AT90USB82 datasheet(29/306 Pages) ATMEL

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7707D–AVR–07/08

AT90USB82/162

6.2.5

Clock Status Register – CLKSTA

• Bit 7-2 - Reserved bits

These bits are reserved and will always read as zero.

• Bit 1 – RCON: RC Oscillator On

This bit is set by hardware to one if the RC Oscillator is running.

This bit is set by hardware to zero if the RC Oscillator is stoped.

• Bit 0 – EXTON: External Oscillator / Low Power Oscillator On

This bit is set by hardware to one if the External Oscillator / Low Power Oscillator is running.

This bit is set by hardware to zero if the External Oscillator / Low Power Oscillator is stoped.

6.3

Clock Sources

The device has the following clock source options, selectable by Flash Fuse bits as shown

below. The clock from the selected source is input to the AVR clock generator, and routed to the

appropriate modules.

Note:

1. For all fuses “1” means unprogrammed while “0” means programmed.

6.3.1

Default Clock Source

The device is shipped with internal RC oscillator at 8.0 MHz and with the fuse CKDIV8 pro-

grammed, resulting in 1.0 MHz system clock. The startup time is set to maximum and time-out

period enabled. (CKSEL = "0010", SUT = "10", CKDIV8 = "0"). The default setting ensures that

all users can make their desired clock source setting using any available programming interface.

6.3.2

Clock Startup Sequence

Any clock source needs a sufficient V

CC to start oscillating and a minimum number of oscillating

cycles before it can be considered stable.

To ensure sufficient V

CC, the device issues an internal reset with a time-out delay (tTOUT) after

the device reset is released by all other reset sources. “On-chip Debug System” on page 45

describes the start conditions for the internal reset. The delay (tTOUT) is timed from the Watchdog

Oscillator and the number of cycles in the delay is set by the SUTx and CKSELx fuse bits. The

Bit

654

RCON

EXTON

CLKSTA

Read/Write

Initial Value

See Bit Description

Table 6-1.

Device Clocking Options Select

(1)

Device Clocking Option

CKSEL3..0

Low Power Crystal Oscillator

1111 - 1000

Reserved

0111 - 0110

Reserved

0101 - 0100

Reserved

0011

Calibrated Internal RC Oscillator

0010

External Clock

0000

Reserved

0001

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