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pikvm/docs/pico_hid.md

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Raspberry Pi Pico HID

!!! warning "This page is under construction"

The Pico HID is a part of DIY PiKVM V1 platform and performs keyboard and mouse emulation. It has excellent compatibility, and by default emulates USB, including two mouse modes: absolute and relative.

The scope of the Pico HID is not limited to V1 platform, it can also be used with V2 and even V3 platform if you need to emulate a PS/2 keyboard and mouse or use legacy multiport KVM switch which does not fully support USB standards.

This page explains how to build, connect and use all the features of the Pico HID.

Making the Pico HID

If you are building the PiKVM V1, then all the necessary components should already be at your fingertips. If you are making the Pico HID for V2 or V3, then here is what you will need:

  • Raspberry Pi Pico board with soldered pins. An official green board is recommended.
  • x1 USB-A to Micro-USB cable.
  • x10 dupont wires female-female.
  • Optional: x1 1N5819 diode. But any similar one will do.

!!! tip "Tip for soldering guru" If you know how to solder, you can buy the Pico without pins and no dupond wires, and just solder everything.

!!! warning The diode is needed to provide the power to the Pico HID regardless of the host state, and prevents backpowering problem. It will allow you to keep the keyboard buttons pressed during the host power cycle, which is important for MacOS to get into the boot menus, for example.

Do not connect the red wire (the `VSYS (Pico) -> 5V (Pi)` line) without a diode.
If you can't find the diode, don't connect this wire at all.

Connect all the parts according to the scheme:

??? example "Simple wiring diagram"

??? example "Electrical schematic diagram for advanced users"

PS/2 Keyboard & Mouse

!!! warning PS/2 is not implemented now. Soon (r) (c) (tm)

If you need PS/2 keyboard and mouse support, you will need a few additional components. The soldering skill will also not be superfluous.

  • x1 3.3V/5V bi-directional logic level shifter like this.
  • Optional: x2 PS/2 cable with male connector (can be cut off from the old keyboard and mouse).

Connect all parts to the Pico HID, and don't forget to enable PS/2 mode support as described in the next paragraph.

??? example "Level shifter and PS/2 wiring"

Make sure that the level shifter pinout matches the scheme, and connect everything according to the [Pico pinout](https://pico.pinout.xyz).

```
                 _________________
                |                 |
Pico GP11 ______| LV1         HV1 |______ PS/2 keyboard data
Pico GP12 ______| LV2         HV2 |______ PS/2 keyboard clock
Pico GP13 ______| LV          HV  |______ PS/2 5V
Pico  GND ______| GND         GND |______ PS/2 GND
Pico GP14 ______| LV3         HV3 |______ PS/2 mouse data
Pico GP15 ______| LV4         HV4 |______ PS/2 mouse clock
                |_________________|

```

You can take the 5V power line from one of the PS/2, for example from the keyboard,
or from both at once, but make sure with a multimeter that it is the same line
on both PS/2 female connectors.

<img src="ps2_pinout.png" />

PS/2 female socket pinout of the keyboard and mouse on the motherboard is the same.
A purple socket is usually responsible for the keyboard, and a green one for the mouse.
If your motherboard only have one port, it's probably universal and can be used either
for the keyboard or for the mouse. Most likely, it is painted in two colors at once.

Use a multimeter to determine the purpose of the wires in your PS/2 cables.

A good idea is to mount the level shifter on top of the Pico, as in this photo:

<img src="ps2_level_shifter_soldering.png" />

Configuring the HID modes

By default, Pico HID emulates a USB keyboard and an absolute or relative mouse (read here about the difference between mouse modes). For most cases, nothing needs to be changed here. However, if you need something special (like Windows 98 support), you can do it without reflashing on the current firmware.

To achieve this, the Pico HID uses a runtime configuration, which is set by connecting some GPIOs with Ground (GND) lines.

Pin name on the Pico board Description
GP2 Enable PS/2 keyboard & mouse support (see below).
GP3 Prefer the PS/2 keyboard over USB when turning on the HID (if PS/2 enabled).
GP4 Prefer the PS/2 mouse over USB (if PS/2 enabled)
GP6 Disable USB keyboard & mouse support. This is useful if you only want to use PS/2.
GP7 Enable the special USB absolute mouse for Windows 98.
GP8 Prefer the relative USB mouse over the absolute one.
GP9 Prefer the Windows 98 USB absolute mouse over the regular absolute one (if enabled).

!!! example To enable Windows 98 absolute mouse, just connect pin GP9 to any GND on the Pico.

Flashing the firmware

To upload the firmware to Pico HID, you can use any computer with a USB port.

  1. Download the latest release of the firmware. The file is called pico-hid.uf2.
  2. Press the white button on the Pico board and plug it using USB cable to the computer.
  3. Release the button.
  4. The Pico board appears as a flash drive on the host computer.
  5. Copy the pico-hid.uf2 file to this flash drive.
  6. Safely disconnect the USB device.

The final steps

Connect the Pico HID to the host computer using the USB cable.

If you are building PiKVM V1, then no further action with the Pico HID is required.

If you are making the Pico HID for V2 or V3, add the following lines to the PiKVM configuration and reboot it.

!!! note "KVMD >= 3.241 is required for the Pico HID"

  • /boot/config.txt

    dtoverlay=spi0-1cs

  • /etc/kvmd/override.yaml:

    kvmd:
    hid:
        type: spi
        chip: 0
        bus: 0
        sw_cs_pin: 7
        sw_cs_per_byte: true
        reset_pin: 25
        reset_inverted: true
        reset_self: true
        power_detect_pin: 16
        power_detect_pull_down: true

Replacing the Arduino HID

!!! warning This section is intended for advanced users of the legacy Arduino HID.

It may seem tempting, but **don't to use the Arduino HID for new PiKVM builds**
just because you have it at your fingertips. Connecting and flashing Arduino
is much more time consuming than Pico. In addition, different Arduino board works
with different voltages, may or may not have SPI (for the Pico, we use SPI to free up
the UART on Raspberry Pi for the console and other useful things), etc.

Using the Pico HID is the recommended fast and standard way in the PiKVM world.

The Pico HID can be used to replace the legacy Arduino HID. Moreover, it can use both Serial (UART) port and SPI. The connection scheme is also noticeably simplified, getting rid of the transistor for the Reset line and level shifter for RX/TX (MOSI/MISO).

??? example "For the Arduino HID over SPI" Throw away the Reset transistor and level shifter, and follow this guide from the very beginning, as if you were connecting Pico HID to PiKVM V2 or V3.

??? example "For the classic Serial (UART) HID" Get rid of the transistor and level shifter, and follow this guide from the very beginning, but the schemes and configs will be slightly different.

* The `GP22` on the Pico is connected directly to the `GND`. This enables UART mode instead of default SPI.

* In the good old PiKVM V0, `GPIO4` [on the Raspberry Pi](https://pinout.xyz) was used for the Reset line.
  Now we recommend to use `GPIO25` for consistency reasons.
  However, you can use `GPIO4` by changing the `reset_pin` value in the config example below.
  On the scheme, this is a yellow wire, the `RUN (Pico) -> GPIO25 (Pi)` line.

??? example "Simple wiring diagram"
    <img src="serial_breadboard.png" />

??? example "Electrical schematic diagram"
    <img src="serial_scheme.png" />

??? example "Configs"
    * Don't add line `dtoverlay=spi0-1cs` to the `/boot/config.txt` file. It's only needed for SPI.

    * `/etc/kvmd/override.yaml`:
        ```yaml
        kvmd:
            hid:
                type: serial
                device: /dev/kvmd-hid
                reset_pin: 25
                reset_inverted: true
                reset_self: true
                power_detect_pin: 16
                power_detect_pull_down: true
        ```