How does a timer count?

How does a timer count? The answer to this question is very simple: A timer always counts up. It doesn’t matter whether the timer is being used as a timer, a counter, or a baud rate generator: A timer is always incremented by the microcontroller.

Programming Tip: Some derivative chips actually allow the program to configure whether the timers count up or down. However, since this option only exists on some derivatives it is beyond the scope of this tutorial which is aimed at the standard 8051. It is only mentioned here in the event that you absolutely need a timer to count backwards, you will know that you may be able to find an 8051-compatible microcontroller that does it.