https://devzone.nordicsemi.com/f/nordic-q-a/64774/sdk-15-3-sd-6-1-1-mtu-negotiation-failsI setup gatt like the following: static void gatt_init(void) { ret_code_t err_code = nrf_ble_gatt_init(&amp;m_gatt, gatt_evt_handler); APP_ERROR_CHECK(err_code); err_code = nrf_ble_gatt_att_mtu_central_set(&amp;m_gatt, NRF_SDH_BLE_GATT_MAX_MTU_SIZE); APP_ERROR_CHECKen-USTelligent Community 13Thu, 13 Aug 2020 10:44:33 GMThttps://devzone.nordicsemi.com/thread/264426?ContentTypeID=1Thu, 13 Aug 2020 10:44:33 GMT137ad170-7792-4731-bb38-c0d22fbe4515:c890c7ed-5a6c-4198-9de3-15356bfd31cbSimonr<p>Hi</p> <p>I know that iOS has an &quot;issue&quot; with achieving 247 MTU, and this seems very similar to that, so my guess is that the macOS doesn&#39;t accept 247 as the MTU size either. This issue (with iOS) is explained in length in <a href="https://devzone.nordicsemi.com/f/nordic-q-a/44825/ios-mtu-size-why-only-185-bytes">this case</a>, and a few workarounds are suggested. You can also check out <a href="https://jimmywongiot.com/2020/06/24/video-clips-of-ble-high-throughput-demo-on-nrf52-series/">this blog post by my colleague Jimmy</a> where he explains how to maximize the BLE throughput in communication between nRF52 and iOS devices.</p> <p>One of the workarounds suggested is that you can delay the MTU request/negotiation to occur at a later point, as iOS devices do not seem to like MTU requests upon connection. The attached project is a modified version of the ble_app_uart example where the MTU requests are delayed and should allow iOS devices to achieve a 247 byte MTU. Please note that this example project has not been through thorough testing, so there might be some hitches to it.</p> <p><a href="https://devzone.nordicsemi.com/cfs-file/__key/support-attachments/beef5d1b77644c448dabff31668f3a47-e2dbb0364e40428bbd92894d360c75e3/ble_5F00_app_5F00_uart.zip">devzone.nordicsemi.com/.../ble_5F00_app_5F00_uart.zip</a></p> <p>Best regards,</p> <p>Simon</p><div style="clear:both;"></div>