Datasheet

ATmega323(L)

1457E–11/01

• Bits 2..0 - CS12, CS11, CS10: Clock Select1, Bit 2,1, and 0

The Clock Select1 bits 2, 1, and 0 define the prescaling source of Timer/Counter1.

The Stop condition provides a Timer Enable/Disable function. The prescaled modes are

scaled directly from the CK oscillator clock. If the external pin modes are used for

Timer/Counter1, transitions on PB1/(T1) will clock the counter even if the pin is config-

ured as an output. This feature can give the user SW control of the counting.

Timer/Counter1 – TCNT1H

and TCNT1L

This 16-bit register contains the prescaled value of the 16-bit Timer/Counter1. To

ensure that both the high and low bytes are read and written simultaneously when the

CPU accesses these registers, the access is performed using an 8-bit temporary regis-

ter (TEMP). This temporary register is also used when accessing OCR1A, OCR1B, and

ICR1. If the main program and also interrupt routines perform access to registers using

TEMP, interrupts must be disabled during access from the main program and interrupt

routines.

TCNT1 Timer/Counter1 Write When the CPU writes to the high byte TCNT1H, the written data is placed in the TEMP

with the byte data in the TEMP register, and all 16-bits are written to the TCNT1

Timer/Counter1 register simultaneously. Consequently, the high byte TCNT1H must be

accessed first for a full 16-bit register write operation.

TCNT1 Timer/Counter1 Read When the CPU reads the low byte TCNT1L, the data of the low byte TCNT1L is sent to

the CPU and the data of the high byte TCNT1H is placed in the TEMP register. When

the CPU reads the data in the high byte TCNT1H, the CPU receives the data in the

TEMP register. Consequently, the low byte TCNT1L must be accessed first for a full 16-

bit register read operation.

Table 19. Clock 1 Prescale Select

CS12 CS11 CS10 Description

0 0 0 Stop, the Timer/Counter1 is Stopped.

001CK

010CK/8

011CK/64

100CK/256

1 0 1 CK/1024

1 1 0 External Pin T1, Falling Edge

1 1 1 External Pin T1, Rising Edge

Bit 151413121110 9 8

$2D ($4D)

MSB TCNT1H

$2C ($4C) LSB TCNT1L

76543210

Read/Write R/W R/W R/W R/W R/W R/W R/W R/W

R/W R/W R/W R/W R/W R/W R/W R/W

Initial Value00000000

00000000