Background LED indication in low power mode
LPUART1 is available in STOP2, you can use the TX output to send a pulse. At 32768 Hz clock frequency, you can set it to as low as 90 baud. Transmitting a 0xFF byte would lower the TX output for 9 times the bit time (start bit + 8 data bits). You can either invert the TX output in the LPUART-&gt;CR2 register or connect the LED between TX and VDD.

Of course it wouldn't work if you are receiving data on LPUART1 while sleeping, because the baud rate would be wrong.