ATMEGA8_08 Datasheet, PDF (105/308 Pages) ATMEL Corporation – 8-bit with 8K Bytes In-System Programmable Flash

Definitions
Timer/Counter
Clock Sources
ATmega8(L)
All interrupts are individually masked with the Timer Interrupt Mask Register (TIMSK). TIFR and
TIMSK are not shown in the figure since these registers are shared by other timer units.
The Timer/Counter can be clocked internally, via the prescaler, or asynchronously clocked from
the TOSC1/2 pins, as detailed later in this section. The asynchronous operation is controlled by
the Asynchronous Status Register (ASSR). The Clock Select logic block controls which clock
source the Timer/Counter uses to increment (or decrement) its value. The Timer/Counter is inac-
tive when no clock source is selected. The output from the clock select logic is referred to as the
timer clock (clk_T2).
The double buffered Output Compare Register (OCR2) is compared with the Timer/Counter
value at all times. The result of the compare can be used by the waveform generator to generate
a PWM or variable frequency output on the Output Compare Pin (OC2). For details, see “Output
Compare Unit” on page 107. The Compare Match event will also set the Compare Flag (OCF2)
which can be used to generate an Output Compare interrupt request.
Many register and bit references in this document are written in general form. A lower case “n”
replaces the Timer/Counter number, in this case 2. However, when using the register or bit
defines in a program, the precise form must be used (i.e., TCNT2 for accessing Timer/Counter2
counter value and so on).
The definitions in Table 41 are also used extensively throughout the document.
Table 41. Definitions
BOTTOM The counter reaches the BOTTOM when it becomes zero (0x00).
MAX
The counter reaches its MAXimum when it becomes 0xFF (decimal 255).
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 0xFF (MAX) or the value stored in the OCR2 Register. The
assignment is dependent on the mode of operation.
The Timer/Counter can be clocked by an internal synchronous or an external asynchronous
clock source. The clock source clk_T2 is by default equal to the MCU clock, clk_I/O. When the AS2
bit in the ASSR Register is written to logic one, the clock source is taken from the Timer/Counter
Oscillator connected to TOSC1 and TOSC2. For details on asynchronous operation, see “Asyn-
chronous Status Register – ASSR” on page 119. For details on clock sources and prescaler, see
“Timer/Counter Prescaler” on page 123.
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