Sending a CSV File over ble Gatt connection

[Thomas]

Hi,

I need to send data from a CSV file on the nordic board(Via fatfs library). I have presently gotten a Gatt service up and running.

I have a mobile application which, can see the gatt service and write to the characteristics. From a specific write command from the mobile device I want the nordic board to send the CSV file over.

I can presently read a single line of the CSV file formatted like this: (Time stamp, data1, data 2, data3, data 4, data 5).

What I need to implement is:

• Reading the whole file but, one or multiple lines at a time due to the size of the CSV
• Changing the read pointer of the SD card (So I know where to continue to read from)
• Send this data to the characteristic
• Read this characteristic data.

My code is based upon :  https://github.com/bjornspockeli/custom_ble_service_example

My CSV file size depends on the length of time I have been storing data from. It could be upto around 1-2mbytes. I beleive I need to set up some kind of notification service. Where I read a line(or several) of the SD card, set the GATT characteristic value to this. Then change the SD card pointer. Wait till the data is received by the mobile application. Then repeat until the file is read full.y

Note I am using SDK 16 NRF52840-dk PCA 10056 S140.

This post is similar and mentions using two characteristics one from commication from the mobile app and, one for receiving the data.

https://devzone.nordicsemi.com/f/nordic-q-a/53987/nrf52840-file-transfer-via-ble

[Edvin]

Hello,

So you want to send a CSV file rom the SD card to the connected mobile phone, right?

If I were you, I would start with the ble_app_uart example, and use ble_nus_data_send() to send your notifications. You can probably do it in the application you have started developing now. Just send the data using notifications.

I didn't really see any questions in your post. Have you attempted this? Did you get stuck somewhere?

Best regards,

Edvin

[Thomas]

Yes I want to send a CSV file from the SD card on the nordic board to a mobile phone. 

Is there an example of this?

I have not used the ble_app_uart example for this. However I do have my own custom service and Gatt connection working. My question is

• How do I change the pointer location for the SD card?
• How do I setup the notifications? As in how do I get them to work and change the value to be sent via this notification to the value of the CSV data?

Also does with ble_app_uart example have a custom service? As I will need to move alot of my functionality too it.

Also how does ble_nus_data_send work?

Is there some guidance for how to setup notifications?

Thanks

[Edvin]

For the UART event handler, the suggestion that the error handler comes with (in this case NRF_ERROR_SVC_HANDLER_MISSING) is not correct. The err_code is 1. You must look at the event description. What sort of type is p_event->data.error_communication? See if you can find the declaration of the error_communication in a header file, and see if you can see what it means when this is set to 1.

Hint: What type is p_event? Right click and click go to definition.

[Thomas]

The type is app_uart_evt_t* is is an APP_UART_COMMUNICATION_ERROR.

I disable the uart in my gatt connection when a connection is made. Then I restart it when I disconnect. This error comes some time after i disconnect.

[Edvin]

I tried to point to the app_uart_evt_t->data.error_communication, which is an uint32_t, but I wanted you to check the description of this event in app_uart.h line 117.

Does it say anything about the value of the error_communication?

From app_uart.h:

uint32_t error_communication; /**< Field used if evt_type is: APP_UART_COMMUNICATION_ERROR. This field contains the value in the ERRORSRC register for the UART peripheral. The UART_ERRORSRC_x defines from nrf5x_bitfields.h can be used to parse the error code. See also the \nRFXX Series Reference Manual for specification. */

You are using the nRF52840, so I suggest you check the ERRORSRC register for the UART on the nRF52840:
https://infocenter.nordicsemi.com/topic/ps_nrf52840/uart.html?cp=4_0_0_5_32_9#topic

A value of 1 means OVERRUN. Perhaps this is to be expected when you disable and/or enable the UART when there is UART data traffic ongoing? Have you tried to ignore the APP_UART_COMMUNICATION_ERROR when you are enabling and disabling the UART?

[Thomas]

How would I go about ignoring it. Or better yet how do I prevent the overrun? This crash occurs seemingly around a couple of minutes after i disconnect.

[Edvin]

Don't pass it into the error handler by commenting out the line saying APP_ERROR_HANDLER(...): 

        case APP_UART_COMMUNICATION_ERROR:
            //APP_ERROR_HANDLER(p_event->data.error_communication);
            break;

[Thomas]

Thanks, that did seem to fix it. I have another question. I am now reading my CSV file by manually each time calling my function send CSV. This is working however, I want it to be automatic. 

Therefore I presume I can loop this until all my data is sent. My question is: Is there a way to confirm each time data is sent it is received before sending the next data?

I need this as I want to send my full log of data. This will be done using the notifications and, checking for a notification value change.

[Edvin]

If you want this to happen fast, you can just queue up as many packets as you like. Queue one using the sd_ble_gatts_hvx function, which you already do, and if that returns NRF_SUCCESS, try to queue another one immediately. Do this as long as it returns NRF_SUCCESS. Eventually, it will return NRF_ERROR_RESOURCES, which means that the buffer is full. When this happens, you must wait for the BLE_GATTS_EVT_HVN_TX_COMPLETE event (add this to your ble_evt_handler()). When you receive this event, you can continue to queue packets until it returns NRF_ERROR_RESOURCES.

Remember that the packet you queue that returns NRF_ERROR_RESOURCES isn't queued, so this would be the first packet you try to queue after the BLE_GATTS_EVT_HVN_TX_COMPLETE event.

[Thomas]

I am not quite sure what you mean by queue. Presently I am sending all my data (10 CSV lines for testing like below)

• Receive notification command
• Loop
  • If flag = true(set by notification timer 10 times a second)
  • Read sd card line
  • send data (ble_cus_send)
  • Check error
  • reset flag
  • Exit condition is all CSV read

This works but is this queuing? If so how can I speed it up atm its 510bytes per second.

[Edvin]

Your function ble_cus_send() is calling sd_ble_gatts_hvx(). This is what I mean by queuing. ble_cus_send() = queuing.

[Thomas]

So therefore, My bullet point code shown above works via queuing?

This means then I don't need a flag to make it loop 10 times a second.

Therefore, I just need to queue up all my data?