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Bluetooth

ZMK's bluetooth functionality allows users to connect their keyboards to hosts using Bluetooth Low Energy (BLE) technology. It also is used for split keyboards to connect the two halves wirelessly.

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Bluetooth 4.2 or newer is required in order to connect to a ZMK keyboard. ZMK implements advanced security using BLE's Secure Connection feature, which requires Bluetooth 4.2 at a minimum. To avoid well-known security vulnerabilities, we disallow using Legacy pairing.

Security​

BLE connections between keyboards and hosts are secured by an initial pairing/bonding process that establishes long term keys (LTK) shared between the two sides, using Elliptic Curve Diffie Hellman (ECDH) for key generation. The same security is used to secure the communication between the two sides of split keyboards running ZMK.

The only known vulnerability in the protocol is a risk of an active man-in-the-middle (MITM) attack exactly during the initial pairing, which can be mitigated in the future using the Numeric Comparison association model. Support for that in ZMK is still experimental, so if you have serious concerns about an active attacker with physical proximity to your device, consider only pairing/bonding your keyboards in a controlled environment.

Profiles​

By default, ZMK supports five "profiles" for selecting which bonded host device should receive the keyboard input.

Connection Management

When pairing to a host device ZMK saves bond information to the selected profile. It will not replace this automatically when you initiate pairing with another device. To pair with a new device select an unused profile with or clearing the current profile, using the &bt behavior on your keyboard.

A ZMK device may show as "connected" on multiple hosts at the same time. This is working as intended, and only the host associated with the active profile will receive keystrokes.

Failure to manage the profiles can result in unexpected/broken behavior with hosts due to bond key mismatches, so it is an important aspect of ZMK to understand.

Bluetooth Behavior​

Management of the bluetooth in ZMK is accomplished using the &bt behavior. Be sure to refer to that documentation to learn how to manage profiles, switch between connected hosts, etc.

Refreshing the HID Descriptor​

Enabling certain features or behaviors in ZMK changes the data structure that ZMK sends over USB or BLE to host devices. This in turn requires HID report descriptors to be modified for the reports to be parsed correctly. Firmware changes that would modify the descriptor include the following:

  • Changing any of the settings under the HID category, including enabling/disabling NKRO or HID indicators
  • Enabling mouse features, such as adding mouse keys to your keymap

While the descriptor refresh happens on boot for USB, hosts will frequently cache this descriptor for BLE devices. In order to refresh this cache, you need to remove the keyboard from the host device, clear the profile associated with the host on the keyboard, then pair again. For Windows systems you might need to follow the instructions in this troubleshooting section below.

Troubleshooting​

Connectivity Issues​

Some users may experience a poor connection between the keyboard and the host. This might be due to poor quality BLE hardware, a metal enclosure on the keyboard or host, or the distance between them. Increasing the transmit power of the keyboard's BLE radio may reduce the severity of this problem. To do this, set the CONFIG_BT_CTLR_TX_PWR_PLUS_8 configuration value in the .conf file of your user config directory as such:

CONFIG_BT_CTLR_TX_PWR_PLUS_8=y

For the nRF52840, the value PLUS_8 can be set to any multiple of four between MINUS_20 and PLUS_8. The default value for this config is 0, but if you are having connection issues it is recommended to set it to PLUS_8 because the power consumption difference is negligible. For more information on changing the transmit power of your BLE device, please refer to the Zephyr docs.

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This setting can also improve the connection strength between the keyboard halves for split keyboards.

Using bluetooth output with USB power​

If you want to test bluetooth output on your keyboard and are powering it through the USB connection rather than a battery, you will be able to pair with a host device but may not see keystrokes sent. In this case you need to use the output selection behavior to prefer sending keystrokes over bluetooth rather than USB. This might be necessary even if you are not powering from a device capable of receiving USB inputs, such as a USB charger.

Issues with dual boot setups​

Since ZMK associates pairing/bond keys with hardware addresses of hosts, you cannot pair to two different operating systems in a dual boot system at the same time. While you can find documented workarounds that involve copying pairing keys across operating systems and use both OS with a single profile, they can be fairly involved and should be followed with caution.

macOS Connected But Not Working​

If you attempt to pair a ZMK keyboard from macOS in a way that causes a bonding issue, macOS may report the keyboard as connected, but fail to actually work. If this occurs:

  1. Remove the keyboard from macOS using the Bluetooth control panel.
  2. Invoke &bt BT_CLR on the keyboard while the profile associated with the macOS device is active, by pressing the correct keys for your particular keymap.
  3. Try connecting again from macOS.

Windows Connected But Not Working​

Occasionally pairing the keyboard to a Windows device might result in a state where the keyboard is connected but does not send any key strokes. If this occurs:

  1. Remove the keyboard from Windows using the Bluetooth settings.
  2. Invoke &bt BT_CLR on the keyboard while the profile associated with the Windows device is active, by pressing the correct keys for your particular keymap.
  3. Turn off Bluetooth from Windows settings, then turn it back on.
  4. Pair the keyboard to the Windows device.

If this doesn't help, try following the procedure above but replace step 3 with one of the following:

  • Restart the Windows device
  • Open "Device Manager," turn on "Show hidden devices" from the "View" menu, then find and delete the keyboard under the "Bluetooth" item