UART FIFO Flush: How to determine that UART TX is completed

[Comodo]

Hi everyone,

in our project we transmit from an android app to Nordic BLE big amount of data (up to 180.000 bytes). I do it by using hvx notifications (MTU 247) and each time I get the notification I try to transmit the data over UART. But we have realized that the uart TX takes too much time and causes packet loss. So I have decieded to store the BLE data and send it after the BLE notifications are completed. Now I face the problem that the UART FIFO (2048) will be full and causes system fault. I need to find out a way where I can determine when the TX transmition is completed and I can flush the TX buffer. I ve searched the uart driver but could find the right function for that. Can anyone help me regarding that?

Thx and Regards

[Andy]

You should get NRFX_UART_EVT_TX_DONE on the event handler.

[Comodo]

You mean "APP_UART_TX_EMPTY" ?

[Andy]

Ah, if you're using the app_uart library then yes. But then I don't understand your problem with flushing the FIFO. I've never used that library, but I assume it flushes the buffers by itself.

[Andy]

Nevermind, just saw the API.

[Comodo]

Hi still got this problem

I fill the Tx buffer and wait until it has been sent but that doesnt happen it stacks there. 

[ovrebekk]

Hi 

Are you able to disable notifications on the characteristic temporarily to avoid the client on the Android side sending more data until the UART has had time to flush the buffers?

Then you could disable the notifications once you have received a certain amount of data, wait for the APP_UART_TX_EMPTY event to occur, and enable notifications again. 

This probably requires the addition of some code in the app to verify if notifications are enabled before uploading data to the Bluetooth stack. 

Best regards
Torbjørn

[Comodo]

Hi, at the moment we have different data sizes depending on setting 2KB to 190KB. We are using two Pixel 3 phones (BLE5 capable) to cross-check the implementation. My collegue who is implementing the android side mentioned that he can set MTU up to 512.  Transfering data between two phones is very quicker (ca. 1000ms). The android BLE implementation has no notification response to keep the speed; so it is not possible to stop notification (2 ms per package). Right now I have a workaround, I store all RX BLE data in to a Buffer (which is limited to 90KB) and send it over uart as heaps, which takes 6 seconds. 

I am not sure if your proposal works more faster or ever implementable (2ms between packages, it should be longer to react at right time). 

Right now I am working on another workaround to speed up the RX-Buffer -> UART (by using bigger heaps, 4K-16K).

[ovrebekk]

Hi

I am not quite sure what you mean by the notification response. There is an onNotificationSent callback issued after the server on the Android side sends a notification, which you should check to ensure the notifications are successfully sent. 

Also, the client on the nRF52 side should always be able to unsubscribe or subscribe to notification updates, doing otherwise would be in violation of the Bluetooth specification. 

An alternative to controlling the data flow by enabling/disabling notifications is to use a separate characteristic to inform the server about how much buffer space you have on the nRF52 side. Then you can update this dynamically, and the Android app can make sure not to send any more data than the buffers in the nRF52 can handle. 

By sending data over the UART as heap I assume you mean you are forwarding a larger buffer to the UART driver, rather than just single bytes?

Best regards
Torbjørn