I am trying to write a pice of code which send to a receiver the frequency to listen.

As per its documentation, it should receive the following sequence:

Example with 123.456 MHz

Command to send (frequency converted in 4 bytes, followed by 2 instruction bytes

0x12 0x23 0x56 0x00 0x01

To be able to memorize six frequencies, I fill an array:

uint32_t mem_freq[6] = {

My concern is that when I do :


Only 1 byte is sent, and it's hard to debug as the serial console displays only ascii characters.

When I use Serial.print instead, mem_freq[2] shows correctly 0x120500

After Majenko's answer, I tried this:

Serial.print(0x123456 >> 16,HEX);
Serial.print(0x123456 >> 8,HEX);

And it outputs:


Is this normal ?

  • use Serial.print(value, HEX) to print on Serial Monitor in hex form arduino.cc/reference/en/language/functions/communication/serial/…
    – Juraj
    Commented Feb 26, 2018 at 18:32
  • encoding 123.456 as 0x12 0x23 0x56 is very exotic. how do you use the frequency? I would send it as decimal number 123456. It fits into uint32_t
    – Juraj
    Commented Feb 26, 2018 at 18:36
  • @Juraj, i think that the receiver hardware uses BCD and requires the above mentioned byte stream for correct operation
    – jsotola
    Commented Feb 26, 2018 at 19:08

2 Answers 2


You need to manually split your values into bytes. You can either do that by storing them as individual bytes (as shown by Ignacio), or by using bit shifting:

Serial.write(mem_freq[mem_index] >> 16);
Serial.write(mem_freq[mem_index] >> 8);

That sends the bits 16-23 (0x12) then bits 8-15 (0x10) followed by bits 0-7 (0x50).

  • Thanks Majenko. I have edited my post to show the result in ascii, and it is strange...
    – Jibeji
    Commented Feb 26, 2018 at 20:33
  • Since you have switched from write to print it can now print the entire number every time. If you want to print it you should also mask the value with & 0xFF.
    – Majenko
    Commented Feb 26, 2018 at 20:34
  • Serial.print((0x123456 >> 16) & 0xFF,HEX);
    – Majenko
    Commented Feb 26, 2018 at 20:35

That's because you need to use an array of arrays, and send three bytes.

uint32_t mem_freq[6][3] = {
  {0x12, 0x10, 0x50},

  Serial.write(mem_freq[mem_index], 3);

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