ATMEGA8-16AI SL383 Atmel, ATMEGA8-16AI SL383 Datasheet - Page 105

• Manufacturer Part Number: ATMEGA8-16AI SL383
• Description: Semiconductors and Actives, Microprocessors, Microcontrollers
• Manufacturer: Atmel

Definitions
Timer/Counter
Clock Sources
2486W–AVR–02/10
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
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. Definitions
The Timer/Counter can be clocked by an internal synchronous or an external asynchronous
clock source. The clock source clk
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
chronous Status Register – ASSR” on page
“Timer/Counter Prescaler” on page
BOTTOM
MAX
TOP
The counter reaches the BOTTOM when it becomes zero (0x00).
The counter reaches its MAXimum when it becomes 0xFF (decimal 255).
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.
T2
).
Table 41
are also used extensively throughout the document.
T2
123.
is by default equal to the MCU clock, clk
119. For details on clock sources and prescaler, see
ATmega8(L)
I/O
. When the AS2
“Asyn-
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