UART Parity Bit and Frame Structure

UART Frame Format

UART (Universal Asynchronous Receiver/Transmitter) sends data in a structured frame that includes the following components:

Start Bit: Always 0; indicates the beginning of the frame.
Data Bits: The actual payload, typically 5 to 8 bits.
Parity Bit (optional): Used for simple error detection.
Stop Bit(s): One or two bits set to 1; mark the end of the frame.

Parity Bit Modes

Even Parity: The parity bit is set such that the total number of 1s (including the parity bit) is even.
Odd Parity: The parity bit ensures the total number of 1s is odd.
No Parity: Parity bit is omitted.

Example: Even Parity Calculation

Let’s consider an example with 0x29:

Binary: 00101001 → Number of 1s = 3 (odd)
Even Parity: Add parity bit 1 → Total 1s = 4 (even)
✅ Result: For even parity, the parity bit for 0x29 is 1

Verilog Implementation with Reduction XOR

logic [7:0] tx_data = 8'b10101010;
parity_bit <= (parity_mode) ? ~^tx_data : ^tx_data;

Using the reduction XOR operator (^), you can easily calculate the parity:

^tx_data returns the XOR of all bits in tx_data
For even parity, parity_bit = ^tx_data
For odd parity, use ~^tx_data

Simulation Behavior

In the simulation waveform, you can clearly observe that the parity bit is transmitted immediately after the data bits. This follows the standard UART sequence.

Simulation waveform under even parity mode

RX-Side Parity Check Logic

parity_bit_rx   <= rx_line; // Received parity bit from UART frame

// Parity bit computed from received data
parity_bit_calc <= parity_mode ? ~^shift_reg : ^shift_reg; 

if (parity_bit_rx != parity_bit_calc)
    rx_error <= 1'b1; // Set error flag if parity doesn't match

The receiver recalculates the expected parity from the received data and compares it with the parity bit received over the wire. If the values differ, it flags a parity error.