CC2540: As central is it possible to send multiple write and read requests to different characteristics within the same connection interval?

Nicolas Veloz Savino

Part Number: CC2540

Using the CC2540 as central connecting to 2 peripherals I want to read 5 and write 7 characteristics (20 bytes each). to the peripherals on each connection events

How can I send all these requests and write requests within 1 connection interval?

As far as I understand, GATT_ReadCharValue will be completed after the GATT_MSG_EVENT is generated with ATT_READ_RSP or ATT_ERROR_RSP.

The same goes with GATT_WriteCharValue.

Do I have to wait for the procedure to be completed before sending a new request? Does this means that it will take 12 connections event to send all these requests?

over 4 years ago

0 Jan over 4 years ago

TI__Mastermind 42320 points

Hello,

I have assigned an expert to help you with your query. In the meantime, could you provide which version of the SDK you are using?

Regards,

Jan

0 Nicolas Veloz Savino over 4 years ago in reply to Jan

Prodigy 30 points

I'm using SDK 1.5.1.1

0 Luis M over 4 years ago in reply to Nicolas Veloz Savino

TI__Expert 5345 points

Nicolas,

Thanks for reaching out. I'm doing some research to answer your question. I'll give you an update later today.

-Luis

0 Evan W over 4 years ago

TI__Mastermind 39960 points

Nicolas,

I don't believe your goal of reading 5 and writing 7 characteristics within 1 connection interval is realistic and possible by BLE. First, the CC2540 only supports ATT payload sizes of up to 20 bytes in a connection so you don't' get the advantage of things like Data Length Extensions that some of our more recent BLE devices get like the CC2640R2 and CC2642.

That said, GATT communication often depends on having acknowledgements and requires multiple connection events to transmit/read data and in addition, you can enqueue lots of packets/requests to send, but there will be a point where you see a blePending return value from your GATT_WriteCharValue which is returned when a response is pending with the server or the GATT sub-procedure is already in progress or ATT request is pending.

You can also look into what the error code of the ATT_ERROR_RSP. Please check Section D.5 Events of the Software Users Guide. I suspect that you are receiving an ATT_ERR_INSUFFICIENT_RESOURCES (0x11) Error code.

0 Luis M over 4 years ago

TI__Expert 5345 points

Nicolas,

Adding t o what Evan already stated, please refer to the User's Guide, section D.3 - Client Commands. In there, you will see that the function GATT_ReadMultiCharValues() is available to read multiple characteristics. A similar function for writing (multiple characteristics) does not exist.

Please note that the read operation will likely be performed in multiple connection events.

-Luis

0 Nicolas Veloz Savino over 4 years ago in reply to Luis M

Prodigy 30 points

Luis,

Thanks for answering.

I'm aware of the ReadMultiCharValues and there's also a PrepareWrite and ExecuteWrite that could help me to accomplish what I want to do, however, there are some things that are not clear to me.

if the MTU is 23, meaning that the max payload is 20 per ATT packet, does that means that the connection event is fixed to sending 23 bytes? I though that inside a connection event you could send multiple ATT packets each of them <= MTU in size as long as you don't overpass the maximum connection length stablished.
If my understanding of question 1 is correct, that you can send multiple ATT packets in a single connection event (each of them <= MTU in size) then, why can't I send multiple read request and write requests as long as they belong to different characteristics? Is this only possible if I'm sending unacknowledgeable data (notifications from the server and write commands from the client)?
The ATT_PrepareWriteReq and ATT_ExecuteWriteReq functions are available in the API, however, it seems the GATT_PrepareWriteReq and GATT_ExecuteWriteReq are only available on testing.
If I try to use the ATT_PrepareWriteReq, ATT_ExecuteWriteReq, ATT_ReadReq, ATT_WriteReq directly, It seems that the SDK won't pass me the events resulting from calling these functions, In my understanding, the blePending response is something the SDK is adding, however you could enqueue several read requests to different characteristics using for example the ATT_ReadReq, and send them all at once in a single connection event. As far as I understand, the ATT_ReadReq and ATT_WriteReq functions won't give me a blePending response (only GATT version of these functions respond with blePending), instead they should be queued and sent in the next connection event (at least the ones that fits inside the connection length established by TGAP_CONN_EST_MAX_CE_LEN)

0 Luis M over 4 years ago in reply to Nicolas Veloz Savino

TI__Expert 5345 points

Nicolas,

I will need to investigate. I'll try to have an answer this week. In the meantime, I would say that the best and most straightforward way to find the answer is to test it directly.

-Luis

0 Jan over 4 years ago in reply to Nicolas Veloz Savino

TI__Mastermind 42320 points

Hello Nicolas,

I apologize for the delay. I am looking into this and will provide an update as soon as possible. As I mentioned in the other thread, I will provide an update by the end of tomorrow. With regards to this thread, do you need clarity on all four queries or were you able to clarify some of them already?

Regards,

Jan

0 Jan over 4 years ago in reply to Nicolas Veloz Savino

TI__Mastermind 42320 points

Hi Nicolas,

You should be able to send more than one packet per connection event. However, the amount that you can send is limited. I do not think this device is able to send that many read and write requests within a single connection event. While researching your query, I came across the following E2E threads.

https://e2e.ti.com/support/wireless-connectivity/bluetooth/f/bluetooth-forum/789363/cc2541-how-many-back-to-back-notification-packets-can-be-transmitted-in-single-connection-interval

https://e2e.ti.com/support/wireless-connectivity/bluetooth/f/bluetooth-forum/634358/rtos-cc2640-connection-events-length

https://e2e.ti.com/support/wireless-connectivity/bluetooth/f/bluetooth-forum/194407/cc2540-multiple-notifications-per-connection-event

https://e2e.ti.com/support/wireless-connectivity/bluetooth/f/bluetooth-forum/353327/cc2540-how-to-send-multiple-packets-in-one-connection-event

https://e2e.ti.com/support/wireless-connectivity/bluetooth/f/bluetooth-forum/346583/cc2540-1-packet-sniffer-capable-or-receiving-more-than-4-notifications-per-connection-event

https://e2e.ti.com/support/wireless-connectivity/bluetooth/f/bluetooth-forum/208288/cc2540---more-than-4-notifications-per-connection-event

https://e2e.ti.com/support/wireless-connectivity/bluetooth/f/bluetooth-forum/549857/cc2640-maximum-att_mtu-size

Please let me know if you find the above threads helpful, if they did not clarify your question, or if you have any additional queries.

Regards.

Jan