Sending byte array over ble on nrf52840

RE: Sending byte array over ble on nrf52840

awneil — Wed, 15 Aug 2018 14:21:35 GMT

[quote userid="71966" url="~/f/nordic-q-a/37480/sending-byte-array-over-ble-on-nrf52840/144243"]There's a ble_nus_c_string_send() in the ble_app_uart_c client example[/quote]

IIRC, the name is misleading:

In 'C', a string is taken to mean an array of characters terminated by NUL.

The ble_nus_c_string_send() function it is not a string send - it just sends a buffer of entirely arbitrary bytes, and takes a second parameter to specify the length.

RE: Sending byte array over ble on nrf52840

howard n2wx — Wed, 15 Aug 2018 14:07:27 GMT

What you're seeing is the classic BLE maximum packet size (MTU in BLE lingo). Larger MTUs are supported by your part when they're negotiated at connection time.

This thread https://devzone.nordicsemi.com/f/nordic-q-a/33541/negotiating-max-mtu-size addresses your part, sdk15, and negotiating the larger MTU..

Without question it's a very steep learning curve. I've been really happy with the nrf52 environment though because the docs are terrific and the sdk very well thought out so even though there's a lot to learn it's rapidly rewarded. Every day is like getting a shot of dopamine. Bet you'll have a similar experience.

RE: Sending byte array over ble on nrf52840

LuckyGinger — Tue, 14 Aug 2018 22:44:15 GMT

I finally found this [1] and this [2] eventually leading me to change this:

    memset(&attr_char_value, 0, sizeof(attr_char_value));

    attr_char_value.p_uuid    = &ble_uuid;
    attr_char_value.p_attr_md = &attr_md;
    attr_char_value.init_len  = sizeof(uint8_t);
    attr_char_value.init_offs = 0;
    attr_char_value.max_len   = sizeof(uint8_t);
    attr_char_value.p_value   = NULL;

To this:

    memset(&attr_char_value, 0, sizeof(attr_char_value));

    attr_char_value.p_uuid    = &ble_uuid;
    attr_char_value.p_attr_md = &attr_md;
    attr_char_value.init_len  = 20;       // <-- changed len to 20
    attr_char_value.init_offs = 0;
    attr_char_value.max_len   = 20;       // <-- changed max len to 20
    attr_char_value.p_value   = NULL;

This seemed to have had the biggest impact on letting me send several bytes at a time. We will see what happens when I need to send several packets. I will definitly look into that BLE_NUS_ENABLED stuff though as 247 bytes at a time would be wildly helpful. My current understanding says the nrf52840 is only able to send 20 byte packets.

It has been a pretty steep learning curve for me. With your comment, I have become aware of the sdk_config.h file and its importance.

[1] https://devzone.nordicsemi.com/f/nordic-q-a/6395/error-using-the-sd_ble_gatts_hvx

[2] https://devzone.nordicsemi.com/f/nordic-q-a/4255/how-to-send-20-bytes-using-sd_ble_gatts_hvx

RE: Sending byte array over ble on nrf52840

howard n2wx — Tue, 14 Aug 2018 22:06:34 GMT

You're so close. If you'd have searched for ble_nus_c_string_send() you'd have found it: There's a ble_nus_c_string_send() in the ble_app_uart_c client example. But note that you might be better off inspecting the ble_app_uart server example and dropping the NUS server into your button server project. The equivalent server project uses ble_nus_data_send() and can send about 247 bytes at a time. By dropping into I mean settomg BLE_NUS_ENABLED to 1 in sdk_config.h, adding ble_nus.c service to the project along with other source files that you'll find in the ble_app_uart project that aren't in blinky along with the corresponding init calls..