Why can not set MAX_MTU_SIZE to 247?

[rolandash]

On 52832, SDK V14.0.0, SD132-5.0

I tried to set NRF_SDH_BLE_GATT_MAX_MTU_SIZE to 247, as from dev-doc i saw that SD132 supports BLE V4.2 DLE.

However it returns:

ble_gatt: sd_ble_gap_data_length_update() (request) on connection 0x0 returned unexpected value 0x13.

When set MAX_MTU_SIZE to 23 this issue disappear.

[rolandash]

@Petter Myhre

Thank you for replying.

I managed to resolve it, at least it looks to. I probably misunderstood the error message, it does not have to be local resource limitation, but from limitations in peer device.

Not using a specific example mixed with my own codes. 52 is acting as peripheral of a iPad Air 2, which according to Apple it supports v4.2. I had thought it is due to 52 local setting limits so i reverted to 23, how ever when I turned on debug log immediately i saw iPad is requesting for a packet length of 185 - i think that's why i can not set PDU size to 247 - as iPad may refuse it, probably because its support of V4.2 is partial.

So in the end as a solution i added PDU size exchange response of 247 to peer's request - as i still have 247 set in SDK_config.h. I still got the same error message but this action seems completed the negotiation procedure and PDU size ends up be set to 185.

LOG:

ble_gatt: Requesting to update ATT MTU to 247 bytes on connection 0x0.

ble_gatt: Requesting to update data length to 251 on connection 0x0.

ble_gatt: sd_ble_gap_data_length_update() (request) on connection 0x0 returned unexpected value 0x13.

ble_gatt: ATT MTU updated to 185 bytes on connection 0x0 (response).

Hope i am understanding the process correctly.

[rolandash]

Yes from the log it looks sd_ble_gap_data_length_update() returned the 0x13 error. If i set the MAX_PDU_SIZE to 23, then the error message disappear, instead i can see following log:

<debug> ble_gatt: Peer is requesting for a length of 185 on connection 0x0.

but then i can only end up with a PDU of 23.

My setting in config:

#ifndef NRF_SDH_BLE_GAP_EVENT_LENGTH
#define NRF_SDH_BLE_GAP_EVENT_LENGTH 3
#endif


// <o> NRF_SDH_BLE_GATT_MAX_MTU_SIZE - Static maximum MTU size.
#ifndef NRF_SDH_BLE_GATT_MAX_MTU_SIZE
#define NRF_SDH_BLE_GATT_MAX_MTU_SIZE 247
#endif

// <o> NRF_SDH_BLE_GATTS_ATTR_TAB_SIZE - Attribute Table size in bytes. The size must be a multiple of 4. 
#ifndef NRF_SDH_BLE_GATTS_ATTR_TAB_SIZE
#define NRF_SDH_BLE_GATTS_ATTR_TAB_SIZE 1408
#endif

Which i did not find anything wrong. The answer i concluded is the only reason i can think about.

However, i not sure how to verify the hypthesis that max PDU size of BLE v4.2 implementation on iPAD air 2 is only 185.