116. Convert Decimal Number to Binary or Hex Without itoa function

Why This Concept Matters in Firmware

In embedded systems, we often need to:

• Display values on LCD/UART
• Transmit raw bytes in binary/hex form
• Parse input commands from strings
• Convert numbers to strings or vice versa without using itoa(), sprintf(), or scanf() (due to memory/speed constraints)

This guide covers how numbers, characters, and byte values are represented and converted — all without library functions.

 Understanding Number Systems

Firmware Tip: You must think in hex/binary when dealing with hardware like registers, communication, or displays.

Integer to ASCII (Decimal) String Conversion

Goal: Convert an integer like 123 → '1' '2' '3'

int num = 123;
char str[5];
int i = 0;

while (num > 0) {
    str[i++] = (num % 10) + '0';  // Convert to ASCII
    num /= 10;
}

// str now contains reverse digits

🧠 ASCII Hack: '0' = 48 → So to convert digit 5 → char '5', just do:

'5' = 5 + '0' = 53

Binary/Hex String to Integer (Parsing)

a) Binary string to int

char *s = "1010";
int val = 0;
for (int i = 0; s[i]; i++)
    val = (val << 1) | (s[i] - '0');  // Bit-by-bit build

b) Hex string to int (e.g., “1A3”)

char *s = "1A3";
int val = 0;
for (int i = 0; s[i]; i++) {
    char c = s[i];
    int digit = (c >= 'A') ? (c - 'A' + 10) : (c - '0');
    val = (val << 4) | digit;
}

Integer to Binary / Hex Output

a) Binary

for (int i = 7; i >= 0; i--)
    printf("%d", (n >> i) & 1);

b) Hex output

char hex[5];
sprintf(hex, "%X", n); // But in firmware, use custom logic

For small 8-bit numbers:

char hex_digit = (value < 10) ? ('0' + value) : ('A' + value - 10);

Convert String to Float (Without atof)

Break into integer and decimal parts manually:

char *s = "123.45";
int int_part = 0;
float frac_part = 0.0f;
int i = 0;

// Parse integer part
while (s[i] != '.' && s[i]) {
    int_part = int_part * 10 + (s[i++] - '0');
}
i++; // skip '.'

// Parse fractional part
float factor = 0.1f;
while (s[i]) {
    frac_part += (s[i++] - '0') * factor;
    factor *= 0.1f;
}

float result = int_part + frac_part;

Nibble Extraction (from 8-bit Register)

A nibble = 4 bits

uint8_t reg = 0xAB;
uint8_t high = (reg >> 4) & 0x0F; // 0xA
uint8_t low  = reg & 0x0F;        // 0xB

Often used in:

• BCD displays
• 7-segment decoding
• Decoding byte-encoded fields
   

Firmware Relevance

Common Pitfalls