Need help regarding custom HID descriptor for nRF51822

[mustafa1802]

Hi,

I am trying to modify the ble_hids_mouse example project to convert the device into a vendor defined HID device. I need to send 64 bytes of custom sensor data (which I am reading over SPI on nRF51) over custom HID so that my application on the PC can access the data using HID APIs. I have tried multiple HID descriptors but fail to get data on the PC side. I am using the HIDAPI library on a Windows 10 platform to read data from the HID device. The hid_open() function returns a valid handle for the HID device (VID,PID) but hid_read() always returns 0 bytes. Error code from ble_hids_inp_rep_send() function on nRF returns 0 (NRF_SUCCESS), so I am assuming there are no errors in sending HID input reports. 

Following are 2 HID descriptors which do not work:

(I)
0x05, 0x01, // Usage Page (Generic Desktop Page)
0x09, 0x00, // Usage (Undefined)
0xA1, 0x01, // Collection (Application)
0x09, 0x00, // Usage (Undefined)
0x16, 0x00, 0x00, // Logical Minimum (0)
0x26, 0xFF, 0x00, // Logical Maximum (255)
0x36, 0x00, 0x00, // Physical Minimum (0)
0x46, 0xFF, 0x00, // Physical Maximum (255)
0x66, 0x00, 0x00, // Unit (None)
0x75, 0x08, // Report Size (8)
0x95, 0x40, // Report Count (64)
0x81, 0x02, // Input
0xC0, // End Collection

(II)

0x06, 0x00, 0xff, // USAGE_PAGE (Vendor Defined Page 1)
0x09, 0x01, // USAGE (Vendor Usage 1)
0xa1, 0x01, // COLLECTION (Application)
0x15, 0x00, // LOGICAL_MINIMUM (0)
0x26, 0xff, 0x00, // LOGICAL_MAXIMUM (255)
0x75, 0x08, // REPORT_SIZE (8)
0x95, 0x40, // REPORT_COUNT (64)
0x09, 0x01, // USAGE (Vendor Usage 1)
0x81, 0x00, // INPUT (Data,Ary,Abs)
0x09, 0x01, // USAGE (Vendor Usage 1)
0x91, 0x00, // OUTPUT (Data,Ary,Abs)
0xc0 // END_COLLECTION

I have adjusted the input report length to 64 bytes at multiple places in the code and modified the report id as per the descriptors. 

However, reducing the length of the input report from 64 bytes to 8 bytes seems to work and I am able to receive 8 bytes of data in my application.

Can you please help to debug this issue.

Regards,

Mustafa 

[Håkon Alseth]

Hi,

 

I have adjusted the input report length to 64 bytes at multiple places in the code and modified the report id as per the descriptors. 

However, reducing the length of the input report from 64 bytes to 8 bytes seems to work and I am able to receive 8 bytes of data in my application.

 

The S130 softdevice has a limit of sending up to 20 bytes of userdata, as described in the specification here (under ATT features):

https://infocenter.nordicsemi.com/topic/com.nordic.infocenter.s130.sds/dita/softdevices/s130/ble_protocol_stack/features_ble_protocol_stack.html?cp=4_7_2_0_9_1

Try limiting your sending size to 20 bytes, and see if that works better.

 

Best regards,

Håkon

[mustafa1802]

Hi, 

Thanks for the prompt reply.

Regarding the max allowable user data per packet, why do we have a limit of 20 bytes ? Is there a way we can increase the limit to a higher value ? If yes, what is the maximum allowable data that can be transferred per data packet ? 

For my application, I need to send at least 80 bytes of data at a time using HID service.

Regards

Mustafa 

[Håkon Alseth]

Its a hard limit of 20 bytes user data in the SoftDevice S130. To get a larger MTU size, you have to use nRF52-series with S132/140 SoftDevice.

 

Kind regards,

Håkon

[mustafa1802]

Is there any limit on the speed at which HID reports can be sent on nrf51 + S130 ? I need to send an HID IN report every 5 ms. 

[Håkon Alseth]

Hi,

 

You need to conform to the BLE connection, which gives you a connection interval (from 7.5 ms to 4 sec), typically asked for by the peripheral device as a range (a min and a max).

If you send data every 5 ms, you must rely on the central device to accept more packets per connection interval. Lets say that you have a connection interval of 10 ms, then you will send 2 packets per interval.

 

Kind regards,

Håkon