ATmega64A datasheet(266/392 Pages) ATMEL

266 / 392 page

266

8160C–AVR–07/09

ATmega64A

25.5.4

Scanning the Clock Pins

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

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

Oscillator, and Ceramic Resonator.

Figure 25-7 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 out-

put 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 Oscillator is not scanned, as this

Oscillator does not have external connections.

Figure 25-7. Boundary-scan Cells for Oscillators and Clock Options

Table 25-3 summaries the scan registers for the external clock pin XTAL1, oscillators with

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

Note:

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 does not change run-time, the

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

Table 25-3.

Scan Signals for the Oscillators

(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

RCOSCEN

RCCK

External RC

OSC32EN

OSC32CK

Low Freq. External Crystal

TOSKON

TOSCK

32 kHz Timer Oscillator

From

Cell

ClockDR

ShiftDR

Cell

FF1

From

Cell

ClockDR

UpdateDR

ShiftDR

Cell

EXTEST

XTAL1/TOSC1

XTAL2/TOSC2

Oscillator

ENABLE

OUTPUT

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