Many register and bit references in this section are written in general form: However, when using the register or bit definitions in a program, the precise form must be used, i.e., TCNT3 for accessing Timer/Counter 3 counter value.

The following definitions are used extensively throughout the section:

Table 1. Definitions
BOTTOM The counter reaches the BOTTOM when it becomes zero (0x0 for 8-bit counters, or 0x00 for 16-bit counters).
MAX The counter reaches its MAXimum when it becomes 0xF (decimal 15, for 8-bit counters) or 0xFF (decimal 255, for 16-bit counters).
TOP The counter reaches the TOP when it becomes equal to the highest value in the count sequence. The TOP value can be assigned to be the fixed value MAX or the value stored in the OCRnA Register. The assignment is dependent on the mode of operation.