Array, hexadecimal and serial write

Question

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] = {
 0x121050,
 0x126500,
 0x120500,
 0x121700,
 0x121825,
 0x118700,
};

My concern is that when I do :

Serial.write(mem_freq[mem_index]);
//Serial.write(0x00);
//Serial.write(0x01);

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);
Serial.print(0x123456,HEX);
Serial.print(0x00,HEX);
Serial.print(0x01,HEX);

And it outputs:

12123412345601

Is this normal ?

• Juraj: use Serial.print(value, HEX) to print on Serial Monitor in hex form arduino.cc/reference/en/language/functions/communication/serial/…
• Juraj: 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
• jsotola: @Juraj, i think that the receiver hardware uses BCD and requires the above mentioned byte stream for correct operation

Answer 1

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

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

• Jibeji: Thanks Majenko. I have edited my post to show the result in ascii, and it is strange...
• Majenko: 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: Serial.print((0x123456 >> 16) & 0xFF,HEX);

Answer 2

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);
 ...