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Device communication protocols

Simhub can communicate in various ways to your devices, some protocols are pre implemented, No matter the protocol they all need you to be able to assign to your MCU a proper PID/VID you own.

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SimHub Standard Serial protocol

The most universal protocol, it can be implemented on any device with just a CDC either by using the Standard Serial Firmware Builder (for atmega32u4) or copying the existing protocol

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Screen pass-through

Both USBD480NX and Vocore offer to drive leds through the screen. In such case you can use this protocol to instruct simhub to use the screen for the LEDs control.

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SimHub Standard HID protocol

When things will get more serious you might want to switch a proper HID implementation,

Simhub can identify the HID interface based on the HID Usage and usage page. The report ID and report length can be specified.

Make sure to properly include the report in your device HID descriptor, descriptors not specifying the report id won't work.

To be able to support multiple device instance, make sure your device exposes an unique HID serial number.

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HID Report Format

This example shows a 64-byte HID report used to update RGB LEDs efficiently. Shorter or longer reports (e.g. 32 or 128 bytes) can be used depending on the microcontroller capabilities, just adapt the maximum number of LEDs per packet.

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Report Layout

Byte
Description
Example Value

1

Report ID

0x68

2

Start LED index

0x00

3

Affected LED count

0x14 (20)

4

Draw flag

  • 0 or 1 if last leds data packet

  • 2 for the optional "on connect" report

  • 3 for the optional "on disconnect" report

0x00

5–7

LED #1 → R, G, B

0xFF 0xFF 0xFF

8–10

LED #2 → R, G, B

0xFF 0xFF 0xFF

…

Next LEDs...

…

62–64

Last LED → R, G, B

0xFF 0xFF 0xFF

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Notes

  • This example assumes 64-byte reports, but it works with any size depending on the HID endpoint.

  • The “Draw” byte (Byte 4) indicates when the update is complete : this allows physical LEDs (e.g. WS2812B) to be refreshed only once per full frame.

  • The Report ID is an example; typically, the report is part of a vendor-specific HID interface.

  • The optional on connect report and on disconnect report* will send a report using 2 or 3 " as a draw flag all other data (RGB, start led, affected led count ...) will be set to zero.

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Warning the on disconnect report can't be guaranteed in case of process being killed or usb issue causing a disconnect for instance. Simhub sends a keep alive "draw" every 5 seconds even if nothing has changed. Using an internal firmware timeout of 6s would be relevant in cases of automations to be triggered after a software disconnect.

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Examples

Example 1 — Updating 2 LEDs

Update the first 2 LEDs (LED #0 → #1):

Example 2 — Updating 22 LEDs (split across packets)

Update the first 20 LEDs (LED #0 → #19):

Update the last 2 LEDs (LED #20 → #21):

Example 3 — Updating 42 LEDs (split across packets)

Update the first 20 LEDs (LED #0 → #19):

Update the next 20 LEDs (LED #20 → #39):

Update the last 2 LEDs (LED #40 → #41):

Example 3 — Optional on connect report

Sends a draw byte set to 2 on connection.

Example 4 — Optional on disconnect report

Sends a draw byte set to 3 before disconnecting.

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