FSC-BT836B, how to get this modules enter low power mode.
The LED pin(#32) will turn to low when the module enters sleep mode. This doesn't mean the module has lost the connection with remote bluetooth.
When module enters sleep, the first packet to be sent to the module through UART is just for wake up, will be discarded. So send a dummy packet before you do normal communication.
Timing:send a dummy packet, wait 100ms, do normal communication.
In the other direction, the first packet from remote bluetooth device could also wake up the module and the first packet would be sent to your MCU correctly, it wouldn't be discarded.