty cycle Oscillator 1 MSB (bits 0-3)
0xfe04: Control register OSC1 (B0: Gate, B1: not used B2: not used B3: not used B4: Triange B5: Sawtooth B6: Pulse B7: Noise)
0xfe05: Attack/Decay ADSR1 (bits 0-3 decay, bits 4-7: attack)
0xfe06: Sustain/Release ADSR1 (bits 0-3 release, bits 4-7: sustain)
0xfe07: Frequency Oscillator 2 LSB
0xfe08: Frequency Oscillator 2 MSB
0xfe09: Pulse wave duty cycle Oscillator 2 LSB
0xfe0a: Pulse wave duty cycle Oscillator 2 MSB (bits 0-3)
0xfe0b: Control register OSC2 (B0: Gate, B1: not used B2: not used B3: not used B4: Triange B5: Sawtooth B6: Pulse B7: Noise)
0xfe0c: Attack/Decay ADSR2 (bits 0-3 decay, bits 4-7: attack)
0xfe0d: Sustain/Release ADSR2 (bits 0-3 release, bits 4-7: sustain)
0xfe0e: Frequency Oscillator 3 LSB
0xfe0f: Frequency Oscillator 3 MSB
0xfe10: Pulse wave duty cycle Oscillator 3 LSB
0xfe11: Pulse wave duty cycle Oscillator 3 MSB (bits 0-3)
0xfe12: Control register OSC3 (B0: Gate, B1: not used B2: not used B3: not used B4: Triange B5: Sawtooth B6: Pulse B7: Noise)
0xfe13: Attack/Decay ADSR3 (bits 0-3 decay, bits 4-7: attack)
0xfe14: Sustain/Release ADSR3 (bits 0-3 release, bits 4-7: sustain)
0xfe15: Master volume

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

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