Unable to update ATT MTU

[Tai]

Hi everyone,

I'm trying to update the ATT MTU with nRF52833 that connects to an IOS app.

For Hardware/software setup, the following is what I'm using:

• Our toolchain is ARM GCC
• Ubuntu 
• SDK 17.0.2
• SD 140

I've taken a look at samples BLE_APP_ATT_MTU_THROUGPUT and BLE_APP_UART where the ATT MTU can be updated. The following code is what I implemented to request ATT MTU. However, I wasn't able to update the ATT MTU. I checked the MTU side on IOS app (Xcode software), it showed the MTU is still 23 bytes only when the app connected to nRF52833. Please advise if I need to do anything to change it. Thanks!

/**@brief Function for handling events from the GATT library. */
static void gatt_evt_handler(nrf_ble_gatt_t * p_gatt, nrf_ble_gatt_evt_t const * p_evt)
{
    if (p_evt->evt_id == NRF_BLE_GATT_EVT_ATT_MTU_UPDATED)
    {
        m_ble_nus_max_data_len = p_evt->params.att_mtu_effective - OPCODE_LENGTH - HANDLE_LENGTH;
        NRF_LOG_INFO("Data len is set to 0x%X(%d)", m_ble_nus_max_data_len, m_ble_nus_max_data_len);
    }
    NRF_LOG_DEBUG("ATT MTU exchange completed. central 0x%x peripheral 0x%x",
                  p_gatt->att_mtu_desired_central,
                  p_gatt->att_mtu_desired_periph);
}

static void gatt_init(void) {
    ret_code_t err_code = nrf_ble_gatt_init(&m_gatt, gatt_evt_handler);
    APP_ERROR_CHECK(err_code);

    err_code = nrf_ble_gatt_att_mtu_periph_set(&m_gatt, NRF_SDH_BLE_GATT_MAX_MTU_SIZE);
    APP_ERROR_CHECK(err_code);
}

[Amanda Hsieh]

Hi, 

Hope this post can help. 

Regards,
Amanda H.

[Tai]

Thanks for the reply, Amanda!

Yeah, I've read that post and implemented some based on what Jimmy suggested. However, I still received only 23 bytes at MTU read by IOS app. Using nRF Connect, I was able to see DLE updated, so I believe the DLE request happened at IOS app, but somehow I can't know if IOS sent a request to nRF chip for MTU request.

[Amanda Hsieh]

Hi,

You can take a look at the BLE_APP_UART which would send mtu exchange requests once it connects with the nRF Connect app. This is I test with the nRF Connect app on iPad with iOS 16.6.

 

Can you test the BLE_APP_UART and your code with the nRF Connect app?

-Amanda H.

[Tai]

Hi Amanda. I got exactly similar what you got by using BLE_APP_UART with the nRF Connect app installed on my Iphone (IOS 16.6.1). There was no request from the Master for MTU exchange. The Slave requested first and then the Master responded with the acceptance of new MTU.

Running my code and connecting with the nRF Connect app, the following is what I got. Somehow, the Master requested the MTU exchange first, then the Slave responded with a new MTU. This behaved differently from BLE_APP_UART. Again, I still saw on Xcode that the MTU length is only 23 bytes as shown in the second figure 

[Amanda Hsieh]

I suggest comparing your code with BLE_APP_UART to find their differences or using BLE_APP_UART as a base to develop the code. 

-Amanda H.  

[Tai]

I finally managed to make it work! Somehow, there is a #if and #endif for NRF_BLE_GATT_MTU_EXCHANGE_INITIATION_ENABLED in nrf_ble_gatt.c file. I didn't define this in my sdk_config.h. Once I defined it, I was able to update MTU length. Thanks Amanda for baring with me!

Edit: I'm using SDK 17.1.0 and it didn't declare NRF_BLE_GATT_MTU_EXCHANGE_INITIATION_ENABLED on sdk_config.h. Hope this helps anyone if they encounter the same problem.

[ask6483]

I got the same issue, it sucks that sdk_config header is not being updated accordingly after sdk update.

There is also other stuff missing in that file like SEGGER RTT, LIBUARTE...