
248
ATmega64A [DATASHEET]
8160D–AVR–02/2013
26.5.4
Scanning the Clock Pins
The AVR devices have many clock options selectable by fuses. These are: Internal RC Oscillator, External RC,
External Clock, (High Frequency) Crystal Oscillator, Low-frequency Crystal Oscillator, and Ceramic Resonator.
Figure 26-7 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 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
Oscillator is not scanned, as this Oscillator does not have external connections.
Figure 26-7. Boundary-scan Cells for Oscillators and Clock Options
Table 26-3 summaries the scan registers for the external clock pin XTAL1, oscillators with XTAL1/XTAL2 connec-
tions 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 Oscilla-
tor 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 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. The INTCAP Fuses are
not supported in the scan-chain, so the boundary scan chain cannot make a XTAL Oscillator requiring internal
capacitors to run unless the fuse is correctly programmed.
Table 26-3.
Scan Signals for the Oscillators
Enable Signal
Scanned Clock Line
Clock Option
Scanned Clock Line
when Not Used
EXTCLKEN
EXTCLK (XTAL1)
External Clock
0
OSCON
OSCCK
External Crystal
External Ceramic Resonator
0
RCOSCEN
RCCK
External RC
0
OSC32EN
OSC32CK
Low Freq. External Crystal
1
TOSKON
TOSCK
32 kHz Timer Oscillator
0
1
DQ
From
Previous
Cell
ClockDR
ShiftDR
To
Next
Cell
To
System
Logic
FF1
0
1
DQ
G
0
1
From
Previous
Cell
ClockDR
UpdateDR
ShiftDR
To
Next
Cell
EXTEST
From
Digital
Logic
XTAL1/TOSC1
XTAL2/TOSC2
Oscillator
ENABLE
OUTPUT