When waiting for EC25, 0xff is received

Hi there,

When my EC25 module first connects, it outputs “RDY” and prepares,
but if I keep waiting, it randomly sends 0xff. (EC25 → My cpu direction)
What kind of problem is this?
Not all modules are like this.

Please disconnect the mcu from the EC25 and separately test the USB or UART port of the module to see if there are still abnormal output characters. If yes, it may be due to an abnormal adaptation of the MCU to the EC25 or an abnormal drive