For the complete documentation index, see llms.txt. This page is also available as Markdown.

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.

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

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.

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.

HID Report Format

The SimHub Standard HID protocol can expose separate reports for LEDs, motors and fans. Each report uses its own report ID and can use the same HID report length configured for the device.

Shorter or longer reports (for example 32, 64 or 128 bytes) can be used depending on the microcontroller capabilities. If the configured report length is too short to carry all channels, SimHub sends multiple packets using the start index, affected channel count and final packet flag.

LED report

The LED report is used to update RGB LEDs efficiently. The default report ID used by the standard firmware builder is 0x68.

Byte
Description
Example value

1

LED Report ID

0x68

2

Start LED index

0x00

3

Affected LED count

0x14 (20)

4

Draw flag: 0 while more LED packets are expected, 1 for the last LED 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 in a 64-byte report: R, G, B

0xFF 0xFF 0xFF

For a 64-byte report, the 4-byte header leaves 60 bytes of payload. Since each LED uses 3 bytes, one packet can carry up to 20 LEDs.

The draw flag allows physical LEDs, such as WS2812B LEDs, to be refreshed only once per full frame. The optional on connect and on disconnect reports use the LED report ID and set the draw flag to 2 or 3; all other data is set to zero.

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.

Motor report

The motor report carries one gain value and one frequency value per motor. The default report ID used by the standard firmware builder is 0x6A.

Byte
Description
Example value

1

Motor Report ID

0x6A

2

Start motor index

0x00

3

Affected motor count

0x02

4

Final packet flag: 1 when this is the last motor packet, otherwise 0

0x01

5-6

Motor #1 gain, unsigned 16-bit big-endian, from 0x0000 to 0xFFFF

0xFF 0xFF

7-8

Motor #1 frequency, unsigned 16-bit big-endian

0x00 0x3C

9-10

Motor #2 gain, unsigned 16-bit big-endian, from 0x0000 to 0xFFFF

0x80 0x00

11-12

Motor #2 frequency, unsigned 16-bit big-endian

0x00 0x78

...

Next motors

...

For a 64-byte report, the 4-byte header leaves 60 bytes of payload. Since each motor uses 4 bytes, one packet can carry up to 15 motors at the communication level. The current SimHub device descriptor implementation limits motor devices to 8 motors.

If the device descriptor does not expose frequency support for motors, firmware can ignore the frequency field.

Fan report

The fan report carries one unsigned 16-bit speed value per fan. The default report ID used by the standard firmware builder is 0x69.

Byte
Description
Example value

1

Fan Report ID

0x69

2

Start fan index

0x00

3

Affected fan count

0x02

4

Final packet flag: 1 when this is the last fan packet, otherwise 0

0x01

5-6

Fan #1 speed, unsigned 16-bit big-endian, from 0x0000 to 0xFFFF

0xFF 0xFF

7-8

Fan #2 speed, unsigned 16-bit big-endian, from 0x0000 to 0xFFFF

0x80 0x00

...

Next fans

...

For a 64-byte report, the 4-byte header leaves 60 bytes of payload. Since each fan uses 2 bytes, one packet can carry up to 30 fans at the communication level. The current SimHub device descriptor implementation limits fan devices to 3 fans.

Examples

Example 1 - Updating 2 LEDs

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

Example 2 - Updating 22 LEDs (split across packets)

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

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

Example 3 - Updating 42 LEDs (split across packets)

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

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

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

Example 4 - Optional on connect report

Sends a draw byte set to 2 on connection.

Example 5 - Optional on disconnect report

Sends a draw byte set to 3 before disconnecting.

Example 6 - Updating 2 motors

Update motor #0 at full gain / 60 Hz and motor #1 at half gain / 120 Hz:

Example 7 - Updating 8 motors

Update the currently supported maximum of 8 motors in one 64-byte report:

Example 8 - Updating 2 fans

Update fan #0 to full speed and fan #1 to half speed:

Example 9 - Updating 3 fans

Update the currently supported maximum of 3 fans in one 64-byte report:

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