Maximum number of characteristics

RE: Maximum number of characteristics

Mahendra Tailor — Mon, 14 Jul 2014 08:06:53 GMT

Hi If I always use VLOC_USER ...

For 128 bit uuids ... (and Characteristics with 0xA property plus 20 byte length)

With 1 svc, I can install 30 characteristics. With 4 svcs it drops to 28 characteistics. with 8 svcs it drops to 25 characteristics.

If I use 16 bit uuid ... (and Characteristics with 0xA property plus 10 byte length), then

With 1 svc, I can install 45 characteristics. With 4 svcs it drops to 44 characteistics. with 8 svcs it drops to 42 characteristics.

This seems to suggest the gatt table is populated with the actual uuid numbers (2 or 16 bytes) and not the uuid handles.

I have not yet moved over to the latest 7.0.0 stack. Anyone care to comment if it is likely to be different?

RE: Maximum number of characteristics

Mahendra Tailor — Sat, 12 Jul 2014 21:31:30 GMT

Hi Thank you for the feedback. I managed to find the issue in my case. I was using VLOC_USER in all the places - even for the CCCD. Once I modified it to just have VLOC_USER for the char values I got further. And yes, I provide p_value as a pointer to a buffer that I manage. What I do is ... unsigned char myCharBufs[600]; int myCharBufFree=0;

And then each time I register a value I copy the data to myCharBufs[myCharBufFree] and then do myCharVufFree += Length_of_char_value;

I have found that the maximum chars I can register is 30 if the uuid for the char is 128 bit. If the uuid is 16 bit , then I can register 44

Hence the absolute maximum one is going to achieve in v6.0.0 is 44.

Which seems to suggest that Nordic are populating the gatt table with the actual uuid values and NOT the 4 byte uuid handles.

Many thanks for the feedback.

Kind regards

RE: Maximum number of characteristics

Ulrich Myhre — Sat, 12 Jul 2014 19:32:13 GMT

See ble_gatts_attr_t here devzone.nordicsemi.com/.../a00270.html You should have to set the p_value to a buffer when using VLOC_USER.

RE: Maximum number of characteristics

Mahendra Tailor — Sat, 12 Jul 2014 11:06:52 GMT

I am using v6.0.0. Does anyone know if the behaviour on newer stack is different?

RE: Maximum number of characteristics

Mahendra Tailor — Sat, 12 Jul 2014 11:05:43 GMT

Another observation I have test code that uses VLOC_STACK and I register chracteristics with various different properties. They all successfully register.

I then change the firmware so that where I use VLOC_STACK I specify VLOC_USER. In this case only the properties with 0xA value succeeds. All the others come back with INVALID_PARAM error code.

Question, why should just changing the location of the memory make that happen?

RE: Maximum number of characteristics

Mahendra Tailor — Sat, 12 Jul 2014 08:28:19 GMT

Hi Thank you for getting back. You say "The rest of the characteristics can then be added with VLOC_USER." Not true because even for VLOC_USER (for everything) it needs to use the 'stack' memory for the uuid and the handles. That is why when I use VLOC_USER for everything I still run out of memory.

A customer has asked for the 33 characteristics. They are king :-) we have to make it happen if in our powers. We pride ourselves in customer service.

Another curious behaviour, which is very annoying as I cannot fathom the reason yet ...

I have a test application where I register a character with property = 0x2A (read/write/indicatable) with VLOC_STACK and it works.

I changed the application so that it has VLOC_USER ( I stress no other change) and now the function sd_ble_gatts_characteristic_add() is returning with an nrf error of 7 (INVALID PARAMETER)

Why does just changing the location memory make the parameters fail?

I can't find any documentation to tell me WHICH parameter is inva

RE: Maximum number of characteristics

Ulrich Myhre — Fri, 11 Jul 2014 22:49:28 GMT

You should be able to use a combination of both.

The stack memory allotted is already reserved for GATT, so you can use that until you run out. The rest of the characteristics can then be added with VLOC_USER.

It seems strange to have that many services and characteristics added though. Are you positive that you need to implement 33 characteristics for your needs?