Question : A UART is configured to transmit 8 bit data, 1 start bit and 1 stop bit. The serial data output is observed on an oscilloscope,which looks like a square wave with frequency of 9600Hz. What is the baud rate and transmitted data?

Answer : The baud rate is 19200 and the data is 55h.

I don't know how to approach this problem. Kindly help and explain.
I only know how the data is 55h. Since the data is a square wave, there are alternating 0s and 1s. But if I start the data sequence with 01010101 , the data is 55. If I start the data sequence as 10101010, the data is AA. How to approach this problem?

 

Draw the waveform. Identify the start bit. Identify the first data bit. Is this lsb or msb? Go from there. Identify how long it takes to transmit one symbol. How many symbols are there per second. That's the baud rate.

 

An alternative approach is to draw the waveform that should result from transmitting 0x55, including the start an stop bits, at 19200 baud, being careful to follow the protocol. What would this look like on a scope. Do the same for 0xAA. What is the difference?