atmega3250v-8auatmega325v-8ai ATMEL Corporation, atmega3250v-8auatmega325v-8ai Datasheet - Page 230

• Manufacturer Part Number: atmega3250v-8auatmega325v-8ai
• Description: Atmega3250 8-bit Microcontroller With In-system Programmable Flash
• Manufacturer: ATMEL Corporation

25.6.3
230
ATmega325/3250/645/6450
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 25-6
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 25-6. Boundary-scan Cells for Oscillators and Clock Options
Table 25-3
XTAL1/XTAL2 connections as well as 32kHz Timer Oscillator.
Table 25-3.
Note:
Enable Signal
EXTCLKEN
OSCON
OSC32EN
From Digital Logic
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
3. The clock configuration is programmed by fuses. As a fuse is not changed run-time, the clock
the internal Oscillator and the JTAG TCK clock. If possible, scanning an external clock is
preferred.
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.
summaries the scan registers for the external clock pin XTAL1, oscillators with
shows how each Oscillator with external connection is supported in the scan chain.
Scan Signals for the Oscillator
Previous
From
Cell
Scanned Clock Line
EXTCLK (XTAL1)
OSCCK
OSC32CK
ShiftDR
0
1
ClockDR
D
UpdateDR
Q
Next
Cell
To
D
G
Q
Clock Option
External Clock
External Crystal
External Ceramic Resonator
Low Freq. External Crystal
EXTEST
0
1
XTAL1/TOSC1
(1)(2)(3)
ENABLE
Oscillator
XTAL2/TOSC2
OUTPUT
Previous
From
Cell
ShiftDR
0
1
ClockDR
Scanned Clock
Line when not
D
FF1
Used
Q
0
1
1
Next
Cell
To
To System Logic
2570L–AVR–08/07