Thanks, we can try those numbers. All but the min_ce_len/max_ce_len are
already configurable by classroom code. I'll need to build a new firmware
to allow changes for min_ce_len/max_ce_len. I'm using the defaults for
those and they are 16 / 768.
On Thu, Apr 9, 2020 at 2:05 AM Ondrej Pilat <ondrej.pi...@ozobot.com> wrote:
> Hi,
>
> I cannot find any documentation about Nimble connection events scheduling.
> I have following scenario:
>
> NRF52 Nimble is in dual role setting. Peripheral device connected to a PC
> with at least BLE 4.2 and MTU more than 153B. Central is connected to 18
> peripheral devices BLE 4.1 with MTU 27B.
>
> Do you think that Nimble can keep interval window 12.5ms with the PC and
> 37.5ms with all peripherals?
>
> My assumption are following:
>
> - only one packet per each connection event
> - send/receive 132B packet between NRF52 and PC each connection event
> - send/receive 20B packet between NRF52 and peripheral each connection
> event
>
> I made following rough computation:
>
> PC <-> NRF52: 132B data + 21B header => (153B * 8) / 1Mbps => 1224 uS +
> 150 uS IFR + 80 uS ACK + 150 uS IFR => 1604 uS * 2 (send/receive) => 3208
> uS
>
> NRF52 <-> peripheral: 20B data + 21B header => (41B * 8) / 1Mbps => 328uS
> + 150 uS IFR + 80 uS ACK + 150uS IFR => 708uS * 2 (send/receive) => 1416 uS
>
> It means one big and six small packets should fit in each PC <-> NRF52
> connection event 3208 + 6 * 1416 = 11 704 < 12.5ms interval window. For 18
> peripherals it means 3 such cycles to serve them all therefore 37.5ms
> interval window between NRF52 and a peripheral.
>
> Are following parameters good to achieve this:
>
> NRF52 <-> PC
>
> static const struct ble_gap_conn_params ble_gap_conn_params_ideal = {
> .scan_itvl = BLE_GAP_SCAN_FAST_INTERVAL_MIN,
> .scan_window = BLE_GAP_SCAN_FAST_WINDOW,
> .itvl_min = 10,
> .itvl_max = 10,
> .latency = 0,
> .supervision_timeout = 500,
> .min_ce_len = 5, // 3.208 / 0.625 = 5.1328
> .max_ce_len = 6,
> };
>
> NRF52 <-> Peripheral
>
> static const struct ble_gap_conn_params ble_gap_conn_params_ideal = {
> .scan_itvl = BLE_GAP_SCAN_FAST_INTERVAL_MIN,
> .scan_window = BLE_GAP_SCAN_FAST_WINDOW,
> .itvl_min = 30,
> .itvl_max = 30,
> .latency = 0,
> .supervision_timeout = 500,
> .min_ce_len = 2, // 1.416 / 0.625 = 2.2656
> .max_ce_len = 3,
> };
>
> Regards
>
> Ondrej
> --
>
