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  • Case ID: 290681
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How to schedule multilink conect event?

user115717

Hi

We are developing a system based on nrf desktop application of nRF Connect SDK 1.90. There are 4 devices  in this system: a dongle, a central device and two peripheral devices. Two peripheral devices connect to the central device with BLE, and the central device connect to the dongle with LLPM. We have managed to set up the connections, and encounter some problems when we tried to minimize the latency.

In our idea, two peripheral devices connect to central device with 7.5ms connect interval, peripheral 1 sends data during first 2.5ms of the connect interval, peripheral 2 sends data during next 2.5ms of the connect interval, and central device sends data during remaining.

However, we found that nrf desktop limit the minimal interval to 10ms with multilink

#if (CONFIG_CAF_BLE_USE_LLPM && (CONFIG_BT_MAX_CONN > 2))
 #define CONN_INTERVAL_BLE_REG		0x0008 /* 10 ms */
#else
 #define CONN_INTERVAL_BLE_REG		0x0006 /* 7.5 ms */
#endif

And below image shows the data timing, we can see there is 12.5ms between rx 0(perpheral 1) and rx 1(perpheral 2), and the inerval of same perpheral is 40ms.

Timespan between perpherals decreased  after changed SDC_DEFAULT_EVENT_LENGTH_US from 7500 to 2500, but same perpheral is still 40ms.

So, my question is how can two periperal send data in same conect interval and descrease same peripheral interval to 10ms?  

  • 0

    Hi,

    In our idea, two peripheral devices connect to central device with 7.5ms connect interval, peripheral 1 sends data during first 2.5ms of the connect interval, peripheral 2 sends data during next 2.5ms of the connect interval, and central device sends data during remaining.

    Based off of your drawing, it looks like there might be a misunderstanding regarding how BLE works. Each connection event, packets can go back and forth between the two connected peers, where one act as central and one act as peripheral. This means, to correct your drawing, the first 2.5 ms there will be alternating TX and RX on Peripheral 1 and on Central, for the communication between those two devices. Then similar for Peripheral 2 and Central the next 2.5 ms. Then TX and RX between Central and Dongle the next 1 ms.

    The connections and connection events themselves are scheduled by the SoftDevice Controller subsystem, according to the documented Scheduling. Depending on the connection parameters, certain schedulings may or may not be possible.

    For instance, you may have to consider the connection event length. Other activity, such as flash access, may lead to skipped connection events due to the CPU stalling during flash operation.

    In general, three connections with 7.5 ms connection interval should be fully possible, at least if the same device act as central for all three connections, the event lengths are short enough, and there is still enough time for successfully scheduling any other timing-activity. (E.g. no concurrent scanning, advertising, etc.)

    Regards,
    Terje

  • 0 in reply to user8164

    Thanks for reply.

    I know that there is alternating TX and RX in one connection event, the central transmit at first 1.25ms, mostly is a polling packet, and peripheral transmit after it.

    The test we did just include the central device and two pheripheral devices, and the central device was in neither advertising nor scanning after two BLE connections were set up.  I am curious why same peripheral interval is 40ms. Any idea?

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