Why I can not set connection interval to 7,5ms?

[rolandash]

Hi,

I find the smallest connection interval I can set on 52840 (and 832 as peer) is 8 units (10ms), and I am not sure if that is reasonable. 

My configuration:

52832 as peripheral, 52840 as central. Both ends using same version of SDK(15.2).

PHY: 1Mbps

LF Clock: RC, 16, 2, 500ppm (for both ends)

Connection params initial setup:

#define MIN_CONN_INTERVAL               MSEC_TO_UNITS(8, UNIT_1_25_MS)            
#define MAX_CONN_INTERVAL               MSEC_TO_UNITS(12, UNIT_1_25_MS)          
#define SLAVE_LATENCY                   0                                           
#define CONN_SUP_TIMEOUT                MSEC_TO_UNITS(1000, UNIT_10_MS)             

BLE gap event length setup 52840

#ifndef NRF_SDH_BLE_GAP_DATA_LENGTH
#define NRF_SDH_BLE_GAP_DATA_LENGTH  112
#endif

// <o> NRF_SDH_BLE_PERIPHERAL_LINK_COUNT - Maximum number of peripheral links. 
#ifndef NRF_SDH_BLE_PERIPHERAL_LINK_COUNT
#define NRF_SDH_BLE_PERIPHERAL_LINK_COUNT 2
#endif

// <o> NRF_SDH_BLE_CENTRAL_LINK_COUNT - Maximum number of central links. 
#ifndef NRF_SDH_BLE_CENTRAL_LINK_COUNT
#define NRF_SDH_BLE_CENTRAL_LINK_COUNT 1
#endif

// <o> NRF_SDH_BLE_TOTAL_LINK_COUNT - Total link count. 
// <i> Maximum number of total concurrent connections using the default configuration.

#ifndef NRF_SDH_BLE_TOTAL_LINK_COUNT
#define NRF_SDH_BLE_TOTAL_LINK_COUNT 3
#endif

// <o> NRF_SDH_BLE_GAP_EVENT_LENGTH - GAP event length. 
// <i> The time set aside for this connection on every connection interval in 1.25 ms units.

#ifndef NRF_SDH_BLE_GAP_EVENT_LENGTH
#define NRF_SDH_BLE_GAP_EVENT_LENGTH 2 //4
#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  112
#endif

BLE gap event length setup 52832

#ifndef NRF_SDH_BLE_GAP_DATA_LENGTH
#define NRF_SDH_BLE_GAP_DATA_LENGTH 112
#endif

// <o> NRF_SDH_BLE_PERIPHERAL_LINK_COUNT - Maximum number of peripheral links. 
#ifndef NRF_SDH_BLE_PERIPHERAL_LINK_COUNT
#define NRF_SDH_BLE_PERIPHERAL_LINK_COUNT 2
#endif

// <o> NRF_SDH_BLE_CENTRAL_LINK_COUNT - Maximum number of central links. 
#ifndef NRF_SDH_BLE_CENTRAL_LINK_COUNT
#define NRF_SDH_BLE_CENTRAL_LINK_COUNT 0
#endif

// <o> NRF_SDH_BLE_TOTAL_LINK_COUNT - Total link count. 
// <i> Maximum number of total concurrent connections using the default configuration.

#ifndef NRF_SDH_BLE_TOTAL_LINK_COUNT
#define NRF_SDH_BLE_TOTAL_LINK_COUNT 2
#endif

// <o> NRF_SDH_BLE_GAP_EVENT_LENGTH - GAP event length. 
// <i> The time set aside for this connection on every connection interval in 1.25 ms units.

#ifndef NRF_SDH_BLE_GAP_EVENT_LENGTH
#define NRF_SDH_BLE_GAP_EVENT_LENGTH 2 //4
#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 112
#endif

And I set GAP event length extension using:

sd_ble_opt_set (BLE_COMMON_OPT_CONN_EVT_EXT, &ext_t);

So in real run, when 840 and 832 are connected, param updated to :

BLE_GAP_EVT_CONN_PARAM_UPDATE: min_intval: 9, max_interval: 9, 
slave_latency: 0, sup_timeout: 40

I was trying to get shorter connection interval, however I found that 8 unit (10ms) is the shortest I can use. If I set min_interval to 6 or 7 units, then when init gap params sd_ble_gap_ppcp_set(&gap_conn_params) will return error code 7(invalid param), until I change min_interval back to 8 units. And even I set min_interval to 8, the actually interval connected is 9 (11.25ms). 

I did some manually calculation, seems 7.5ms should be fine with above param setting. But why I can not set to it in real run?

Thanks.

[run_ar]

Hi,

Remember that the softdevice also need time for pre- and post-processing. See: Bluetooth Low Energy central connection processor usage

I would not expect to be able to handle 8 links within 7.5 ms. You can find more information on scheduling here: Connection timing as a Central

[rolandash]

Any further hints?

thanks.

[run_ar]

For the peripheral links we also add time for the worst case window widening, calculated based on supervision timeout and LF clock ppm setting.

Not sure what the minimum is for 2 peripheral links and 1 central link, but remember that the link timing is controlled by two different clocks for central and peripheral, so you will get a few collisions regardless.

[rolandash]

thank you run_ar

I can understand that SD needs to prepare for worst case. Is there a way that I tell SD that condition is not that bad so I can use more tight timing?

For example, if I use a 20ppm LF XTAL instead of 500ppm RC, will that help SD to use shorter interval?

[run_ar]

I am sorry for the late answer. I don't think reducing the ppm value will enough to be able to reduce one unit, but it might. Eventually the Central will calculate how much time is needed, and choose an appropriate interval.