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I need to send data from a processing sketch to an Arduino to pass to a function that takes a uint32_t. How to I send the data in Processing to be run on the Arduino

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I recommend studying up on bitwise shifts (<< and >>), along with the bitwise OR and AND. They're what you're going to need, especially since atoi() would only give you a 16 bit, signed integer.

Let's say that your variable's binary is this (split up into 4 segments):

10010010 10110101 01010010 10010100

Sending

uint32_t myVarCopy = myVar;
for(int i = 0; i < 4; i++) {
    Serial.write(myVarCopy & 255);
    myVarCopy = myVarCopy >> 8;
}

The first line in the loop modifies the variable by bitshifting right eight to allow us to read the data eight bits at a time. We have to, whenever sending data through the serial port, "trim" any data that is in the 9th digit or above using the & 255 part. This allows us to split this int up into four bytes.

How the myVarCopy variable would look over the four iterations:

After 1: 00000000 10010010 10110101 01010010 (sent 10010100)
After 2: 00000000 00000000 10010010 10110101 (sent 01010010)
After 3: 00000000 00000000 00000000 10010010 (sent 10110101)
After 4: 00000000 00000000 00000000 00000000 (sent 10010010)

Reading

uint32_t buffer = 0;
for(int i = 0; i < 4; i++) {
    buffer = (buffer >> 8) | (Serial.read() << 24);
}

This code works by adding the newest data onto the digits 25-32 after it moves everything else over eight bits.

This is how the buffer variable would look over the four iterations:

After 1: 10010100 00000000 00000000 00000000
After 2: 01010010 10010100 00000000 00000000
After 3: 10110101 01010010 10010100 00000000
After 4: 10010010 10110101 01010010 10010100

Depending on how Processing works, you might have to worry about little/big endian encoding to ensure that your data is in the right order. Warning: I haven't tested these yet as an IDE isn't installed on the computer I'm typing this on. Also, I don't know the exact function calls for Processing, so it might not compile.

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