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., TCNT0 for accessing Timer/Counter0 counter value.

The following definitions are used throughout the section:

Table 1. Definitions
Constant Description
BOTTOM The counter reaches the BOTTOM when it becomes zero (0x00 for 8-bit counters, or 0x0000 for 16-bit counters).
MAX The counter reaches its Maximum when it becomes 0xFF (decimal 255, for 8-bit counters) or 0xFFFF (decimal 65535, 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 OCR0A Register. The assignment is dependent on the mode of operation.