I don't know Flash Magic being I've never had a need to use it, but if it is able to keep sending out its stream of U's for a couple hundred of milliseconds whilst it looks for an echo then you could try this suggestion...

Use two of the reset generators where the first one is triggered by the RTS line from the Flash Magic PC. The output of that reset generator connects to the MCU reset line. Connect the 2nd reset generator so that it triggers when the first one times out. Add a gate to the serial line that carries the U's to the MCU and drive the other side of the gate with the output of the second reset generator. This will mask the serial stream to the MCU till a couple hundred msec after the reset is complete.

You could build a more robust version of this by OR'ing the outputs of both reset generators together before applying the result to the serial stream gate.

Make sure to select the gating logic such that the serial data in line to the MCU is forced to a stop-bit-level whenever the gate logic is in the cut-off state.

:-)

Michael Karas