[Mike Betts]

Originally Posted by mightywombat

What does the volatile declaration mean?
ie (from kevin.org/frc timer demo)

volatile unsigned long msClock =0;

Volatile is an ANSI C storage qualifier that tells the compiler that a value in that storage location can not be trusted.

From the MPLAB® C18 C Compiler User’s Guide:

2.9.2.2 INTERRUPT SERVICE ROUTINES
...
Global variables that are accessed by both an ISR and mainline functions should be declared volatile.

What does this mean? Let's assume you are doing a simple math operation. One operand is in a register and the other is a memory location. The compiler will not use the memory location for a volatile variable. This becomes very important for a complex mathematical line such as:

x = x * m + b; // where x is a variable modifiable by an ISR.

In this example, if x is stored with x*m and the ISR modifies x before x+b occurs, you get the wrong answer... The volatile keyword prevents this from happening.

Regards,