pico-zxspectrum

48k/128k ZX Spectrum for Raspberry Pico Pi RP2040

This project is intended to be relatively easy to breadboard or prototype in some other way. It's just for fun and not a highly accurate emulation; hopefully it is good enough to be enjoyable.

Features

DVI/HDMI (Wren's Amazing PicoDVI)
LCD support (ST7789/ILI9341 320x240)
VGA video (RGB332, RGB222, RGBY1111)
USB keyboard & Joysticks
PS/2 keyboard
Martix keyboard
HDMI/PWM/I2S DAC audio for 48k buzzer and AY-3-8912
Audio input (load from tape)
12 quick save slots
Load from .z80 snapshot files
Read from .tap & .tzx tape files
On screen menu system
Kempston and Sinclair joystick emulation
Kempston mouse emulation

Updates

23/11/24

Better beeper sound quality (Mark II)

18/11/24

Better beeper sound quality (Should improve most builds)

16/10/24

Published a selection of RP2350 builds
There are now two firmware folders uf2-rp2040 and uf2-rp2350-arm-s

If you have a board for which you would like a RP2350 build please raise an issue and I will try to add it.

29/09/24

Removed dependency on modified TinyUSB library
Unmodified pico-sdk can be now used
New library for USB joystick/mouse/keyboard. See https://github.com/fruit-bat/pico-hid-host

18/09/24

Added support for NES/SNES joypads on MURMULATOR firmware
New library to support reading NES/SNES joypads. See https://github.com/fruit-bat/pico-nespad

more...

Boards

Click on the images below for more information ...

Board	Notes
HDMI breadboard	Some breadboard HDMI options
VGA breadboard	Some breadboard VGA options
PICO ZX48/128	ZX Spectrum 48K replacement board by Bobricius
PICOZX LCD	ZX Spectrum with LCD and VGA by Bobricius
PICOZX	ZX Spectrum with VGA by Bobricius
RetroVGA	VGA micro-computer by Bobricius
PicomputerMax	LCD micro-computer by Bobricius
PicomputerZX	ZX Spectrum with LCD by Bobricius
Pimoroni Pico DV	Pimoroni's Pico DV board
Pimoroni Pico VGA	Pimoroni's Pico VGA board
HDMI + key matrix	Breadboard with key matrix support
ArnoldUK	Arnold UK's board
MURMULATOR	Micro with VGA/HDMI and more
RP2040-PiZero	Waveshare RP2040-PiZero Development Board

Screen shots

Audio samples

Firmware targets

Pre-built binary targets can be copied directly to a Pico Pi. They can be downloaded from the links on the page for the board below or found in the uf2 folder.

Before attempting to update the firmware on your Pico make sure power supplies and any USB host devices have been disconnected (hub, keyboard, joysticks, etc.).

Push and hold the BOOTSEL button and plug your Pico into the USB port of your computer. Release the BOOTSEL button after your Pico is connected. It will mount as a Mass Storage Device called RPI-RP2. Drag and drop the UF2 file onto the RPI-RP2 volume.

On hardware with a faceplate the button is usually accessible through a small hole; the PICOZX Lcd board has the Pico facing inwards and you will need to reach a finger inside to press the bootsel button.

e.g. for the HDMI breadboard wiring show above use:

cp ZxSpectrumBreadboardHdmi4PinAudio.uf2 /media/pi/RPI-RP2/

Audio output

There are three techniques for audio output.

A mixture of digital sound and PWM output, which comes in three variants.
A DAC connected to the Pico using I2S.
Audio over HDMI to the display.

PWM/Digital Audio

PWM audio output comes in 3 variants, using 1, 2 and 4 pin:

Label	1 Pin	2 Pin	4 Pin
RP AUDIO1	Buzzer & AY-3-8912 PWM	AY-3-8912 PWM	AY-3-8912 Channel A PWM
RP AUDIO2	-	Buzzer	Buzzer
RP AUDIO3	-	-	AY-3-8912 Channel B PWM
RP AUDIO4	-	-	AY-3-8912 Channel C PWM

High frequencies need to be filtered out of the PWM audio output and mixed with the Spectrum's digital audio. Here are some sample designs. Please note they are not carefully designed but made from components I found lying around. If you create a particularly nice sounding design please let me know and I will add it to the documentation.

Separating out the Spectrum buzzer from the AY-3-8912 improves the fidelity of the Spectrum beeps.

The best audio is achieved by having separate pins for the Spectrum buzzer and AY-3-8912 A,B & C channels.

The sound is actually quite good from the 4 pin filter and at some point I will do a stero version.

Designs that only have a single GPIO pin available can have the audio mixed digitally:

I2S DAC Audio

The emulation can drive a PCM5100A DAC for line out audio over I2S. It uses the RP_DAC_DATA, RP_DAC_BCLK and RP_DAC_LRCLK pin on the Pico. Currently, only tested on the Pimoroni Pico DV board.

HDMI audio

HDMI builds can now output 44.1kHz audio to the display.

Video output

DVI/HDMI

The following circuit shows roughly how to connect an HDMI socket; I have always used a breakout board...

...but this is what I think it boils down to:

So far, there are three supported VGA configurations, which can be found in the various build targets. They have all been designed with a combination of plagiarism and guesswork, so please let me know if you have better versions and I will update this document.

VGA RGBY 1111

Although this is the most complicated, it is my favourite as it only uses 5 pins on the Pico. The display is slightly paler than the other two versions, which is easier on the eyes.

See this CMakeLists.txt for an example configuration.

VGA RGB 222

See this CMakeLists.txt for an example configuration.

VGA RGB 332

See this CMakeLists.txt for an example configuration.

PS/2 Keyboards

The emulator targets can accept input from a PS/2 keyboard wired to RP_PS2_DATA and RP_PS2_CLK. A suggested circuit is shown below:

The resistors and Zeners are there in case the keyboard contains a pull-up resistor to 5v on either the data or clock lines; the data and clock lines are, in theory, open-collector with no pull-up.

I'm told most PS/2 keyboards can be run at 3.3v and the the extra components become redundant... but I've not tried with mine. You may find the Pico struggles to deliver enough power at 3.3v for the SD card writes and running a keyboard.

Currently there is no toggling on the lock keys (caps/num lock) and the indicator leds are not used.

Audio Input

Due to a great deal of help from badrianiulian, here is a suggested audio input circuit:

Suggestions to improve this circuit are appreciated and please post them here.

Components

Pico pinout

Display considerations

Firstly, the emulation does not have a screen buffer, and there is not enough RAM left to add one. Whichever dsiplay you use, it will not have exactly the same timing as the old PAL monitors. This is particularly noticable on 60Hz displays with programs on the Spectrum that try to synchronize their output with the display, or use the frame rate to control the speed of music. I'm gradually adding 50Hz modes to firmware for various boards.

Joysticks

There is basic support for connecting USB joysticks and have them appear as Kempston or Sinclair joysticks to the Spectrum.

For joysticks to work they have to provide a HID report, which is usually the case for generic PC joypads. Some manufacturers make joysticks that don't report their behaviour and hence require custom drivers; these are not going to work.

Basically, I don't know how many different joysticks will be usable. If you are having trouble raise an issue and attach a HID report descriptor from your device and I will have a look at it.

To get this to work I have created a library which works alongside TinyUSB

https://github.com/fruit-bat/pico-hid-host

Build

Make a folder in which to clone the required projects e.g.

mkdir ~/pico
cd ~/pico

Clone the projects from github:

Using git protocol:

git clone git@github.com:fruit-bat/pico-hid-host
git clone git@github.com:fruit-bat/pico-nespad.git
git clone git@github.com:fruit-bat/pico-extras.git
git clone git@github.com:fruit-bat/PicoDVI.git
git clone git@github.com:fruit-bat/pico-zxspectrum.git
git clone git@github.com:pimoroni/pimoroni-pico.git
git clone git@github.com:fruit-bat/pico-dvi-menu
git clone git@github.com:fruit-bat/pico-emu-utils
git clone git@github.com:redcode/Z80.git
git clone git@github.com:redcode/Zeta.git

...or using https protocol:

git clone https://github.com/fruit-bat/pico-hid-host
git clone https://github.com/fruit-bat/pico-nespad.git
git clone https://github.com/fruit-bat/pico-extras.git
git clone https://github.com/fruit-bat/PicoDVI.git
git clone https://github.com/fruit-bat/pico-zxspectrum.git
git clone https://github.com/pimoroni/pimoroni-pico.git
git clone https://github.com/fruit-bat/pico-dvi-menu
git clone https://github.com/fruit-bat/pico-emu-utils
git clone https://github.com/redcode/Z80.git
git clone https://github.com/redcode/Zeta.git

Edit:

pimoroni-pico/drivers/fatfs/ffconf.h

and set FF_USE_FIND to 1

#define FF_USE_FIND            1

Switch to the audio branch of PicoDVI

cd PicoDVI
git checkout audio
cd -

Perform the build:

mkdir build
cd build

# $1 board
# $2 platform
build_group() {
  mkdir -p "$1_$2"
  pushd "$1_$2"
  cmake -DPICO_COPY_TO_RAM=0 -DPICO_PLATFORM=$2 -DPICO_BOARD=$1 ../..
  make -j4 relevant
  find . -name '*.uf2' -exec cp '{}' "../../uf2-$2" \;
  popd
}

# Pimoroni Pico VGA Demo Base with a Pico RP2040
build_group vgaboard rp2040
# Pimoroni Pico VGA Demo Base with a Pico2 RP2350 (Untested)
build_group vgaboard rp2350-arm-s
# Adafruit feather with built in RP2040
build_group adafruit_feather_rp2040 rp2040
# Various Pi Pico RP2040 boards
build_group pico rp2040
# Various Pi Pico2 RP2350 boards
build_group pico2 rp2350-arm-s

Copy the relevant version to your board, which can be located with:

find . -name '*.uf2'

e.g.

cp ./bin/breadboard_hdmi/ZxSpectrumBreadboardHdmi.uf2 /media/pi/RPI-RP2/

Prepare an SD card

I have used the following SD card formats:


2 GB FAT16
2 GB FAT32
4 GB W95 FAT32 (LBA)

The table shows the folders used by the emulator on the SD card. If not already present, they will be created as the emulator starts up.

Folder	Contents
zxspectrum/snapshots	Put your .z80 snapshot files in here.
zxspectrum/snapshots/quicksaves	Folder for quick saves.
zxspectrum/tapes	Folder for .tap and .tzx tape files.

USB and PS2 Keyboard mappings

Key	Action
AltGr	Symbol
F1	Toggle on screen menu
F3	Toggle mute
F4	Toggle the Z80 moderator. Cycles through 3.5Mhz, 4.0Mhz and unmoderated
F8	Reload current snap
F9	previous snap
F10	next snap
F11	Reset as 48k Spectrum
F12	Reset as 128k Spectrum
LCtrl + F1-F12	Quick save (LCtrl+F1 = save slot 1, LCtrl+F2 = save slot 2, etc)
LAlt + F1-F12	Quick load (LAlt+F1 = load slot 1, LAlt+F2 = load slot 2, etc)

Debug

Pico pin	Pico GPIO	Adapter wire
1	GP0	White
2	GP1	Green
3	GND	Black

tio -m ODELBS /dev/ttyUSB0

Thanks to

CarlK for the super no OS FAT FS for Pico
Damien G for maintaining and publishing some wonderful 8-bit fonts
Wren for the amazing PicoDVI
hathach for the embeded USB library TinyUSB
Lin Ke-Fong for the Z80 emulator
Pimoroni for lots of useful libraries
badrianiulian for help testing and design work on the audio circuitry
redcode for the Z80 emulator
Javavi for adding support for the MURMULATOR platform
ikjordan for his audio additions to PicoDVI

References

Z80 Test suite
Wren's Amazing PicoDVI
Z80 file format documentation
Fonts by DamienG
breakintoprogram - Screen memory layout
breakintoprogram - keyboard layout
breakintoprogram - interrupts
worldofspectrum - 48k ZX Spectrum reference
worldofspectrum - 128k ZX Spectrum reference
worldofspectrum - AY-3-8912 reference
JGH Spectrum ROM
48k ZX Spectrum service manual
GOSH ZX Spectrum ROM
Cassette input circuit design
ZX Spectrum ROM Images
AY-3-8912 - manual
AY-3-8912 - synth
USB HID 1.1
ST7789 LCD driver reference
RGB for 128k ZX Spectrum
PS/2 vs HID keyboard codes
PCM 5100A DAC
RP2040 Datasheet
Z80 Instruction set with XYH
Z80 Instruction set
Site with some WAV files
ZX Modules Software
TZX format
A decoder for ZX format
Circuit diagram editor

Interesting projects

Hermit Retro Products

fruit-bat / pico-zxspectrum

readme

pico-zxspectrum

Features

Updates

Boards

Screen shots

Audio samples

Firmware targets

Audio output

PWM/Digital Audio

I2S DAC Audio

HDMI audio

Video output

DVI/HDMI

VGA RGBY 1111

VGA RGB 222

VGA RGB 332

PS/2 Keyboards

Audio Input

Components

Pico pinout

Display considerations

Joysticks

Build

Prepare an SD card

USB and PS2 Keyboard mappings

Debug

Thanks to

References

Interesting projects