How to increase the sampling rate of the ble_app_uart__saadc_timer_driven__scan_mode example?

[Michael01101]

Hi,

in my project I have a custom board with a nRF52832 chip that reads SAADC data and transmits this data (peripheral role). This data is received by a nRF52840-DK board (central role) and written into a COM-Port of a pc. I flashed the "ble_app_uart_c" example into the nRF52840-DK and the "ble_app_uart__saadc_timer_driven__scan_mode" example into the custom board with the nRF52832 chip (nRF5_SDK_17.0.0_9d13099). This configuration works fine. The SAADC data gets transmitted by the nRF52832 custom board, received by the nRF52840-DK and written into the COM-Port of my PC.

My problem is the SAADC sampling rate (SAADC_SAMPLE_RATE) of the "ble_app_uart__saadc_timer_driven__scan_mode" example. By default the SAADC sampling rate is 250ms (4Hz). I need to increase this sampling rate to 1ms (1000Hz). But if I choose the SAADC_SAMPLE_RATE < 10ms (>100Hz) an error occured and the chip resets and tries to reconnect. I receive the following error code if I debug the ble_app_uart__saadc_timer_driven__scan_mode example:

which is produced by the ble_nus_data_send(...) function in main.c:

Unfortunaly, I am not able to find a error description for this error code.

Can you please tell me, how to increase the sampling rate of the "ble_app_uart__saadc_timer_driven__scan_mode" example to 1ms (=1000Hz)?

[Karl Ylvisaker]

Hello,

"ble_app_uart__saadc_timer_driven__scan_mode" example into the custom board with the nRF52832 chip (nRF5_SDK_17.0.0_9d13099).

I am unfamiliar with this code, what example is this - where did you find this project code?
Is this code a merge between the saadc peripheral example, modified to sample multiple channels, and the ble_app_uart example?

My problem is the SAADC sampling rate (SAADC_SAMPLE_RATE) of the "ble_app_uart__saadc_timer_driven__scan_mode" example. By default the SAADC sampling rate is 250ms (4Hz). I need to increase this sampling rate to 1ms (1000Hz).

Could you share with me the SAADC configuration, and how you are modifying it?
It would also be good to see how you are using the SAADC - how the timer is set up, etc.
Please use the "Insert -> Code" option when sharing code here on DevZone.

I receive the following error code if I debug the ble_app_uart__saadc_timer_driven__scan_mode example:

Unfortunaly, I am not able to find a error description for this error code.

Could you ensure that DEBUG is defined in your preprocessor defines?
The included image shows how you can check this, or add DEBUG if it is missing.
Please run the program again with DEBUG defined, to see the proper error message.

Can you please tell me, how to increase the sampling rate of the "ble_app_uart__saadc_timer_driven__scan_mode" example to 1ms (=1000Hz)?

Yes, depending on your answer to my above question we will get started on changing the sampling rate to 1000 Hz, no problem.

Looking forward to resolving this issue together,

Best regards,
Karl 

[Michael01101]

Hi,

thank you so much for your reply.

1. I found this project code here: 

https://github.com/NordicPlayground/nRF52-ADC-examples/tree/master/ble_app_uart__saadc_timer_driven__scan_mode

You are right, this code is a merge between the saadc peripheral example, modified to sample four channels, and the ble_app_uart example.

2. Here you can see the SAADC configuration in the "ble_app_uart__saadc_timer_driven__scan_mode" example:

#define SAADC_SAMPLES_IN_BUFFER         4
#define SAADC_SAMPLE_RATE               250                                         /**< SAADC sample rate in ms. */               

volatile uint8_t state = 1;

static const nrf_drv_timer_t   m_timer = NRF_DRV_TIMER_INSTANCE(3);
static nrf_saadc_value_t       m_buffer_pool[2][SAADC_SAMPLES_IN_BUFFER];
static nrf_ppi_channel_t       m_ppi_channel;
static uint32_t                m_adc_evt_counter;

void timer_handler(nrf_timer_event_t event_type, void* p_context)
{

}


void saadc_sampling_event_init(void)
{
    ret_code_t err_code;
    err_code = nrf_drv_ppi_init();
    APP_ERROR_CHECK(err_code);
    
    nrf_drv_timer_config_t timer_config = NRF_DRV_TIMER_DEFAULT_CONFIG;
    timer_config.frequency = NRF_TIMER_FREQ_31250Hz;
    err_code = nrf_drv_timer_init(&m_timer, &timer_config, timer_handler);
    APP_ERROR_CHECK(err_code);

    /* setup m_timer for compare event */
    uint32_t ticks = nrf_drv_timer_ms_to_ticks(&m_timer,SAADC_SAMPLE_RATE);
    nrf_drv_timer_extended_compare(&m_timer, NRF_TIMER_CC_CHANNEL0, ticks, NRF_TIMER_SHORT_COMPARE0_CLEAR_MASK, false);
    nrf_drv_timer_enable(&m_timer);

    uint32_t timer_compare_event_addr = nrf_drv_timer_compare_event_address_get(&m_timer, NRF_TIMER_CC_CHANNEL0);
    uint32_t saadc_sample_event_addr = nrf_drv_saadc_sample_task_get();

    /* setup ppi channel so that timer compare event is triggering sample task in SAADC */
    err_code = nrf_drv_ppi_channel_alloc(&m_ppi_channel);
    APP_ERROR_CHECK(err_code);
    
    err_code = nrf_drv_ppi_channel_assign(m_ppi_channel, timer_compare_event_addr, saadc_sample_event_addr);
    APP_ERROR_CHECK(err_code);
}


void saadc_sampling_event_enable(void)
{
    ret_code_t err_code = nrf_drv_ppi_channel_enable(m_ppi_channel);
    APP_ERROR_CHECK(err_code);
}


void saadc_callback(nrf_drv_saadc_evt_t const * p_event)
{
    if (p_event->type == NRF_DRV_SAADC_EVT_DONE)
    {
        ret_code_t err_code;
        uint16_t adc_value;
        uint8_t value[SAADC_SAMPLES_IN_BUFFER*2];
        uint16_t bytes_to_send;
     
        // set buffers
        err_code = nrf_drv_saadc_buffer_convert(p_event->data.done.p_buffer, SAADC_SAMPLES_IN_BUFFER);
        APP_ERROR_CHECK(err_code);
						
        // print samples on hardware UART and parse data for BLE transmission
        printf("ADC event number: %d\r\n",(int)m_adc_evt_counter);
        for (int i = 0; i < SAADC_SAMPLES_IN_BUFFER; i++)
        {
            printf("%d\r\n", p_event->data.done.p_buffer[i]);

            adc_value = p_event->data.done.p_buffer[i];
            value[i*2] = adc_value;
            value[(i*2)+1] = adc_value >> 8;
        }

         // Send data over BLE via NUS service. Create string from samples and send string with correct length.
        uint8_t nus_string[50];
        bytes_to_send = sprintf(nus_string, 
                                "CH0: %d\r\nCH1: %d\r\nCH2: %d\r\nCH3: %d",
                                p_event->data.done.p_buffer[0],
                                p_event->data.done.p_buffer[1],
                                p_event->data.done.p_buffer[2],
                                p_event->data.done.p_buffer[3]);

        err_code = ble_nus_data_send(&m_nus, nus_string, &bytes_to_send, m_conn_handle);
        if ((err_code != NRF_ERROR_INVALID_STATE) && (err_code != NRF_ERROR_NOT_FOUND))
        {
            APP_ERROR_CHECK(err_code);
        }
	
        m_adc_evt_counter++;
    }
}


void saadc_init(void)
{
    ret_code_t err_code;
	
    nrf_drv_saadc_config_t saadc_config = NRF_DRV_SAADC_DEFAULT_CONFIG;
    saadc_config.resolution = NRF_SAADC_RESOLUTION_12BIT;
	
    nrf_saadc_channel_config_t channel_0_config =
        NRF_DRV_SAADC_DEFAULT_CHANNEL_CONFIG_SE(NRF_SAADC_INPUT_AIN4);
    channel_0_config.gain = NRF_SAADC_GAIN1_4;
    channel_0_config.reference = NRF_SAADC_REFERENCE_VDD4;
	
    nrf_saadc_channel_config_t channel_1_config =
        NRF_DRV_SAADC_DEFAULT_CHANNEL_CONFIG_SE(NRF_SAADC_INPUT_AIN5);
    channel_1_config.gain = NRF_SAADC_GAIN1_4;
    channel_1_config.reference = NRF_SAADC_REFERENCE_VDD4;
	
    nrf_saadc_channel_config_t channel_2_config =
        NRF_DRV_SAADC_DEFAULT_CHANNEL_CONFIG_SE(NRF_SAADC_INPUT_AIN6);
    channel_2_config.gain = NRF_SAADC_GAIN1_4;
    channel_2_config.reference = NRF_SAADC_REFERENCE_VDD4;
	
    nrf_saadc_channel_config_t channel_3_config =
        NRF_DRV_SAADC_DEFAULT_CHANNEL_CONFIG_SE(NRF_SAADC_INPUT_AIN7);
    channel_3_config.gain = NRF_SAADC_GAIN1_4;
    channel_3_config.reference = NRF_SAADC_REFERENCE_VDD4;				
	
    err_code = nrf_drv_saadc_init(&saadc_config, saadc_callback);
    APP_ERROR_CHECK(err_code);

    err_code = nrf_drv_saadc_channel_init(0, &channel_0_config);
    APP_ERROR_CHECK(err_code);
    err_code = nrf_drv_saadc_channel_init(1, &channel_1_config);
    APP_ERROR_CHECK(err_code);
    err_code = nrf_drv_saadc_channel_init(2, &channel_2_config);
    APP_ERROR_CHECK(err_code);
    err_code = nrf_drv_saadc_channel_init(3, &channel_3_config);
    APP_ERROR_CHECK(err_code);	

    err_code = nrf_drv_saadc_buffer_convert(m_buffer_pool[0],SAADC_SAMPLES_IN_BUFFER);
    APP_ERROR_CHECK(err_code);   
    err_code = nrf_drv_saadc_buffer_convert(m_buffer_pool[1],SAADC_SAMPLES_IN_BUFFER);
    APP_ERROR_CHECK(err_code);
}

/**@brief Application main function.
 */
int main(void)
{
    bool erase_bonds;

    // Initialize.
    uart_init();
    log_init();
    timers_init();
    buttons_leds_init(&erase_bonds);
    power_management_init();
    ble_stack_init();
    gap_params_init();
    gatt_init();
    services_init();
    advertising_init();
    conn_params_init();

    saadc_sampling_event_init();
    saadc_init();
    saadc_sampling_event_enable();

    // Start execution.
    printf("\r\nUART started.\r\n");
    NRF_LOG_INFO("Debug logging for UART over RTT started.");
    advertising_start();

    // Enter main loop.
    for (;;)
    {
        idle_state_handle();
    }
}

I tried to modify the SAADC_SAMPLE_RATE to 1. If I set the SAADC_SAMPLE_RATE to 1, I receive the error which I already mentioned.

3. Where can I find your included image for checking or adding DEBUG? How can I ensure that DEBUG is defined in my preprocessor defines?

Thank you in advance.

[Michael01101]

Hello,

thank you for your response.

Fortunately, it looks like the MTU exchange is already implemented in the "ble_app_uart_c" and the "ble_app_uart__saadc_timer_driven__scan_mode" example. After I run the program on nRF52832 custom board (peripheral), I immediately receive the following in the debug terminal:

<info> app: ATT MTU exchange completed. central 0xF7 peripheral 0xF7
<info> app: Data len is set to 0xF4(244)

At the moment the following changes are applied to the "ble_app_uart__saadc_timer_driven__scan_mode" example on the peripheral side in main.c:

#define MIN_CONN_INTERVAL               MSEC_TO_UNITS(7.5, UNIT_1_25_MS)
#define MAX_CONN_INTERVAL               MSEC_TO_UNITS(10 , UNIT_1_25_MS)

#define SAADC_SAMPLE_RATE               1

static int16_t counter = 1;

static int16_t ch1_values[8];
static int16_t ch2_values[8];
static int16_t ch3_values[8];
static int16_t ch4_values[8];

void saadc_callback(nrf_drv_saadc_evt_t const * p_event)
{
    if (p_event->type == NRF_DRV_SAADC_EVT_DONE)
    {
        ...
        if (counter <= 8){
            ch1_values[counter-1] = p_event->data.done.p_buffer[0];
            ch2_values[counter-1] = p_event->data.done.p_buffer[1];
            ch3_values[counter-1] = p_event->data.done.p_buffer[2];
            ch4_values[counter-1] = p_event->data.done.p_buffer[3];
        }
        
        if (counter == 8){     
                        
            counter = 0;
            
            uint8_t nus_string[200];                                                                                
            bytes_to_send = sprintf(nus_string,                                                               
                                  "%d%d%d%d_%d%d%d%d_%d%d%d%d_%d%d%d%d_%d%d%d%d_%d%d%d%d_%d%d%d%d_%d%d%d%d\r\n",
                                  ch1_values[0],
                                  ch2_values[0],
                                  ch3_values[0],
                                  ch4_values[0],
                                  ch1_values[1],
                                  ch2_values[1],
                                  ch3_values[1],
                                  ch4_values[1],
                                  ch1_values[2],
                                  ch2_values[2],
                                  ch3_values[2],
                                  ch4_values[2],
                                  ch1_values[3],
                                  ch2_values[3],
                                  ch3_values[3],
                                  ch4_values[3],
                                  ch1_values[4],
                                  ch2_values[4],
                                  ch3_values[4],
                                  ch4_values[4],
                                  ch1_values[5],
                                  ch2_values[5],
                                  ch3_values[5],
                                  ch4_values[5],
                                  ch1_values[6],
                                  ch2_values[6],
                                  ch3_values[6],
                                  ch4_values[6],
                                  ch1_values[7],
                                  ch2_values[7],
                                  ch3_values[7],
                                  ch4_values[7]);
        
            err_code = ble_nus_data_send(&m_nus, nus_string, &bytes_to_send, m_conn_handle);
            if ((err_code != NRF_ERROR_INVALID_STATE) && (err_code != NRF_ERROR_NOT_FOUND))
            {
                APP_ERROR_CHECK(err_code); 
            }
        }
	counter++;
	}
}

Basically, I save the SAADC readings for 7ms. In the next millisecond I transmit the actual SAADC readings and the saved SAADC readings of the last 7ms. On the central side I receive every 8ms 8 samples of each channel. This works for quite a few seconds or sometimes for one or two minutes.

If it stopps after a few seconds I get the following error:

<error> app: ERROR 4 [NRF_ERROR_NO_MEM] at C:\SDK\SDKv17\nRF5_SDK_17.0.0_9d13099\examples\ble_peripheral\ble_app_uart__saadc_timer_driven__scan_mode_optimized_third\main.c:237
PC at: 0x0002A58D

which is caused by the following function:

static void nus_data_handler(ble_nus_evt_t * p_evt)
{
...
    err_code = app_uart_put(p_evt->params.rx_data.p_data[i])
}    

And sometimes I receive the following error:

<error> app: ERROR 19 [NRF_ERROR_RESOURCES] at C:\SDK\SDKv17\nRF5_SDK_17.0.0_9d13099\examples\ble_peripheral\ble_app_uart__saadc_timer_driven__scan_mode_optimized_third\main.c:833
PC at: 0x0002A4F1

which is caused by the ble_nus_data_send(...) function.

I don´t think, that this is the ideal solution for transmitting my SAADC readings of 4 channels with a sample rate of 1ms (1000Hz). So here are my questions:

1. Why do I get sometimes a NRF_ERROR_RESOURCES error and sometimes a NRF_ERR_NO_MEM error?

2. What is the reason for a NRF_ERR_NO_MEM error?

3. Should I call the ble_nus_data_send(...) function more often than in a 8ms interval (Because my minimal connection interval is 7.5ms)?

4. Is there a better way to transmit the SAADC readings, when using a sample rate of 1ms(1000Hz) and four channels? Do you have any suggestions?

Thank you very much in advance.

[Karl Ylvisaker]

Hello,

Michael01101 said:

Basically, I save the SAADC readings for 7ms. In the next millisecond I transmit the actual SAADC readings and the saved SAADC readings of the last 7ms. On the central side I receive every 8ms 8 samples of each channel. This works for quite a few seconds or sometimes for one or two minutes.

Please remember that the shortest valid connection interval is in fact 7.5 ms. 

Michael01101 said:

1. Why do I get sometimes a NRF_ERROR_RESOURCES error and sometimes a NRF_ERR_NO_MEM error?

When troubleshooting why a function returned a certain error, it is all written in the specific function documentation. From your error log it seems this is line 237 of main.c
If this is app_uart_put, returning NRF_ERROR_NO_MEM we take a look at the functions documentation and see that this error is returned when:

If no more space is available in the TX buffer. NRF_ERROR_NO_MEM may occur if flow control is enabled and CTS signal is high for a long period and the buffer fills up.

This indicates that messages are being placed in the TX buffer faster than they are being sent over UART. To resolve this, you will either have to reduce the size or frequency of your transfers, or increase the UART transmission frequency.

Doing the same for ble_nus_data_send we see that it returns the error code NRF_ERROR_RESOURCES returned by sd_ble_gatts_hvx, which happens when:

Too many notifications queued. Wait for a BLE_GATTS_EVT_HVN_TX_COMPLETE event and retry.

So, it seems you are still queuing the notifications faster than they are being sent. This makes sense if you are in fact queuing a notification every 7 ms, since that will cause a 0.5 ms shift every 7.5 ms. Eventually, you will run out of memory, resulting in the error.

Michael01101 said:

4. Is there a better way to transmit the SAADC readings, when using a sample rate of 1ms(1000Hz) and four channels? Do you have any suggestions?

I think this should be fine, but you will need to make sure that there at least is a match or even some leeway for every transfer.

To visualize and confirm the current issue, you can write out a messages to the log with the contents of your buffers when the program is running, and how often you are calling ble_nus_data_send vs. how often you are receiving the BLE_GATTS_EVT_HVN_TX_COMPLETE event. I think you will then see that you are generating and queuing data faster than you are processing it. You should see that the buffers / mismatch keeps growing until an error is eventually generated.

Best regards,
Karl

[Michael01101]

Hello,

sorry for my late response.

I implemented a counter (counterTX), which counts how often a BLE_GATTS_EVT_HVN_TX_COMPLETE event happend.

I did this in ble_nus.c:

void ble_nus_on_ble_evt(ble_evt_t const * p_ble_evt, void * p_context)
{
    if ((p_context == NULL) || (p_ble_evt == NULL))
    {
        return;
    }

    ble_nus_t * p_nus = (ble_nus_t *)p_context;

    switch (p_ble_evt->header.evt_id)
    {
        case BLE_GAP_EVT_CONNECTED:
            on_connect(p_nus, p_ble_evt);
            break;

        case BLE_GATTS_EVT_WRITE:
            on_write(p_nus, p_ble_evt);
            break;

        case BLE_GATTS_EVT_HVN_TX_COMPLETE:

            counterTX++;
            NRF_LOG_INFO("TX_COMPLETE: %d", counterTX);

            on_hvx_tx_complete(p_nus, p_ble_evt);

            break;

        default:
            // No implementation needed.
            break;
    }
}

But even in the unmodified "ble_app_uart__saadc_timer_driven__scan_mode" example on the peripheral side, there is never a BLE_GATTS_EVT_HVN_TX_COMPLETE event happening. The counterTX never increases.

1. Is this the right place for counting the BLE_GATTS_EVT_HVN_TX_COMPLETE events?

2. Why are no BLE_GATTS_EVT_HVN_TX_COMPLETE events happening in this example?

Thank you very much in advance.

[Karl Ylvisaker]

Hello,

Michael01101 said:

sorry for my late response.

 No problem at all, do not worry about it.

Michael01101 said:

1. Is this the right place for counting the BLE_GATTS_EVT_HVN_TX_COMPLETE events?

Yes, if this is the event handler that is provided during the "ble_stack_init" call.
The code that is setting this up looks like this:

    // Register a handler for BLE events.
    NRF_SDH_BLE_OBSERVER(m_ble_observer, APP_BLE_OBSERVER_PRIO, ble_evt_handler, NULL);

If you have changed the name of the event handler, or created a new one, you will need to register it as an observer to the BLE events, so that it will receive them.
I see from the github repository code that the name of the event handler is ble_evt_handler but that the name of the event handler you are referring to in your reply is ble_nus_on_ble_evt - have you made sure this event handler is registered as an observer to the ble events?

Looking forward to solving this issue together,

Best regards,
Karl

[Michael01101]

Hello,

thank you for your fast reply.

I tried to count the BLE_GATTS_EVT_HVN_TX_COMPLETE events in the wrong event handler. Now I am using the right event handler and I am able to count the BLE_GATTS_EVT_HVN_TX_COMPLETE events.

I implemented a counter which increases when ble_nus_data_send() is called, and decreases when a BLE_GATTS_EVT_HVN_TX_COMPLETE event is received. 

Overall the counter is pretty stable. The counter rarely increases and decreases by 1 or 2.  Sometimes after 5-30 minutes the counter increases by 3. That is enough to cause an overflow (NRF_ERROR_RESOURCES). It looks like, that the buffer is pretty small.

I think I am able to handle this by increasing the buffer size. I already tried to increase NRF_SDH_BLE_GAP_EVENT_LENGTH, which is 6 by default, but this does not help.

So here are my questions:

1. How can I increase the buffer size of the ble_nus_data_send() or the sd_ble_gatts_hvx() function?

2. Do you suggest to increase the buffer size?

Thank you very much in advance.

[Karl Ylvisaker]

Hello,

Michael01101 said:

thank you for your fast reply.

No problem at all, I am happy to help.

Michael01101 said:

I tried to count the BLE_GATTS_EVT_HVN_TX_COMPLETE events in the wrong event handler. Now I am using the right event handler and I am able to count the BLE_GATTS_EVT_HVN_TX_COMPLETE events.

Great, I am glad to hear that it correctly identified and resolved the issue.

Michael01101 said:

Do you suggest to increase the buffer size?

It sounds to me like the case here is a somewhat stable buffer, that is strictly increasing as time passes.
This leads me to believe that by increasing the buffer size you will just increase the time it takes for an overflow to occur.
So, instead, I would look at what is causing the buildup in the buffer and how the this can be alleviated. For example, I mentioned earlier that you have a constant 0.5 ms data buildup per connection interval, since you are queuing a notification every 8 ms.
Did you make any changes based on my previous comments regarding the 0.5 ms data buildup per connection interval?

By the way, did you also take a look at the contents of the contents of the BLE_GATTS_EVT_HVN_TX_COMPLETE - it contains the number of notifications transmissions completed. It might be useful to take a look at using this. Could you let me know if this value always is 1, or does it sometimes increase?

Best regards,
Karl

[Michael01101]

Hello,

thank you for your quick response.

Regarding the 0.5ms data builup:

Sorry, I should have told you this.

I made the following changes to the code:

#define MIN_CONN_INTERVAL               MSEC_TO_UNITS(22.5, UNIT_1_25_MS)
#define MAX_CONN_INTERVAL               MSEC_TO_UNITS(24 , UNIT_1_25_MS)
#define SAADC_SAMPLE_RATE               1

void saadc_callback(nrf_drv_saadc_evt_t const * p_event)
{
    if (p_event->type == NRF_DRV_SAADC_EVT_DONE)
    {
        ...
        if (counter == 24){     
                        
            counter = 0;
            bytes_to_send = 120;
    
            if (send_enable == 1){
            
                err_code = ble_nus_data_send(&m_nus, ch_values, &bytes_to_send, m_conn_handle);
                
                if ((err_code != NRF_ERROR_INVALID_STATE) && (err_code != NRF_ERROR_NOT_FOUND)){
                    APP_ERROR_CHECK(err_code); 
                }
                counter_Send++;
            }
    
            NRF_LOG_INFO("Counter Send: %d",counter_Send);
    
        }
	    counter++;
        m_adc_evt_counter++;
    }
}

I save the saadc readings for 24ms. So I call the ble_nus_data_send() function every 24ms and send them all togheter (10Bit*4Channels*24=120Byte). "send_enable" is TRUE when it is connected successfully to the central.

The MIN_CONN_INTERVAL and the MAX_CONN_INTERVAL are equal on the central side.

Regarding the number of notifications transmissions:

Counter von TX_COMPLETE: ble_gatts_evt_hvn_tx_complete_t::count. It gets printed every time a BLE_GATTS_EVT_HVN_TX_COMPLETE event happend.

Counter Send: The counter which increases when ble_nus_data_send() function is called and decreases if a BLE_GATTS_EVT_HVN_TX_COMPLETE event is received.

    ble_gatts_evt_hvn_tx_complete_t const * p_evt_complete = &p_ble_evt->evt.gatts_evt.params.hvn_tx_complete;

    switch (p_ble_evt->header.evt_id)
    {
        case BLE_GATTS_EVT_HVN_TX_COMPLETE:
            NRF_LOG_INFO("Counter von TX_COMPLETE: %d",p_evt_complete->count);
            counter_Send--;
            break;
            
        ...

<info> app: Counter von TX_COMPLETE: 1
<info> app: Counter Send: 14
<info> app: Counter von TX_COMPLETE: 1
<info> app: Counter Send: 14
<info> app: Counter von TX_COMPLETE: 1
<info> app: Counter Send: 14
<info> app: Counter Send: 15
<info> app: Counter von TX_COMPLETE: 1
<info> app: Counter Send: 15
<info> app: Counter von TX_COMPLETE: 1
<info> app: Counter Send: 15
<info> app: Counter von TX_COMPLETE: 1
<info> app: Counter von TX_COMPLETE: 1
<info> app: Counter Send: 14
<info> app: Counter von TX_COMPLETE: 1
<info> app: Counter Send: 14
<info> app: Counter von TX_COMPLETE: 1
<info> app: Counter Send: 14

It looks like, that a a notification gets buffered for a short amount of time. Then two notifications were sended in one interval.

So here are my questions:

1. Does it make sense to call the ble_nus_data_send() function every 24ms. Because 24ms is not a multiple of the 1,25ms tick?

2. How should I choose the MIN_CONN_INTERVAL and the MAX_CONN_INTERVAL?

3. It would be possible for me to save SAADC readings up to 100ms before sending them to the central. Do you have any suggestions how to implement this with the ble_nus_data_send() function.

Thank you very much in advance.

[Karl Ylvisaker]

Hi, 

Michael01101 said:

Sorry, I should have told you this.

I made the following changes to the code:

Thank you for telling me.

Michael01101 said:

Then two notifications were sended in one interval.

This is correct, and can happen.
If your data shift was in the other direction, - 0.5 ms, then this would make sure that your buffer never overflowed - regardless of "when" the function call to send is placed.

Michael01101 said:

1. Does it make sense to call the ble_nus_data_send() function every 24ms. Because 24ms is not a multiple of the 1,25ms tick?

In short; no. If you are not receiving an error then I suppose the SoftDevice goes for "the next best thing". Have you monitored your connection interval while you are in a connection, using the nRF Sniffer?

Michael01101 said:

2. How should I choose the MIN_CONN_INTERVAL and the MAX_CONN_INTERVAL?

What are your applications real-time requirements? I see that you are ok with a delay up to 100 ms before sending, this gives us a lot of leeway.
If you want it to match exactly ( receive the same number of samples from all channels in each notification ), it would have to be a multiple of 20 ms ( 16 x 1.25 ms ) . This would mean that you get 100 bytes ready to transfer, which is well within the MTU size you currently have set.

Alternatively, you could have the notification sent with a negative data shift, by for example queuing a notification every 12th sampling ( 12 ms ) with a 10 ms connection interval. This will ensure that the buffer does not overflow, since you are processing data faster than you are creating it. This also fits within your 100 ms requirement.

What do you think about either of these alternatives?

In general, please remember that shorter connection intervals increases power consumption. So, if you device is battery powered ( which I suspect that it is not? ), then you would want to pick the largest possible connection interval.

Best regards,
Karl

[Michael01101]

Hello,

thank you for your response.

Karl Ylvisaker said:

Have you monitored your connection interval while you are in a connection, using the nRF Sniffer?

I haven´t done this yet. I tried to use the nRF52840 dongle for the nRF Sniffer, but unfortunaly the dongle is not supportet by the nRF Sniffer. I already ordered a nRF52-DK for this, which will arrive soon.

Karl Ylvisaker said:

I see that you are ok with a delay up to 100 ms before sending, this gives us a lot of leeway.

A delay up to 100ms is okey.

Karl Ylvisaker said:

What do you think about either of these alternatives?

I would prefer the 20ms alternative. I implemented this with a MIN_CONN_INTERVAL and a MAX_CONN_INTERVAL of 20ms and it works fine.

I also tried to use an 40ms (32 x 1,25ms) interval (MIN_CONN_INTERVAL and MAX_CONN_INTERVAL of 40ms). So every 40ms the ble_nus_data_send() function gets called with 200 bytes of data. This should be within my MTU size. But nearly immediatly after a connection is established between central und peripheral, I receive a NRF_ERROR_RESOURCES. Although I receive for every call of the ble_nus_data_send() function an BLE_GATTS_EVT_HVN_TX_COMPLETE event and it doesn´t looks like a buffer overflow.

Do you know why this is happening?

Karl Ylvisaker said:

please remember that shorter connection intervals increases power consumption

My goal is the largest possible connection interval for lower power consumption.

Thank you very much in advance.

[Karl Ylvisaker]

Hello,

Michael01101 said:

thank you for your response.

No problem at all, I am happy to help!

Michael01101 said:

I haven´t done this yet. I tried to use the nRF52840 dongle for the nRF Sniffer, but unfortunaly the dongle is not supportet by the nRF Sniffer. I already ordered a nRF52-DK for this, which will arrive soon.

That is great! The nRF Sniffer is a very powerful tool to wield when developing BLE applications.
It makes it a lot easier to see what is actually going on in your links.

Michael01101 said:

I would prefer the 20ms alternative. I implemented this with a MIN_CONN_INTERVAL and a MAX_CONN_INTERVAL of 20ms and it works fine.

I am glad to hear that is is working as intended now!

Michael01101 said:

nearly immediatly after a connection is established between central und peripheral, I receive a NRF_ERROR_RESOURCES. Although I receive for every call of the ble_nus_data_send() function an BLE_GATTS_EVT_HVN_TX_COMPLETE event and it doesn´t looks like a buffer overflow.

Do you know why this is happening?

An exempt from the sd_ble_gatts_hvx function ( which is used in ble_app_uart_send ) reads:

The number of Handle Value Notifications that can be queued is configured by ble_gatts_conn_cfg_t::hvn_tx_queue_size When the queue is full, the function call will return NRF_ERROR_RESOURCES. A BLE_GATTS_EVT_HVN_TX_COMPLETE event will be issued as soon as the transmission of the notification is complete.

What is your queue size defined to?

Michael01101 said:

My goal is the largest possible connection interval for lower power consumption.

Then I would go for the largest possible payload with the longest acceptable connection interval, to minimize active radio time.
Have you seen our Online Power Profiler? It is very useful to see the difference in estimated power consumption for different connection parameters.

Best regards,
Karl