UART RX missing bytes -event buffer length and offset mismatch-NRF Connect SDK

[Midhunjac]

Hi, greetings all !!

I am developing an application based on ble_uart_peripheral example on nrf52dk_nrf52832 using NRF Connect SDK2.1. In my application, I have to receive bytes from an external client via UART. The problem is, at high baud rate, some of my bytes get missed. For example, while debugging, the evt->data.rx.offset shows 4 but evt->data.rx.len shows 2. So, when i try to read the buffer, based on offset value, I miss bytes. HW flow control is not an option for me since that would involve changes to my client as well. Is there anything else I can do to get this working ? I thought it might be an overrun case, but that was not the error cause.

Any ideas ?

[Jørgen Holmefjord]

Hi,

Do you expect offset and len to always be the same? As described in uart_event_rx struct documentation: "The data represented by the event is stored in rx.buf[rx.offset] to rx.buf[rx.offset+rx.len]. That is, the length is relative to the offset." Is this how you read out data from the buffer in the event?

Best regards,
Jørgen

[Midhunjac]

Hi Jorgen,

maybe i am not making myself clear here. This is my code.

case UART_RX_RDY:
		LOG_DBG("UART_RX_RDY");
		buf = CONTAINER_OF(evt->data.rx.buf, struct uart_data_t, data);
		buf->len += evt->data.rx.len;
		if (disable_req) {
			return;
		}
		uint8_t data_t = *(buf->data+evt->data.rx.offset);
		packetDecode(data_t,eSrc_UART);

		if ((evt->data.rx.buf[buf->len - 1] == '\n') ||
		    (evt->data.rx.buf[buf->len - 1] == '\r')) {
			disable_req = true;
			uart_rx_disable(uart);
		}
		
		break;

This is part of the default uart_cb() that was part of the peripheral uart example. I am expecting that when each byte is received, I will get this interrupt and i will get that in data_t variable. Please correct me if my understanding is wrong or if there is a better implementation.
I dont feel that the logic is wrong though. Since, when there is a 1s gap between each byte transmitted from my client, I get all the bytes with no hassle in the data_t variable.

Please advise. Thanks for your time.

[Jørgen Holmefjord]

Do I read your code correctly that you will only assign one byte to data_t? If the CPU is blocked by other higher priority tasks to provide the UART RX event immediately when the data is received, the event can contain multiple bytes (given by the len struct member variable). You need to loop through the buffer from offset to offset+len to make sure you get all received bytes.

[Midhunjac]

yes. my requirement is to receive and process byte by byte. If my UART thread is given highest priority, do you think maybe this event will work as expected ? with one byte each ? how can I give my UART RX the highest priority ? I am not doing much other than collecting the data and a little bit of work on it. So, i dont mind it blocking some other communication, i think.

[Jørgen Holmefjord]

This is not the expected way that the async UART should work, but you can try to set CONFIG_BT_NUS_UART_BUFFER_SIZE=1, as this is what determines the buffer size used in the sample. Note that this may cause overrun errors if the CPU is blocked from providing a new buffer to the UARTE peripheral before ~4 bytes are received. I would be much more safe to have a larger buffer size and iterate through the received data when you get the events.