Raspberry Pi Pico HID

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

Full list of features:

Feature	Enabled by default
USB Keyboard, absolute & relative mouse	Yes
USB Absolute Mouse for Windows 95
PS/2 Keyboard & mouse

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 a 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.

!!! note "KVMD >= 3.241 is required for 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 all that 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 gurus" If you know how to solder, you can buy the Pico without pins and just solder everything without needig the dupond wires.

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

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

Connect all the parts according to this scheme:

??? example "Simple wiring diagram"

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

PS/2 Keyboard & Mouse

??? example "Additional steps for PS/2 support"

If you need PS/2 keyboard and mouse support, you will need a few additional components.
Soldering skills will also come in handy.

* *x1* 3.3V/5V bi-directional logic level shifter [like this](https://learn.sparkfun.com/tutorials/bi-directional-logic-level-converter-hookup-guide/).
* Optional: *x2* PS/2 cable with male connector (can be salvaged from the an keyboard or mouse).

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 use a multimeter to make sure that both PS/2 female 
connectors have the same line.

<img src="ps2_pinout.png" />

PS/2 female socket pinout on the motherboard is the same for the keyboard and the mouse.
A purple socket usually corresponds to the keyboard, and a green one to the mouse.
If your motherboard only has one port, it's probably universal and can be used for both
the keyboard and the mouse. Most likely, it will be painted both colors.

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" width="300" />

!!! note "Don't forget to enable PS/2 mode support as described in the next paragraph"

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 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 is enabled).
`GP4`	Prefer the PS/2 mouse over USB (if PS/2 is 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.

Download the latest release of the firmware. The file is called pico-hid.uf2.
Press and hold the white button on the Pico board.
While still holding the button, plug it in the computer using a USB cable.
Release the button.
The Pico board appears as a flash drive on the host computer.
Copy the pico-hid.uf2 file to this flash drive.
Safely eject the USB device.

The final steps

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

If you are building PiKVM V1, 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:

/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 the 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 work
with different voltages, they 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. It can use both Serial (UART) port and the SPI. The connection scheme is also much simpler, getting rid of the transistor for the Reset line and the 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 the 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
        ```

9.0 KiB Raw Blame History