ATmega165 datasheet(219/336 Pages) ATMEL

219 / 336 page

219

ATmega165/V

2573G–AVR–07/09

Scanning the Clock Pins

The AVR devices have many clock options selectable by fuses. These are: Internal RC

Oscillator, External Clock, (High Frequency) Crystal Oscillator, Low-frequency Crystal

Oscillator, and Ceramic Resonator.

Figure 104 shows how each Oscillator with external connection is supported in the scan

chain. The Enable signal is supported with a general Boundary-scan cell, while the

Oscillator/clock output is attached to an observe-only cell. In addition to the main clock,

the timer Oscillator is scanned in the same way. The output from the internal RC Oscilla-

tor is not scanned, as this Oscillator does not have external connections.

Figure 104. Boundary-scan Cells for Oscillators and Clock Options

Table 86 summaries the scan registers for the external clock pin XTAL1, oscillators with

XTAL1/XTAL2 connections as well as 32kHz Timer Oscillator.

Notes:

1. Do not enable more than one clock source as main clock at a time.

2. Scanning an Oscillator output gives unpredictable results as there is a frequency drift

between the internal Oscillator and the JTAG TCK clock. If possible, scanning an

external clock is preferred.

3. The clock configuration is programmed by fuses. As a fuse is not changed run-time,

the clock configuration is considered fixed for a given application. The user is advised

to scan the same clock option as to be used in the final system. The enable signals

are supported in the scan chain because the system logic can disable clock options

in sleep modes, thereby disconnecting the Oscillator pins from the scan path if not

provided.

Table 86. Scan Signals for the Oscillator

(1)(2)(3)

Enable Signal

Scanned Clock Line

Clock Option

Scanned Clock

Line when not

Used

EXTCLKEN

EXTCLK (XTAL1)

External Clock

OSCON

OSCCK

External Crystal

External Ceramic Resonator

OSC32EN

OSC32CK

Low Freq. External Crystal

From

Cell

ClockDR

ShiftDR

Cell

To System Logic

FF1

From

Cell

ClockDR

UpdateDR

ShiftDR

Cell

EXTEST

From Digital Logic

XTAL1/TOSC1

XTAL2/TOSC2

Oscillator

ENABLE

OUTPUT

ATmega165 Datasheet(PDF) 219 Page - ATMEL Corporation

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