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CHAPTER 11 SERIAL INTERFACE
145
(b) Parity types and operation
The parity bit is used to detect a bit error in the communication data. Normally, the same parity bit is
used on the transmitting side and the receiving side. With even parity and odd parity, a one-bit (odd
number) error can be detected. With 0 parity and no parity, an error cannot be detected.
(i)
Even parity
At transmission
The transmission operation is controlled so that the number of bits with a value of "1" in the
transmission data including parity bit may be even. The parity bit value should be as follows.
The number of bits with a value of "1" is an odd number in transmission data : 1
The number of bits with a value of "1" is an even number in transmission data: 0
At reception
The number of bits with a value of "1" in the reception data including parity bit is counted, and if
the number is odd, a parity error is generated.
(ii) Odd parity
At transmission
Conversely to the even parity, the transmission operation is controlled so that the number of bits
with a value of "1" in the transmission data including parity bit may be odd. The parity bit value
should be as follows.
The number of bits with a value of "1" is an odd number in transmission data : 0
The number of bits with a value of "1" is an even number in transmission data: 1
At reception
The number of bits with a value of "1" in the reception data including parity bit is counted, and if
the number is even, a parity error is generated.
(iii) 0 parity
When transmitting, the parity bit is set to "0" irrespective of the transmission data.
At reception, a parity bit check is not performed. Therefore, a parity error is not generated,
irrespective of whether the parity bit is set to "0" or "1".
(iv) No parity
A parity bit is not added to the transmission data. At reception, data is received assuming that
there is no parity bit. Since there is no parity bit, a parity error is not generated.