RE: I have problem when i use nRF52840 to make a touchpad

Hieu — Fri, 13 Jun 2025 18:50:31 GMT

[quote user="Harvey.Zhang"]Do you mean to use an endpoint using touchpad ID and another endpoint for rest?[/quote]

We are working with HID Service over GATT, a BLE feature, and not USB HID, right? Unlike USB with endpoints, in BLE GATT, we are working with HID Service. What I mean is creating two HID Service instances, in a way that if you would perform service discovery on the device, you will see two HID Services.

[quote user="Harvey.Zhang"]

So we need 426 bytes for whole touchpad report descriptor.

It seems need to simplify the report descriptor.

[/quote]

If you can optimize the descriptor size to under 512, that would be the best solution.

RE: I have problem when i use nRF52840 to make a touchpad

Harvey.Zhang — Fri, 13 Jun 2025 08:32:26 GMT

Oh! thank you very much.

Do you mean to use an endpoint using touchpad ID and another endpoint for rest?

It looks can reduce the size of a single endpoint.

This is the descriptor of one finger.

	/** 1st finger  73 */  
	0x05, 0x0d, 						/** USAGE_PAGE (Digitizers) */  
    0x09, 0x22,                         /**   USAGE (Finger) */  
    0xa1, 0x02,                         /**   COLLECTION (Logical)  */  
    0x15, 0x00,                         /**       LOGICAL_MINIMUM (0) */  
    0x25, 0x01,                         /**       LOGICAL_MAXIMUM (1) */  
    0x09, 0x47,                         /**       USAGE (Confidence) */  
    0x09, 0x42,                         /**       USAGE (Tip switch) */  
    0x95, 0x02,                         /**       REPORT_COUNT (2) */  
    0x75, 0x01,                         /**       REPORT_SIZE (1) */  
    0x81, 0x02,                         /**       INPUT (Data,Var,Abs) */ 
    0x95, 0x01,                         /**       REPORT_COUNT (1) */ 
    0x75, 0x02,                         /**       REPORT_SIZE (3) */ 
    0x25, 0x02,                         /**       LOGICAL_MAXIMUM (2) */ 
    0x09, 0x51,                         /**       USAGE (Contact Identifier) */ 
    0x81, 0x02,                         /**       INPUT (Data,Var,Abs) */ 
    0x75, 0x01,                         /**       REPORT_SIZE (1) */ 
    0x95, 0x04,                         /**       REPORT_COUNT (3) */ 
    0x81, 0x03,                         /**       INPUT (Cnst,Var,Abs) */ 
    0x05, 0x01,                         /**       USAGE_PAGE (Generic Desk.. */ 
    0x15, 0x00,                         /**       LOGICAL_MINIMUM (0) */ 
    0x26, 0x13, 0x0f,                   /**       LOGICAL_MAXIMUM (4095)  //3859 */ 
    0x75, 0x10,                         /**       REPORT_SIZE (16) */ 
    0x55, 0x0e,                         /**       UNIT_EXPONENT (-2) */ 
    0x65, 0x13,                         /**       UNIT(Inch,EngLinear) */ 
    0x09, 0x30,                         /**       USAGE (X) */ 
    0x35, 0x00,                         /**       PHYSICAL_MINIMUM (0) */ 
    0x46, 0xad, 0x01,                   /**       PHYSICAL_MAXIMUM (429,4.29inch)   800=0x0320 */ 
    0x95, 0x01,                         /**       REPORT_COUNT (1) */ 
    0x81, 0x02,                         /**       INPUT (Data,Var,Abs) */ 
    0x46, 0xD8, 0x00,                   /**       PHYSICAL_MAXIMUM (216,2.16inch)  0x0226 */ 
    0x15, 0x00,                         /**       LOGICAL_MINIMUM (0) */ 
    0x26, 0xe3, 0x07,                   /**       LOGICAL_MAXIMUM (2019) */ 
    0x09, 0x31,                         /**       USAGE (Y) */ 
    0x81, 0x02,                         /**       INPUT (Data,Var,Abs) */ 
    0xc0,                               /**    END_COLLECTION */

We have five finger and touchpad report id, this is total 373 bytes.

And that need to be added for the scanning time, keys and the number of contacts, as follow,

/**  scan time& buttons. end input. +57 */ 
    0x05, 0x0d, 						/** USAGE_PAGE (Digitizers) */ 
    0x55, 0x0C,                         /**    UNIT_EXPONENT (-4) */ 
    0x66, 0x01, 0x10,                   /**    UNIT (Seconds) */ 
    0x47, 0xff, 0xff, 0x00, 0x00,      /**     PHYSICAL_MAXIMUM (65535) */ 
    0x27, 0xff, 0xff, 0x00, 0x00,         /**  LOGICAL_MAXIMUM (65535) */ 
    0x75, 0x10,                           /**  REPORT_SIZE (16) */ 
    0x95, 0x01,                           /**  REPORT_COUNT (1) */ 
    0x05, 0x0d,                         /**    USAGE_PAGE (Digitizers) */ 
    0x09, 0x56,                         /**    USAGE (Scan Time) */ 
    0x81, 0x02,                           /**  INPUT (Data,Var,Abs) */ 
    0x09, 0x54,                         /**    USAGE (Contact count) */ 
    0x25, 0x7f,                           /**  LOGICAL_MAXIMUM (127) */ 
    0x95, 0x01,                         /**    REPORT_COUNT (1) */ 
    0x75, 0x08,                         /**    REPORT_SIZE (8) */ 
    0x81, 0x02,                         /**    INPUT (Data,Var,Abs) */ 
    0x05, 0x09,                         /**    USAGE_PAGE (Button) */ 
    0x09, 0x01,                         /**    USAGE_(Button 1) */ 
    0x09, 0x02,                         /**    USAGE_(Button 2) */ 
    0x09, 0x03,                         /**    USAGE_(Button 3) */ 
    0x25, 0x01,                         /**    LOGICAL_MAXIMUM (1) */ 
    0x75, 0x01,                         /**    REPORT_SIZE (1) */ 
    0x95, 0x03,                         /**    REPORT_COUNT (3) */ 
    0x81, 0x02,                         /**    INPUT (Data,Var,Abs) */ 
    0x95, 0x05,                          /**   REPORT_COUNT (5) */ 
    0x81, 0x03,                         /**    INPUT (Cnst,Var,Abs) */

This is total 53 bytes.

So we need 426 bytes for whole touchpad report descriptor.

It seems need to simplify the report descriptor.

RE: I have problem when i use nRF52840 to make a touchpad

Hieu — Wed, 11 Jun 2025 19:28:44 GMT

Hello Harvey.Zhang,

615 bytes is longer than 512, which is the maximum length of a HID Service. In this case, you need to split your record into two half, and support two HIDS instances. Refer to section 2.5 of the HID over GATT Profile Specification.

However, I experimented with it a little bit 2 years ago and found that Android devices don't handle two HIDS instances very well. It might be safer to see if you can minimize the descriptor further.

I am not familiar with a touchpad report descriptor, but I know for sure there have been mouse and keyboard report descriptor done successfully. Does a touchpad require far more reports than a mouse?

Hieu