Macro-Based Register Config Helper

Code

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

#define SET_ENABLE(val)   ((val & 0x01) << 0)
#define SET_MODE(val)     ((val & 0x03) << 1)
#define SET_SPEED(val)    ((val & 0x07) << 3)

uint16_t build_register(uint8_t enable, uint8_t mode, uint8_t speed) {
    uint16_t reg = 0;
    reg |= SET_ENABLE(enable);
    reg |= SET_MODE(mode);
    reg |= SET_SPEED(speed);
    // RESERVED bits (6–7) are left as 0
    return reg;
}

int main() {
    uint8_t enable, mode, speed;
    scanf("%hhu %hhu %hhu", &enable, &mode, &speed);
    printf("%u\n", build_register(enable, mode, speed));
    return 0;
}

Solving Approach

🧠 Solving Approach in 4 Clear Steps

1. Understand the Register Layout
   • Break down each field: number of bits, starting position, and constraints (e.g., reserved bits must be 0).
   • Example: MODE is 2 bits starting at bit 1 → mask = 0x03, shift = << 1.
2. Define Bitfield Macros

   • Use masking and shifting to isolate and position each field:

     #define SET_MODE(val) ((val & 0x03) << 1)
     

   • This ensures only valid bits are used and placed correctly.
3. Pack the Register Value

   • Use bitwise OR (|) to combine all fields into a single register:

     reg |= SET_ENABLE(enable) | SET_MODE(mode) | SET_SPEED(speed);
     

4. Validate Reserved Bits

   • Ensure reserved bits are untouched or explicitly cleared:

     reg &= ~(0x03 << 6);  // Clear bits 6–7
     

🔧 Embedded Best Practices

• Always use & masks to avoid overflow or invalid values.
• Use const or macros for field widths and positions to improve maintainability.
• Comment each macro with its field name and bit position.
• For complex registers, consider using bitfield structs (with caution for portability).

This approach works whether you're configuring TCCR0A, ADCSRA, or a custom peripheral register. Want to see how this maps to real datasheet specs or auto-generate macros from a register map? I can help with that too!