Designing Embedded Hardware, 2nd Edition

Error Detection

In any transfer of data over a potentially noisy medium (such as a serial cable), the possibility of errors exists. To detect such errors, many serial systems implement parity as a simple check for the validity of the data. The parity bit of a byte to be transmitted is calculated by the sending UART and included with the byte as part of the transmission. The receiving UART also calculates the parity bit for the byte and compares this against the parity bit received. If they match, the receiver assumes that everything is fine. If they do not, the receiver then knows that something went amiss and that an error exists.

There are several types of parity, the main two being even parity and odd parity . In any byte of data, there is either an even number of "1" bits or an odd number of "1" bits. An extra bit (the parity bit) is added to the byte to make the number of "1" bits even (even parity) or odd (odd parity). For successful transmission, both the receiver and transmitter must be set for the same type of parity generation. There is no protocol for establishing common parity settings between UARTs; it must be done manually at either end.

So for the binary sequence %01000000, the parity bit would be "1" for even parity and "0" for odd parity. Similarly, for %11111111, the parity bit would be "0" if we were using even parity and "1" if we had odd parity. The generation and detection of parity is done automatically by dedicated hardware within the UART. It's not something ...