You can use a typical trick used in cases as your, encoding multibyte character literals in a single unsigned int, that on Arduino uno and Mega 2560 is 16 bits in little endian format.
With reference to C standard ISO/IEC 9899:201x § "6.4.4.4 Character constants".
The subparagraph 10 explain our case:
An integer character constant has type int. The value of an integer
character constant containing a single character that maps to a
single-byte execution character is the numerical value of the
representation of the mapped character interpreted as an integer. The
value of an integer character constant containing more than one
character (e.g., 'ab'), or containing a character or escape sequence
that does not map to a single-byte execution character, is
implementation-defined. If an integer character constant contains a
single character or escape sequence, its value is the one that results
when an object with type char whose value is that of the single
character or escape sequence is converted to type int.
Where 'implementation-defined' in our case is managed as described below.
In this case the multibyte char constant 'bt' can be coded as the 16 bits integer 0x6274, where 'b'=0x62 and 't'=0x74.
The compiler should also be smart enough to convert the multibyte char sequence in the int value.
In the following snippet we consider that the char array msg holds the received message, and we use a simple and functional switch statement (that requires an integer value) casting the msg variable to an unsigned integer.:
char msg[10];
...
switch (*((unsigned int *)msg))
{
case 'tb': //Note the reverse order of command characters due to endianess
int value = atoi(msg+2); //Convert number to int
.... //do something
break;
.... //other cases
}
casting the msg pointer variable to a pointer to unsigned integer the compiler will interpret the first 2 chars as the integer in the way explained above, and act the switch based on their value.
The following sample use your code modified for use of a switch. It assumes that the command has a fixed length equal to MAX_MSG_LEN (2 characters for command, 2 characters for the value and the end of message):
// Bluetooth module used - HC-06
#include <SoftwareSerial.h>
SoftwareSerial BlueTooth(5, 6); // (TXD, RXD) of HC-06
#define MAX_MSG_LEN 5 //Max message length
#define OFFSET_TO_VALUE //Offset in input buffer to value
char BT_input[10]; // to store input characters received via BT.
void setup()
{
pinMode(13, OUTPUT); // Arduino Board LED Pin
BlueTooth.begin(9600);
}
void loop()
{
if (BlueTooth.available())
{
/*
* Read in the message up to the '!'
*/
int i=0;
do
{
BT_input[i] = (BlueTooth.read());
} while (i<MAX_MSG_LEN && BT_input[i++]!='!');
/*
* If message length is exactly what we expect
* we can process the message.
* Note that because of endianess the command
* chare are rversed.
*/
if (i == MAX_MSG_LEN)
{
switch (*((unsigned int *)BT_input))
{
case 'tb': // command 'bt'
process_brigthness(atoi(BT_input + OFFSET_TO_VALUE));
break;
case 'no': // command 'on'
{
digitalWrite(13, HIGH);
BlueTooth.println("Now LED is ON");
break;
}
case 'fo': // command 'of' for off
{
digitalWrite(13, LOW);
BlueTooth.println("Now LED is OFF");
break;
}
default: // unknown command
{
unknown_command();
break;
}
}
}
else
{
/*
* Process communication error
*/
communication_error();
}
}
}
Or using an union of the input stream and the command structure:
// Bluetooth module used - HC-06
#include <SoftwareSerial.h>
SoftwareSerial BlueTooth(5, 6); // (TXD, RXD) of HC-06
#define MAX_MSG_LEN 5 //Max message length
#define OFFSET_TO_VALUE //Offset in input buffer to value
union tag_BT_input // to store input characters received via BT.
{
char stream[MAX_MSG_LEN];
struct
{
unsigned int cmd; //Command
char val[2]; //value
char eom; //End of message marker '!'
}msg;
} BT_input;
void setup()
{
pinMode(13, OUTPUT); // Arduino Board LED Pin
BlueTooth.begin(9600);
}
void loop()
{
if (BlueTooth.available())
{
/*
* Read in the message up to the '!'
*/
int i=0;
do
{
BT_input.stream[i] = (BlueTooth.read());
} while (i<MAX_MSG_LEN && BT_input.stream[i++]!='!');
/*
* If message length is exactly what we expect
* we can process the message.
* Note that because of endianess the command
* chare are rversed.
*/
if (i == MAX_MSG_LEN)
{
switch (BT_input.msg.cmd)
{
case 'tb': // command 'bt'
process_brigthness(atoi(BT_input.msg.val));
break;
case 'no': // command 'on'
{
digitalWrite(13, HIGH);
BlueTooth.println("Now LED is ON");
break;
}
case 'fo': // command 'of' for off
{
digitalWrite(13, LOW);
BlueTooth.println("Now LED is OFF");
break;
}
default: // unknown command
{
unknown_command();
break;
}
}
}
else
{
/*
* Process communication error
*/
communication_error();
}
}
}
