Read long characteristic as ble central

RE: Read long characteristic as ble central

run_ar — Wed, 15 Feb 2017 15:57:52 GMT

Please see the Message sequence chart variant 2 offset !=0, here.

RE: Read long characteristic as ble central

Jackxl — Wed, 15 Feb 2017 10:42:13 GMT

Where I have to multiple read ? I read in for-loop when I service discover but I get only first packet.

RE: Read long characteristic as ble central

run_ar — Wed, 25 Nov 2015 19:42:18 GMT

Yes, unfortunately you will have to do multiple reads using offset to get all the data. Only 20 bytes of actual data is transfered on air in each packet, the rest is protocol overhead. So you have to do three reads in total to get 45 bytes.

RE: Read long characteristic as ble central

RomainQ — Mon, 23 Nov 2015 19:30:32 GMT

2- no, I need the characteristics full bytes: my peripheral advertise a characteristic with let's say 45 bytes. On the central side, on HVX I only get 20 bytes.

RE: Read long characteristic as ble central

run_ar — Mon, 23 Nov 2015 12:05:51 GMT

Yes, I agree this should have been stated in the API documentation as well. I'll create a suggestion for that.
The 7 bytes is protocol overhead. Is there a particular reason why you want that forwarded to your application?

RE: Read long characteristic as ble central

RomainQ — Mon, 23 Nov 2015 10:22:32 GMT

1 - what is disturbing: the com buffer is limited to 20 bytes max, this is not clearly visible in the doc. 2 - the data will be forwarded: my peripheral sent 27 bytes, softdevice only send me 20 bytes without warnings it truncated it. I need to know the peripheral did try to send me more data !

I don't want to argue too much about it as the manual handle with read works. But a new user can easily fall in the problems I found.

RE: Read long characteristic as ble central

run_ar — Mon, 23 Nov 2015 10:18:17 GMT

This would be possible, but I'm not sure I see the big difference between this solution and the actual implementation except that in this case the application would be force to handle the callback, instead of just stop reading if desired.

"What is disturbing is that the notification data limit is not easy to see when you discover the SDK"
Not sure i understand what you mean by this.

"And you don't see how many bytes were sent by the peripheral vs how many bytes you actually receive in the central."
The data received by the Softdevice will be forwarded to the applicaiton.

RE: Read long characteristic as ble central

RomainQ — Mon, 23 Nov 2015 09:00:35 GMT

Thanks for replying. But what about keeping a small buffer and giving several calls to a callback with offset. What is disturbing is that the notification data limit is not easy to see when you discover the SDK. And you don't see how many bytes were sent by the peripheral vs how many bytes you actually receive in the central.

RE: Read long characteristic as ble central

run_ar — Mon, 23 Nov 2015 08:50:30 GMT

We had an internal discussion how to do this, and it was decided that this should resemble the size of the payload sent on air. This way we avoid having to allocate a lot of memory internally in the Softdevice to handle long reads, instead this has to be handled by the application. This way it's easier to manage memory usage in a wide variety of applications. The size can vary quite a lot, and the application doesn't necessarily know how many packets will be sent on air. Alternatively, if we did this procedure in the Softdevice, we would need large buffers both in the Softdevice and in the application.

RE: Read long characteristic as ble central

shibshab — Sun, 22 Nov 2015 18:19:47 GMT

Ah, I see, did not see that part of your question. I have no knowledge of the motivation behind this design choise :)

RE: Read long characteristic as ble central

RomainQ — Sun, 22 Nov 2015 18:13:20 GMT

Hi, Thanks, I did that and my project is working. I'm just wondering why the SoftDevice itself is not doing it ...