Is the teller value more than 1 byte. If it is and you are changing the value in an interrupt then you have to disable interrupts when you fetch the multibyte value in the mainline code. Otherwise the mainline code may be in the process of reading part of the multi-byte value when the interrupt comes along and changes parts that may have already been read.

This is a common opps when writing in C when it takes multiple machine instructions to access multi byte data types.

Michael Karas