The HCI UART is the only interface on the BCM20737S that can be used to program the device (physical pins 18/19)
Attention should be paid to the HCI UART Rx line as the state of this line during reset determines whether or not the part comes up in Application or Programming mode.
WICED Sense uses the SiP module, so follow the reference schematic for it (TAG3 board uses the SoC): WICED Sense Reference Design Schematic (PDF)
Additionally, there is a pre-compiled minidriver required for programming, so you will need to use the SDK or the command line tools described here: Programming the TAG2/TAG3 Board using command line tools
On a custom board, you will most likely not implement the Serial to USB converter that's used on the TAG3, and in this case, you will want to purchase the EXACT FTDI external serial to USB cable that is recommended here on the forum (programming thread noted above): http://www.mouser.com/ProductDetail/FTDI/TTL-232RG-VREG3V3-WE/?qs=D1%2FPMqvA103YESiuBsLf1Q%3D%3D
