ATtiny88 datasheet(26/266 Pages) ATMEL

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8008C–AVR–03/09

ATtiny48/88

6.1.4

Analog to Digital Converter Clock – clk

ADC

The ADC is provided with a dedicated clock domain. This allows halting the CPU and I/O clocks

in order to reduce noise generated by digital circuitry. This gives more accurate ADC conversion

results.

6.1.5

High-Speed Two-Wire Interface Clock – clk

TWIHS

The TWI clock controls the operation of the Two-Wire Interface module, when operated in high-

speed mode. In practice, this clock is identical to the source clock of the device. See “Bit Rate

Generator Unit” on page 130.

6.2

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.

6.2.1

Default Clock Source

The device is shipped with internal oscillator at 8.0 MHz and with the fuse CKDIV8 programmed,

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

enabled (CKSEL = 0b10, SUT = 0b10, CKDIV8 = 0). The default setting ensures that all users

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

6.2.2

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. “System Control and Reset” on page 39

describes the start conditions for the internal reset. The delay (t

TOUT) is timed from the Watchdog

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

selectable delays are shown in Table 6-2. The frequency of the Watchdog oscillator is voltage

Table 6-1.

Device Clocking Options

(1)

CKSEL1..0

Device Clocking Option

External Clock

Reserved

Calibrated Internal Oscillator

Internal 128 kHz Oscillator

ATtiny88 Datasheet(PDF) 26 Page - ATMEL Corporation

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