CC2640R2F: BTools error - GAP Device Initialization Timeout

Hi, I am using CC2640R2F in one of my circuit design. FYI, the UART_TX and UART_RX of my CC2640R2F is connected to the UART_RX and UART_TX of an MCU. When I tried to launch the BTool (with the correct COM port setting and the use of level shifter board), I got the following error message:

GAP Device Initialization Timeout.

Device May Not Function Properly.

Also, I notice that the current drops from 25 mA to 9 mA (at 3.3V) when I launched BTool. I assume 25 mA is the total current needed to power both CC2640R2F and the MCU. 

However, when I isolated my CC2640R2F from the MCU (i.e. removing the UART connection between them), I manage to launch the BTool successfully. May I know why did this happen? FYI, my design requires the UART (TX and RX) of MCU and CC2640R2F to be connected.

Please advise.

Thanks.

Best Regards,

Zhi Wei