37. Construct UART Data Frame with Parity Bit

#include <stdio.h>
#include <stdint.h>

// UART Control Register Struct: 8-bit total
typedef struct {
    uint8_t parity_enable : 1;   // 0 = disabled, 1 = enabled
    uint8_t parity_type   : 1;   // 0 = even, 1 = odd
    uint8_t reserved      : 6;   // unused
} UART_Control;

// Count number of 1s in lower 7 bits of data
int count_ones(uint8_t data) {
    int count = 0;
    for (int i = 0; i < 7; i++) {
        if (data & (1 << i)) {
            count++;
        }
    }
    return count;
}

// Construct UART frame based on control register settings
uint8_t create_uart_frame(uint8_t data, UART_Control *ctrl) {
    data = data & 0x7F;  // Ensure 7-bit data

    if (ctrl->parity_enable == 0) {
        return data;  // No parity, MSB is 0
    }

    int ones = count_ones(data);
    int parity_bit;

    if (ctrl->parity_type == 0) {
        // Even parity: parity bit is 1 if ones are odd
        parity_bit = (ones % 2 == 0) ? 0 : 1;
    } else {
        // Odd parity: parity bit is 1 if ones are even
        parity_bit = (ones % 2 == 0) ? 1 : 0;
    }

    return (parity_bit << 7) | data;  // Frame = parity_bit at MSB + data
}

int main() {
    uint8_t data;
    scanf("%hhu", &data);  // 7-bit input

    uint8_t parity_enable, parity_type;
    scanf("%hhu %hhu", &parity_enable, &parity_type);

    UART_Control ctrl;
    ctrl.parity_enable = parity_enable;
    ctrl.parity_type = parity_type;

    uint8_t frame = create_uart_frame(data, &ctrl);
    printf("frame = 0x%02X", frame);

    return 0;
}

• data & 0x7F masks data to 7 bits.
• 1s are counted using loop to determine parity.
• Parity bit is calculated based on parity type.
• Final frame is parity_bit << 7 | data.