# V684. Value of variable is not modified. It is possible that '1' should be present instead of '0'. Consider inspecting the expression.

The analyzer has detected a suspicious expression which is used to change certain bits of a variable, but the variable actually remains unchanged.

Here is an example of suspicious code:

    MCUCR&=~(0<<SE);

This code is taken from the firmware for the ATtiny2313 microcontroller. The SE bit must be set to one so that the microcontroller switches to sleep mode when receiving the SLEEP command. To avoid accidental switch to sleep mode, it is recommended to set the SE bit to one immediately before calling the SLEEP command and reset it after wake-up. It is this reset on wake-up that the programmer wanted to implement. But he made a typo causing the value of the MCUCR register to remain unchanged. So it appears that although the program works, it is not reliable.

This is the fixed code:

    MCUCR&=~(1<<SE);

Note. Sometimes the V684 warning generates a set of multiple false positives. These are usually triggered by large and complex macros. See the corresponding section of the documentation to find out the methods of suppressing false positives in macros.

This diagnostic is classified as:

CWE-682

You can look at examples of errors detected by the V684 diagnostic.