AT43USB353M datasheet(41/95 Pages) ATMEL

41 / 95 page

AT43USB353M

3307B–USB–4/04

The Stop condition provides a Timer Enable/Disable function. The CK down divided modes

are scaled directly from the 12 MHz input clock.

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 register (TEMP). This tem-

porary 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 dis-

abled during access from the main program and from interrupt routines if interrupts are

allowed from within interrupt routines.

• TCNT1 Timer/Counter1 Write:

When the CPU writes to the high byte TCNT1H, the written data is placed in the TEMP regis-

ter. Next, when the CPU writes the low byte TCNT1L, this byte of data is combined with the

byte data in the TEMP register, and all 16 bits are written to the TCNT1 Timer/Counter1 regis-

ter 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.

The Timer/Counter1 is realized as an up or up/down (in PWM mode) counter with read and

wri te a ccess . If T i m er/Co un ter1 is w ritte n to a nd a cl ock sou rce i s se le cted , th e

Timer/Counter1 continues counting in the timer clock cycle after it is preset with the written

value.

Bit

$2D ($4D)

MSB

–

TCNT1H

$2C ($4C)

––

–

LSB

TCNT1L

Read/Write

R/W

Initial Value

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