Hi Saji,

The answer to your question is: A framing error with data 0x66 (assuming the standard 1 start, 1 stop, no parity and 8 data bits).

Remember that the start bit is a high to low transition. It will assume it is sync'd to the data then. A 2x square wave will then present a low to the UART when it is expecting a high stop bit.

Dennis