308
7679H–CAN–08/08
AT90CAN32/64/128
Figure 23-7.
Boundary-scan Cells for Oscillators and Clock Options
summaries the scan registers for the external clock pin XTAL1, oscillators with
XTAL1/XTAL2 connections as well as external Timer2 clock pin TOSC1 and 32kHz Timer2
Oscillator.
Notes:
1. Do not enable more than one clock source as 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 main clock configuration is programmed by fuses. As a fuse is not changed run-time, the
main 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 sup-
ported 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.
23.6.5
Scanning the Analog Comparator
The relevant Comparator signals regarding Boundary-scan are shown in
Boundary-scan cell from
is attached to each of these signals. The signals are
described in
.
The Comparator need not be used for pure connectivity testing, since all analog inputs are
shared with a digital port pin as well.
Table 23-5.
Scan Signals for the Oscillators
Enable Signal
Scanned Clock Line
Clock Option
Scanned Clock Line
when not Used
EXTCLKEN
EXTCLK (XTAL1)
External Main Clock
0
OSCON
OSCCK
External Crystal
External Ceramic Resonator
1
OSC32EN
OSC32CK
Low Freq. External Crystal
1
TOSKON
TOSCK
32 kHz Timer2 Oscillator
1
0
1
D
Q
From
Previous
Cell
ClockDR
ShiftDR
To
Next
Cell
To System Logic
FF1
0
1
D
Q
D
Q
G
0
1
From
Previous
Cell
ClockDR
UpdateDR
ShiftDR
To
Next
Cell
EXTEST
From Digital Logic
XTAL1 / TOSC1
XTAL2 / TOSC2
Oscillator
ENABLE
OUTPUT
Содержание AVR AT90CAN128
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