Parity Checking

Parity Checking I

In modern computers, we represent data digitally because doing so provides more resistance to noise than analog representations do. However, it doesn't mean errors don't occur, and often times when two devices communicate digitally they will implement some form of error-checking. This lets each device determine if the data it recieved is invalid, if so, either discard it or ask it to be retransmitted. Depending on the method, some forms of error-checking can also serve as error-correction - where the error is not only detected, but can also be fixed.

One of the simplest error-checking techniques is the addition of a parity bit with a message.

Ahead of time, the transmitting and receiving parties will have agreed to use either even or odd parity in their messages. This means both devices expect each message to have either an even or odd number of 1's, depending on their choice of even or odd parity. To comply with this, each devices adds a 'parity bit' to the end of their message. This parity bit is set to 0 or 1 in order to match the even/odd parity agreement made earlier. Here's some examples:

• If both devices agree to use even parity, and one wants to send the data 'b1111001, it will set the parity bit to 1 to have an even number of 1's in the message. The resulting message will be 'b11110011.
• If both devices agree to use odd parity, and one wants to send the data 'b1100001, it will set the parity bit to 0 to have an odd number of 1's in the message. The resulting message will be 'b11000010.

You'll write logic to check the parity of these messages by completing the parity_checker module below with strictly combinational logic. The module outputs a single bit parity_out, which is 1 if the 8-bit input data_in has even parity, and 0 if it has odd parity.

Note this same function could be used to both generate and check the parity bit of a message.

module parity_checker( input wire[7:0] data_in, output logic parity_out); endmodule

Loading...

Parity Checking II

This is great! However, there's a bunch of protocols that implement parity-checking, and they use messages of a variety of widths. To make our module more portable, let's refactor it to accept an input of an arbitrary width.

Refactor parity_checker below so that it has a parameter WIDTH that dictates how large the the input (data_in) port is. Have it default to a width of 8 bits. parity_out should still just be one bit, regardless of the size of the input.

Loading...