ATmega88 datasheet(136/340 Pages) ATMEL

136 / 340 page

136

7530H–AVR–02/09

ATmega48/88/168 Automotive

15.1.2

Definitions

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 the following table are also used extensively throughout the section.

15.2

Timer/Counter Clock Sources

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, clkI/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 154. For details on clock sources and prescaler, see

“Timer/Counter Prescaler” on page 155.

15.3

Counter Unit

The main part of the 8-bit Timer/Counter is the programmable bi-directional counter unit. Figure

15-2 shows a block diagram of the counter and its surrounding environment.

Figure 15-2. Counter Unit Block Diagram

Signal description (internal signals):

count

Increment or decrement TCNT2 by 1.

direction

Selects between increment and decrement.

clear

Clear TCNT2 (set all bits to zero).

clk

Timer/Counter clock, referred to as clk

T2 in the following.

top

Signalizes that TCNT2 has reached maximum value.

bottom

Signalizes that TCNT2 has reached minimum value (zero).

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 OCR2A Register. The assignment is depen-

dent on the mode of operation.

DATA BUS

TCNTn

Control Logic

count

TOVn

(Int.Req.)

top

bottom

direction

clear

TOSC1

T/C

Oscillator

TOSC2

Prescaler

clk

I/O

clk

ATmega88 Datasheet(PDF) 136 Page - ATMEL Corporation

ATmega88 Datasheet(HTML) 136 Page - ATMEL Corporation

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 ...More

Datasheet Download

Link URL