235
8023F–AVR–07/09
ATmega325P/3250P
23.5.3
Scanning the Clock Pins
The AVR devices have many clock options selectable by fuses. These are: Internal RC Oscilla-
tor, External Clock, (High Frequency) Crystal Oscillator, Low-frequency Crystal Oscillator, and
Ceramic Resonator.
Figure 23-5 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 23-5. Boundary-scan Cells for Oscillators and Clock Options
Table 23-2 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 disconnect-
ing the Oscillator pins from the scan path if not provided.
Table 23-2.
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
1
OSC32EN
OSC32CK
Low Freq. External Crystal
1
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