UART (Universal Asynchronous Receive-Transmit)

pins – RX, TX, GND          -              full duplex
one master, one slave – one to one
each uart device has a shift register, to covert serial data to parallel and parallel data to serial
separate interface devices are used to convert logic level signals, example max232
example of standard of voltage signaling is rs232
idle is high, to tell that line and transmitter or device at other end is not damaged
so, start bit is low
then 0 through 7 bits of data
and one or two stop bits, which are high
why extra stop bit sometimes? to correct the clock error
when low is sent more than a character time, it is treated as break condition, can be used as attention signal.
clock signal runs at multiple of data rate, typically 8 times the bit rate, in other words every bit is sampled atleast 8 times!
bit should last for atleast half of the bit time to be considered as a valid bit
uart hardware generally resynchronize their internal clocks on each change of data line or at the falling edge of the start bit
uart uses two different registers for transmit and receive
in usart synchronous mode, clock data is recovered from data stream and no start/stop bits are used
maximum standard serial port speed is 9600 bits per second if the operating system has a 1 millisecond interrupt latency
typically upto 256000 i.e. 256kbps/250k

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