What layer of BLE stack datasheet mentions in Crystal tolerance section?

[Dennis Melentyev]

Hi,

NRF52840 datasheet section "5.4.4.2 64 MHz crystal oscillator (HFXO)" mentions crystal tolerances being dependent on packet size.

It is not clear to me, what layer of BLE protocol it refers to: L2CAP? ATT?

We're using BLE ATT to exchange rather long packets (very often over 200 bytes), with ATT_MTU set to smaller values ( experimenting in the range of 32-180).

Could someone please confirm (better officially), that those 200 bytes mentioned in datasheet relate to ATT_MTU (L2CAP layer), not ATT one?

Sincerely,

Dennis Melentyev

[Kenneth]

Hi,

This refer to packet length, so >200bytes refer to the entire PDU, including LL header, L2CAP header, ATT data and optional MIC.

Kenneth

[Dennis Melentyev]

Hi Kenneth,

Still, can't get it: ATT data in your answer is a whole ATT layer packet, or fragmented down to ATT_MTU size physical layer packets?

Rephrasing the question: If I set ATT_MTU to 100 bytes and send 250 bytes of data over ATT protocol, will the packet size referenced to in datasheet be <200?

[Kenneth]

Dennis Melentyev said:

or fragmented down to ATT_MTU size physical layer packets?

That is correct.

Kenneth

[Dennis Melentyev]

Thank you!