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 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.

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

Prepare the SD card

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Note: The image supplied with the release here may be outdated, please check the development build on the main CP/M-65 repository if you want the latest version.

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 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.

UART registers

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

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

USB host registers

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

GPIO registers

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

Sound generator registers

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

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 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