SAADC Sampling Frequency to 10 ms

[PinkSquirrel]

Hi, I am fairly new to the NRF and was wondering if there was a way to increase the sampling rate to 100Hz.  I have used the ble_app_uart example to configure this code:

This is the Header File 

#ifndef _BLUETOOTH_H
#define _BLUETOOTH_H

#include <stdint.h>
#include <string.h>
#include "nordic_common.h"
#include "nrf.h"
#include "ble_hci.h"
#include "ble_advdata.h"
#include "ble_advertising.h"
#include "ble_conn_params.h"
#include "nrf_sdh.h"
#include "nrf_sdh_soc.h"
#include "nrf_sdh_ble.h"
#include "nrf_ble_gatt.h"
#include "nrf_ble_qwr.h"
#include "app_timer.h"
#include "ble_nus.h"
#include "app_uart.h"
#include "app_util_platform.h"
#include "bsp_btn_ble.h"
#include "nrf_pwr_mgmt.h"
#include "nrf_drv_saadc.h"
#include "nrf_drv_ppi.h"
#include "nrf_drv_timer.h"
#include "nrf_log.h"

#if defined (UART_PRESENT)
#include "nrf_uart.h"
#endif
#if defined (UARTE_PRESENT)
#include "nrf_uarte.h"
#endif

#include "gpio_control.h"
#include "temperature.h"

/* Define */
#define APP_BLE_CONN_CFG_TAG            1                                           /**< A tag identifying the SoftDevice BLE configuration. */

#define DEVICE_NAME                     "nRF_Peripheral"                           /**< Name of device. Will be included in the advertising data. */
#define NUS_SERVICE_UUID_TYPE           BLE_UUID_TYPE_VENDOR_BEGIN                  /**< UUID type for the Nordic UART Service (vendor specific). */

#define APP_BLE_OBSERVER_PRIO           3                                           /**< Application's BLE observer priority. You shouldn't need to modify this value. */

#define APP_ADV_INTERVAL                64                                          /**< The advertising interval (in units of 0.625 ms. This value corresponds to 40 ms). */

#define APP_ADV_DURATION                18000                                       /**< The advertising duration (180 seconds) in units of 10 milliseconds. */

#define MIN_CONN_INTERVAL               MSEC_TO_UNITS(7.5, UNIT_1_25_MS)             /**< Minimum acceptable connection interval (20 ms), Connection interval uses 1.25 ms units. */
#define MAX_CONN_INTERVAL               MSEC_TO_UNITS(25, UNIT_1_25_MS)             /**< Maximum acceptable connection interval (75 ms), Connection interval uses 1.25 ms units. */
#define SLAVE_LATENCY                   0                                           /**< Slave latency. */
#define CONN_SUP_TIMEOUT                MSEC_TO_UNITS(250, UNIT_10_MS)             /**< Connection supervisory timeout (4 seconds), Supervision Timeout uses 10 ms units. */
#define FIRST_CONN_PARAMS_UPDATE_DELAY  APP_TIMER_TICKS(5000)                       /**< Time from initiating event (connect or start of notification) to first time sd_ble_gap_conn_param_update is called (5 seconds). */
#define NEXT_CONN_PARAMS_UPDATE_DELAY   APP_TIMER_TICKS(30000)                      /**< Time between each call to sd_ble_gap_conn_param_update after the first call (30 seconds). */
#define MAX_CONN_PARAMS_UPDATE_COUNT    3                                           /**< Number of attempts before giving up the connection parameter negotiation. */


#define UART_TX_BUF_SIZE                256                                         /**< UART TX buffer size. */
#define UART_RX_BUF_SIZE                256                                         /**< UART RX buffer size. */

#define SAADC_SAMPLES_IN_BUFFER         4
#define SAADC_SAMPLE_RATE               1                                      /**< SAADC sample rate in ms. */               

BLE_ADVERTISING_DEF(m_advertising);                                                 /**< Advertising module instance. */


void uart_init(void);
void buttons_leds_init(bool * p_erase_bonds);
void ble_stack_init(void);
void gap_params_init(void);
void gatt_init(void);
void services_init(void);
void conn_params_init(void);
void saadc_sampling_event_init(void);
void saadc_sampling_event_enable(void);
void advertising_start(void);
void saadc_init(void);
void advertising_init(void);


#endif

This is the C file:

In this code, I am trying to sample the 4 channels at 100Hz, but when I do, I get the NRF_RESOURCES error. 

In the ble_gatts.h I have also changed the BLE_GATTS_HVN_TX_QUE_SIZE_DEFAULT to 8 along with 

#ifndef NRF_SDH_BLE_GAP_EVENT_LENGTH
#define NRF_SDH_BLE_GAP_EVENT_LENGTH 6
#endif

// <o> NRF_SDH_BLE_GATT_MAX_MTU_SIZE - Static maximum MTU size.
#ifndef NRF_SDH_BLE_GATT_MAX_MTU_SIZE
#define NRF_SDH_BLE_GATT_MAX_MTU_SIZE 247
#endif

It seems that I am getting the error from the buffer overflow, but I am lost as to how to address this problem. Any pointers or help would be greatly appreciated. 

Thankyou!

[Karl Ylvisaker]

Hello,

It seems like you are describing two different issues here which we should deal with separately.

Issue 1 regards the sampling rate of the SAADC. Are you unable to achieve the 10 ms sampling period for the SAADC? What sampling rate do you get with your current configuration?

Issue 2 regards the possibility of having all the samples sent over the BLE connection, and the generation of the NRF_ERROR_RESOURCES error. This error is generated whenever you attempt to queue more notifications than there is buffer allocated (configured with BLE_GATTS_HVN_TX_QUE_SIZE_DEFAULT). So in order to resolve this we will need to balance how many notifications that are sent each connection event, how often the connection events occur, and how long the connection events are. Could you start by increasing your NRF_SDH_BLE_GAP_EVENT_LENGTH to 20, and which value you are seeing in the count variable of the BLE_GATTS_EVT_HVN_TX_COMPLETE event? Please also include what connection interval was used in the test.

Furthermore, I see from your included code that you are trying to use the BLE_GATTS_EVT_HVN_TX_COMPLETE event in the SAADC callback - unless you have registered the SAADC callback as an observer to BLE events deliberately you will not receive this event in the SAADC callback.
Instead, you will receive it in your ble_evt_handler or similar.

Looking forward to resolving these issues together!

Best regards,
Karl

[PinkSquirrel]

Karl Ylvisaker said:

Issue 1 regards the sampling rate of the SAADC. Are you unable to achieve the 10 ms sampling period for the SAADC? What sampling rate do you get with your current configuration?

Hi, currently I am able to get a stable connection with a rate of 25 Hertz with a SAADC_SAMPLE_RATE of 40ms 

Karl Ylvisaker said:

Could you start by increasing your NRF_SDH_BLE_GAP_EVENT_LENGTH to 20, and which value you are seeing in the count variable of the BLE_GATTS_EVT_HVN_TX_COMPLETE event? Please also include what connection interval was used in the test.

#define MIN_CONN_INTERVAL               MSEC_TO_UNITS(7.5, UNIT_1_25_MS)             /**< Minimum acceptable connection interval (20 ms), Connection interval uses 1.25 ms units. */
#define MAX_CONN_INTERVAL               MSEC_TO_UNITS(25, UNIT_1_25_MS)             /**< Maximum acceptable connection interval (75 ms), Connection interval uses 1.25 ms units. */
#define SLAVE_LATENCY                   0                                           /**< Slave latency. */
#define CONN_SUP_TIMEOUT                MSEC_TO_UNITS(250, UNIT_10_MS)             /**< Connection supervisory timeout (4 seconds), Supervision Timeout uses 10 ms units. */
#define FIRST_CONN_PARAMS_UPDATE_DELAY  APP_TIMER_TICKS(5000)                       /**< Time from initiating event (connect or start of notification) to first time sd_ble_gap_conn_param_update is called (5 seconds). */
#define NEXT_CONN_PARAMS_UPDATE_DELAY   APP_TIMER_TICKS(30000)                      /**< Time between each call to sd_ble_gap_conn_param_update after the first call (30 seconds). */
#define MAX_CONN_PARAMS_UPDATE_COUNT    3                                      

Yes my min max conn interval is shown here. Im really sorry but how would I get the count value for BLE_GATTS_EVT_HVN_TX_COMPLETE? 

Karl Ylvisaker said:

Furthermore, I see from your included code that you are trying to use the BLE_GATTS_EVT_HVN_TX_COMPLETE event in the SAADC callback - unless you have registered the SAADC callback as an observer to BLE events deliberately you will not receive this event in the SAADC callback.
Instead, you will receive it in your ble_evt_handler or similar.

Oh, I understand now. Thankyou!

Oh thankyou so much for your help. I changed the  NRF_SDH_BLE_GAP_EVENT_LENGTH to 20. Things brings up another issue; is there a way to check the notification rate in respect to the increase in sampling frequency? To my knowledge I might need to use a sniffer. 

Again thankyou so much for you help!

[Karl Ylvisaker]

Hello again,

PinkSquirrel said:

Oh thankyou so much for your help.

No problem at all, I am happy to help! :) 

PinkSquirrel said:

currently I am able to get a stable connection with a rate of 25 Hertz with a SAADC_SAMPLE_RATE of 40ms 

Aha, I see, thanks for clarifying!
By stable I assume you mean that it does not reset due to the NRF_ERROR_RESOURCES error? If so, this is merely a symptom of the NRF_ERROR_RESOURCES, and I would think that you do in fact achieve the 100 Hz sampling rate when you set it to 10 ms - it is just that you do not get to see if since the NRF_ERROR_RESOURES immediately triggers a reset.

PinkSquirrel said:

Yes my min max conn interval is shown here. Im really sorry but how would I get the count value for BLE_GATTS_EVT_HVN_TX_COMPLETE? 

You could also try to set the _MIN and _MAX connection interval both to 7.5 ms (or to another predetermined value), so that you know exactly which connection interval you are working with. Right now, the central may choose 7.5 ms all the way up to 25 ms without the application knowing / adjusting to it.
The count value is passed as a part of the received event. You can read it out straight from the BLE_GATTS_EVT_HVN_TX_COMPLETE event when it is received. The count value tells us how many notifications were sent during the last connection interval.

PinkSquirrel said:

I changed the  NRF_SDH_BLE_GAP_EVENT_LENGTH to 20. Things brings up another issue; is there a way to check the notification rate in respect to the increase in sampling frequency? To my knowledge I might need to use a sniffer. 

Notification rate can be checked with the count variable from the BLE_GATTS_EVT_HVN_TX_COMPLETE event, or indeed with a sniffer. The sniffer is what I would usually recommend because it gives the complete picture with all the details of what is happening on-air during the connection.
Are you familiar with the sniffer tool already? It is a powerful tool to wield when working with BLE. If you could provide a sniffer trace of the communication between the devices when this happens that would be very useful.
Please note that the sniffer needs to be listening for the peripheral device specifically before the connection is established in order for it to follow into the connection - you can choose which device to listen for specifically in the Wireshark dropdown menu (the default value is to listen for All advertising devices).

Best regards,
Karl

[PinkSquirrel]

Oh ok thankyou so much again for your help!. I apologize to ask this again though; If i were to change the NRF_SDH_BLE_GAP_EVENT_LENGTH again would I be able to acheive 200 Hz with a sample rate of 5 ms? 

[Karl Ylvisaker]

PinkSquirrel said:

Oh ok thankyou so much again for your help!. I apologize to ask this again though;

No need to apologize - I am happy to help!

PinkSquirrel said:

If i were to change the NRF_SDH_BLE_GAP_EVENT_LENGTH again would I be able to acheive 200 Hz with a sample rate of 5 ms? 

It is likely not enough to change NRF_SDH_BLE_GAP_EVENT_LENGTH to resolve the NRF_ERROR_RESOURCES error, but when this is resolved it should be no problem at all to achieve 200 Hz sampling frequency. The maximum sampling frequency of the SAADC is 200 kHz (only SAADC, not accounting for BLE transmission of the results).
So I suggest that we first resolve the NRF_ERROR_RESOURCES like discussed in the previous comment, and then move on to increasing the sample rate of the SAADC.

Best regards,
Karl

[PinkSquirrel]

Karl Ylvisaker said:

It is likely not enough to change NRF_SDH_BLE_GAP_EVENT_LENGTH to resolve the NRF_ERROR_RESOURCES error, but when this is resolved it should be no problem at all to achieve 200 Hz sampling frequency. The maximum sampling frequency of the SAADC is 200 kHz (only SAADC, not accounting for BLE transmission of the results).
So I suggest that we first resolve the NRF_ERROR_RESOURCES like discussed in the previous comment, and then move on to increasing the sample rate of the SAADC.

Ah I appreciate it! I changed the NRF_SDH_BLE_GAP_EVENT_LENGTH to 20 for 100Hz and it works fine now. Kinda kicking myself for not knowing such a simple fix of the SDK :/ .... but thankyou so much for the assistance. After 100Hz sampling rate was successful, I changed the Sample Rate to 5ms and it returned the same NRF_ERROR_RESOURCES(Error 19). Would I need to change the min and max conn interval to achieve 200Hz? 

Thankyou!