Best way to pass variables between Arduino and PC over serial? - c++

Question

I've written an Arduino sketch which reads data from a remote control receiver and returns a value between 0 and 1023 for that channel. I basically want to send this data (something in the format of channel:value, eg, Channel 1 : 1023, Channel 2 : 511) to a PC program (which I plan to write myself).
The most efficient way I can think to do this is to use two bytes of data, with the first 6 bits representing the channel (2^6 = 64 possible channels, way more than I need), and the last ten representing the value (2^10 = 1024, perfect). But I'm not sure on the best way to implement this in C++, or if this is even the most ideal way to do this. So:
What is the best way to craft individual bytes and work with binary numbers in C++? Preferably storing the values in memory as such (ie, no bool arrays, where each index takes up it's own byte). Two bytes of data is more than enough for what I need.
Is this the easiest/simplest/most efficient/recommended way to implement what I am trying to achieve? I basically want to pass variables as is between programs, are there any other ways to do this?

Answer 1

You can declare a packed struct to hold these two values:
struct chan_value_t
{
uint8_t channel : 6;
uint16_t value : 10;
};
But to send it as two bytes, you'll need to either (1) "union" it with a two-byte array:
union chan_value_t
{
struct {
uint8_t channel : 6;
uint16_t value : 10;
};
uint8_t bytes[2];
};
chan_value_t cv;
void setup()
{
Serial.begin( 9600 );
cv.channel = 2;
cv.value = 800;
for (int i=0; i<sizeof(cv.bytes); i++) {
Serial.print( cv.bytes[i], HEX );
Serial.print( ' ' );
}
Serial.println();
}
void loop() {}
(The struct is anonymous when nested in this union; the union has the name.)
Or (2) cast a pointer to the struct to a pointer to bytes:
struct chan_value_t {
uint8_t channel : 6;
uint16_t value : 10;
};
chan_value_t cv;
void setup()
{
Serial.begin( 9600 );
cv.channel = 2;
cv.value = 800;
uint8_t *bytes = (uint8_t *) &cv; // cast &cv to a pointer to bytes
for (int i=0; i<sizeof(cv); i++) {
Serial.print( bytes[i], HEX );
Serial.print( ' ' );
}
Serial.println();
}
void loop() {}
They both print the hexadecimal value of the bytes: 0x02 and 0xC8. 800 is 0x320, shifted left by 6 bits is 0xC800.
To send this to the PC, you may want to start with a special character sequence and finish with a checksum of some sort (Fletcher checksum is easy). Then it's easy to throw away garbage characters and know when there are transmission errors.

Answer 2

This is aimed at your no. 2 question.
OSC (OpenSoundControl) is a convenient way to send messages across different platforms and devices. Libraries exist for most platforms.
You could use the library OSC for Arduino and implement your own solution to the specification or using a library that fits your context.
The message you mention could be sent as /channel/1 /value/1023