ATMEga64L ATMEL Corporation, ATMEga64L Datasheet - Page 262

• Manufacturer Part Number: ATMEga64L
• Description: 8-bit AVR Microcontroller with 64K Bytes In-System Programmable Flash
• Manufacturer: ATMEL Corporation

Scanning the Clock Pins
262
ATmega64(L)
The AVR devices have many clock options selectable by fuses. These are: Internal RC
Oscillator, External RC, External Clock, (High Frequency) Crystal Oscillator, Low-fre-
quency Crystal Oscillator, and Ceramic Resonator.
Figure 131 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 131. Boundary-scan Cells for Oscillators and Clock Options
Table 103 summaries the scan registers for the external clock pin XTAL1, oscillators
with XTAL1/XTAL2 connections as well as 32 kHz Timer Oscillator.
Table 103. Scan Signals for the Oscillators
Notes:
Enable
Signal
EXTCLKEN
OSCON
RCOSCEN
OSC32EN
TOSKON
Previous
From
Cell
ShiftDR
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
3. The clock configuration is programmed by fuses. As a fuse does not change run-time,
0
1
between the internal Oscillator and the JTAG TCK clock. If possible, scanning an
external clock is preferred.
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
ClockDR
D
Scanned Clock
Line
EXTCLK (XTAL1)
OSCCK
RCCK
OSC32CK
TOSCK
UpdateDR
Q
Next
Cell
To
D
G
Q
EXTEST
0
1
Clock Option
External Clock
External Crystal
External Ceramic Resonator
External RC
Low Freq. External Crystal
32 kHz Timer Oscillator
XTAL1/TOSC1
ENABLE
(1)(2)(3)
Oscillator
XTAL2/TOSC2
OUTPUT
Previous
From
Cell
Scanned Clock Line
ShiftDR
0
1
when Not Used
ClockDR
2490G–AVR–03/04
D
0
0
0
1
0
FF1
Q
Next
Cell
To