nRF52840 - DLE update issue.

[AKV]

Hi,

We are facing some issues with the DLE update.

The given table shows that the softdevice set data length and time.

Event Length (ms)	ATT MTU (bytes)	Data Length (bytes)	Tx and Rx Time (us)
2.5	60	64	737
2.5	70	74	737
2.5	80	78	737
2.5	90	78	737
3.75	100	104	1362
3.75	120	124	1362
3.75	140	144	1362
3.75	160	156	1362
3.75	170	156	1362

Parameters being set are,

Connection interval = 15ms.

Scan window = 6.25ms.

Advertising interval = 152.5ms

Peripheral Role Count = 1

Central Role Count = 2

1. Why is the softdevice is set constant time for event length (737us for 2.5 event length and 1362us for 3.75ms)?

If the MTU is 170 bytes then the Tx time = 170+18 (LL header) = 188 *8 =1504us.

and then we can add IFS as well so the total time is 1504+150= 1654us. 

Total ttime for both Tx and Rx = 1654 * 2 =3308us.

We have getting the NRF_ERROR_RESOURCES when trying to set the DLE (174/1654) manually by calling the function sd_ble_gap_data_length_update(connHandle, &ble_gap_data_length_params, nullptr).

2. The DLE requested time is less than the event length (3.75ms) then why it's giving the error?

[Einar Thorsrud]

Hi,

The SoftDevice will need to fit all connections and scanning, see Connection timing with Connection Event Length Extension.

By the way, do you use the SoftDevice handler module in the SDK? If so it calls sd_ble_cfg_set() with the event length from NRF_SDH_BLE_GAP_EVENT_LENGTH in sdk_config.h.

[Srinivasan C]

Hi Einar, thanks for the links .

Still have a question on the relation between the Event Length  and the Tx / Rx Timing.  If we have an Event length of 2.5ms

the Tx and Rx lengths can be 1.1ms each  ( removign the 150uS CIFS ).  This should have corresponded to about 137 bytes.

But we are seeing about 78 bytes only.    This seems to suggest a time about 750usec being used up for other activities ( this seems to fit when we tried with 3.75ms Event length ).  Where does this extra 750us come from ? Is this the radio prepare time ?

[Einar Thorsrud]

Hi,

This is expected. Before the connection event starts, the SoftDevice reserves 183 us for configuring the radio. After the the last packet has been received, it  reserve 350 us for post processing. In addition comes 200 us for scheduling. This adds up to the numbers you have seen. Also note that the SoftDevice will not start a new packet pair if there is not enough available time.

[Srinivasan C]

Thanks for the clarification. Will the situation be any different if we switch to nRF Connect ?

[Einar Thorsrud]

The numbers are currently a bit smaller with nRF Connect SDK. These numbers are caused by implementation details though, and can change at any time with new releases so you should not rely on this.