On a protoboard

Before the PCB existed, ZX-V was built on a protoboard with Pico 2 boards and a few breakout modules, and that is how its firmware has been tested, one Pico at a time. The two Picos have not yet been tested working together, so the complete machine described here is still untested. This page explains how to build it that way.

The PCB is the recommended way (The board): it has everything on it, the fast signals (HDMI and the link between the Picos) are short and well routed, and you only solder a few parts. A protoboard is fine for experimenting, or if you already have the parts, but the wiring is long and it needs care.

You can build it in stages and test each one (Building it step by step): the ULA Pico alone already shows a picture.

Parts#

The Pico pins#

Both Picos use the same pin numbers for the link (GP4 to GP4, GP26 to GP26...), which makes the wiring easier. Physical pin numbers are counted round the Pico as usual: pin 1 (GP0) and pin 40 (VBUS) are at the micro-USB end. Here are all the pins ZX-V uses:

Note that GP12 to GP19 mean different things on each Pico: on the ULA they are the HDMI output, on the CPU the debug port, USB and clock. Do not join them.

Power#

All the grounds must be joined. There are two ways to power it:

The simplest, for testing: each Pico from its own micro-USB, from the computer or from two USB chargers. Join the grounds of the two Picos. The USB-A port takes its 5 V from the CPU Pico's VBUS (pin 40), which is enough for a keyboard and a mouse; with a powered hub, better.

A single 5 V supply (like the PCB):

• The supply feeds VSYS (pin 39) of each Pico through a Schottky diode (the anode to the supply, the cathode, the end with the stripe, to VSYS). This way you can plug in a Pico's micro-USB at the same time (to update it, or for the serial port) without its 5 V going back into the supply.
• The supply also feeds the USB-A's 5 V (VBUS) through the 500 mA resettable fuse.

If you feed the supply into the CPU Pico's VBUS (pin 40) instead, without diodes, you must disconnect the supply before plugging in that Pico's micro-USB (to flash it, or for the serial port), or its 5 V would reach your computer.

The link between the two Picos#

Short wires (at most about 20 cm), with at least three ground wires spread among the data wires. Each signal has a 33 Ω resistor in series, placed next to the Pico that sends it. With very short wires you can try without the resistors; if the link shows errors (see No picture), add them.

• Without the sound line there is a picture but no sound.
• Without the return channel everything works, but programs are always told the output is HDMI at 60 Hz, and the CPU's report warns about it.

HDMI#

On the ULA Pico. One 240 Ω resistor in series with each of the eight signals, as close to the Pico as possible. If your breakout has its pins labelled (D0+, CK−...), just follow the Signal column.

• Keep the wires short (under 10 cm) and each pair (+ and −) together, or twisted. These signals carry hundreds of millions of bits per second.
• Pin 18 (5 V): many TVs need it to detect that something is connected. Take it from the ULA's VBUS (pin 40) when the ULA is powered by its micro-USB, or from your 5 V supply. The PCB puts a Schottky diode in this line so the TV cannot feed power back into the board.
• Use a short HDMI cable, without adapters.

Audio#

On the ULA Pico: GP2 (pin 4) is the left channel and GP3 (pin 5) the right. Each channel goes through a two-stage filter and a capacitor to the jack:

GP2 ──[1 kΩ]──┬──[10 kΩ]──┬──|+ 10 µF|──┬── jack: tip (left)
            10 nF        1 nF         100 kΩ
              │           │            │
             GND         GND          GND

The same for GP3, to the jack's ring (right). The jack's sleeve goes to GND (pin 3 or 8). The 10 µF capacitor is polarised: its + goes towards the Pico. The output is line level, for powered speakers or a TV's audio input.

For a quick test, a 1 kΩ resistor and the 10 µF capacitor are enough, although you will hear some hiss.

The mode button and the link LED#

On the ULA Pico.

The button needs no resistor: the Pico has one inside. What the button and the LED do is explained in The firmware.

The SD card#

On the CPU Pico. A 3.3 V microSD module (or a 5 V one, with a regulator and level shifters, powered from 3.3 V):

The activity LED: from GP22 (pin 29), a 330 Ω resistor and an LED to ground. It lights while the machine uses the card or the clock.

The keyboard, mouse and gamepads (USB-A)#

On the CPU Pico. A female USB-A connector (a breakout, or the end of a cut USB extension cable):

D+ and D− go straight to the Pico, or through a 22 Ω resistor each (the PCB has them); no other resistors are needed. Plug in a keyboard, or a hub with the keyboard, a mouse and one or two gamepads.

The clock (optional)#

On the CPU Pico. Without a clock the machine works, but forgets the date and time, the language and the keyboard layout when switched off.

The easiest is a DS3231 module for 3.3 V, with its CR2032 cell (a 5 V module with a regulator also works if its pull-up resistors go to 3.3 V):

A PCF8563 (or BM8563) chip, as on the PCB, connects the same way, but needs more parts around it: a 32.768 kHz crystal with a 12.5 pF load next to the chip, 4.7 kΩ pull-up resistors from SDA and SCL to 3.3 V, and a BAT54C diode pair that feeds the chip from the 3.3 V and from the CR2032 cell, with a 100 nF capacitor. The firmware looks for a DS3231 first and then for a PCF8563, and uses the one it finds.

Reset button and debug port (optional)#

• Reset: a push button from the RUN pin (pin 30) of each Pico to ground restarts it. The PCB joins both RUN pins to a single button.
• Debug serial port: the CPU Pico's GP12 (pin 16) is TX and GP13 (pin 17) is RX, at 921600 baud and 3.3 V. Cross them to a 3.3 V USB-serial adapter (TX to RX). The same messages come out of the micro-USB anyway.

Building it step by step#

1. The ULA Pico alone. Wire the HDMI and the audio, and flash the zxv_ula.uf2 test (The firmware). You should see some demos and hear music, on the screen and on the jack. Add the mode button and the LED, and try the modes.
2. The two Picos. Wire the link, flash zxv_cpu.uf2 on the CPU Pico and zxv_ula_link.uf2 on the ULA Pico. The BASIC should appear on the screen. Open the CPU Pico's serial port with a terminal: what you type there reaches the machine, and every 5 seconds it writes a report. (This is the step that has not been tested yet: if it fails, the report on the serial port is the place to start.)
3. The SD card, the USB-A and the clock, one at a time, checking each in the report (card, USB, clock) and from the BASIC: CAT for the card, typing for the keyboard, and CLOCK 2026,10,20,18,30 then PRINT CLOCK 3;":";CLOCK 4 for the clock.

If something does not work, see If there is no picture or no sound.