nRF52840 UARTE0 EasyDMA receives more bytes than expected

Question

Greetings!

I'm currently making a project that involves using UART in EasyDMA Mode (via Zephyr Async API).

I have noticed an interesting behavior when you try to receive via UARTE0 (provided by Interface/Debug chip VCOM).

For example, I'm enabling the reception with

uart_rx_enable(uart, rx_buf, 10, SYS_FOREVER_US);

After that I'm sending >10, let's say 20 bytes. After 10 bytes received I get an UART_RX_RDY callback, everything is very fine, but if I will restart the reception with

uart_rx_enable(uart, rx_buf, 10, SYS_FOREVER_US);

I will instantly get one more UART_RX_RDY callback. Buffer will contain that second 10 bytes part.

My question is: Who is actually responsible for such a behavior? Is this some shadow UART buffer in RAM? If so, how can I control its size? Because I don't need it to buffer more than I have set up. Also I'm thinking about the Interface chip, probably if it has received something via VCOM, it stores some data inside in a hidden buffer.

If I switch to UARTE1, no such behavior is observed. Only UARTE0 via USB vcom.

Simple snippet for testing, some minimal editing may be needed:

#include <zephyr/kernel.h>

#include <zephyr/drivers/uart.h>

static void uart_cb(const struct device *dev, struct uart_event *evt, void *user_data)

{

switch (evt->type) {

case UART_RX_RDY:

printk("RX READY\n");

printk("%d \n", evt->data.rx.len);

break;

}

}

static uint8_t rx_buf[10] = {0}; //A buffer to store incoming UART data

const struct device *uart = DEVICE_DT_GET(DT_NODELABEL(uart0));

int main(void)

{

int err = 0;

if (!device_is_ready(uart)) {

return err;

}

err = uart_callback_set(uart, uart_cb, NULL);

if (err) {

return err;

}

uart_rx_enable(uart, rx_buf, 10, SYS_FOREVER_US);

while (1)

{

k_msleep(5000); // Mash your keyboard here, make > 10 bytes in these 5 seconds

uart_rx_enable(uart, rx_buf, 10, SYS_FOREVER_US);

}

return 0;

}

Thanks in advance,

BR

Vlad