When using Arduino, you often want to print the hex value of a 64-bit value such as a uint64_t, consisting of sixteen hex digits. For example, if you have uint64_t val = 169557370125;, you intend to print 000000277a68250d.

In Arduino you can do that using Serial.printf() with %08lx08lx as format specifier, splitting the uint64_t in two uint32_t instances and printing those one after another:

Serial.printf("%08lx%08lx\r\n",
    ((uint32_t)((val >> 32) & 0xFFFFFFFF)),
    ((uint32_t)(val & 0xFFFFFFFF)));

See How to print 32-bit uint32_t as eight hex digits in Arduino for more information on what %x means and why we need to use the 08 in %08x as a printf format specifier.

When using Arduino, you often want to print the hex value of a 32-bit value such as a uint32_t, consisting of eight hex digits. For example, if you have uint32_t val = 9177025;, you intend to print 008C07C1.

In Arduino you can do that using Serial.printf() with %08lx as format specifier. Furthermore, you typically want to invert the byte order of the uint32_t using __builtin_bswap32()since it’s more inuitive to write the hex value MSB-first (big-endian) while most hardware platforms represent the uint32_t as LSB-first (little-endian):

Serial.printf("val = %08lx\r\n", __builtin_bswap32(val));

When using printf, %x means to format the value as hex whereas l tells printf() that the argument is a long (32 bit) as opposed to an int (16 bit). 08 means to pad the value with 0s up to a length of 8 digits. If you would format 9177025 using just %x, it would print 008C07C1  instead of 008C07C1. This is why you need to use %08lx instead.

When using Arduino, you often want to print the hex value of a 16-bit value such as a uint16_t, consisting of four hex digits. For example, if you have uint16_t val = 2022;, you intend to print 07E6.

In Arduino you can do that using Serial.printf() with %04x as format specifier.

Furthermore, you typically want to invert the byte order of the uint16_t using __builtin_bswap16()since it’s more inuitive to write the hex value MSB-first (big-endian) while most hardware platforms represent the uint16_t as LSB-first (little-endian):

Serial.printf("val = %04x\r\n", __builtin_bswap16(val));

When using printf, %x means to format the value as hex. 04 means to pad the value with 0s up to a length of 4 digits. If you would format 2022 using just %x, it would print 7E6  instead of 07E5. This is why you need to use %04x instead.

In embedded programming, you often want to print the hex value of a byte, consisting of two hex digits. For example, if you have uint8_t val = 14;, you intend to print 0x0E.

In Arduino you can do that using Serial.printf() with %02x as format specifier:

Serial.printf("val = %02x\r\n", val);

When using printf, %x means to format the value as hex. 02 means to pad the value with 0s up to a length of 2 digits. If you would format 14 using just %x, it would print E  instead of 0E. This is why you need to use %02x.

The correct order to call Wire commands in Arduino is:

Wire.beginTransmission(MY_I2C_ADDR);
Wire.write(addr);
Wire.endTransmission();
Wire.requestFrom(MY_I2C_ADDR, 1); // Request one byte
delay(5); // Wait for data to be available
uint8_t value = Wire.read();

So you call Wire.endTransmission() after Wire.write() and call Wire.requestFrom() directly after Wire.endTransmission()