ATMEGA169_14 Datasheet, PDF (80/365 Pages) ATMEL Corporation – Non-volatile Program and Data Memories

Definitions
Timer/Counter Clock
Sources
Counter Unit
event will also set the Compare Flag (OCF0A) which can be used to generate an Output
Compare interrupt request.
Many register and bit references in this section are written in general form. A lower case
“n” replaces the Timer/Counter number, in this case 0. A lower case “x” replaces the
Output Compare unit number, in this case unit A. However, when using the register or
bit defines in a program, the precise form must be used, i.e., TCNT0 for accessing
Timer/Counter0 counter value and so on.
The definitions in Table 48 are also used extensively throughout the document.
Table 48. Definitions
BOTTOM The counter reaches the BOTTOM when it becomes 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 OCR0A Register. The
assignment is dependent on the mode of operation.
The Timer/Counter can be clocked by an internal or an external clock source. The clock
source is selected by the Clock Select logic which is controlled by the Clock Select
(CS02:0) bits located in the Timer/Counter Control Register (TCCR0A). For details on
clock sources and prescaler, see “Timer/Counter0 and Timer/Counter1 Prescalers” on
page 93.
The main part of the 8-bit Timer/Counter is the programmable bi-directional counter unit.
Figure 28 shows a block diagram of the counter and its surroundings.
Figure 28. Counter Unit Block Diagram
DATA BUS
TOVn
(Int.Req.)
TCNTn
count
clear
direction
Control Logic
clk_Tn
bottom
top
Clock Select
Edge
Detector
Tn
( From Prescaler )
Signal description (internal signals):
count
Increment or decrement TCNT0 by 1.
direction Select between increment and decrement.
clear
Clear TCNT0 (set all bits to zero).
clk_Tn
top
Timer/Counter clock, referred to as clk_T0 in the following.
Signalize that TCNT0 has reached maximum value.
bottom Signalize that TCNT0 has reached minimum value (zero).
80 ATmega169/V
2514P–AVR–07/06