Switchable serial bridge (Bluetooth or USB -> Telemetry)

Question

So I've got a simple serial bridge (wireless telemetry -> bluetooth).

Now what I would like to do is to be able to use either USB or bluetooth to access the telemetry module (Not at the same time). Also Ideally when using USB the power would come from USB, and when using bluetooth power will come from the battery.

I guess I'm going to need some way of isolating power from USB so it only powers the telemetry (I've got a switch to turn the whole thing on and off in between the battery and the regulator at the moment). Also stop the serial connection going the 'wrong way' from usb to bluetooth?

Is there a way to do this with just discrete components?

Thanks

Answer 1

Look at the Arduino UNO Rev 3 schematic. They have a similar setup with a pair of series resistors on the RX and TX between the USB IC and the main microcontroller, with a direct connection to any shields. This allows the shield to be the primary serial connection. So add a resistor between the Bluetooth IC and the telemetry IC. The USB IC will take precedence.

As for power, the Uno Rev 3 uses a comparator and a P-channel MOSFET to only allow one power source. Not completely discrete but close enough.