Rainbow Electronics ATmega128L - Manual (Page 251)

251
ATmega128(L)
2467B–09/01
Figure 127. Boundary-scan Cells for Oscillators and Clock Options
Table 102 summaries the scan registers for the external clock pin XTAL1, oscillators
with XTAL1/XTAL2 connections as well as 32 kHz 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 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 can not make a XTAL oscillator requiring internal capacitors to run unless
the fuse is correctly programmed.
Scanning the Analog
Comparator The relevant Comparator signals regarding Boundary-scan are shown in Figure 128.
The Boundary-scan cell from Figure 129 is attached to each of these signals. The sig-
nals are described in Table 103.
The Comparator need not be used for pure connectivity testing, since all analog inputs
are shared with a digital port pin as well.
Table 102. Scan Signals for the Oscillator (1)(2)(3)
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
RCOSCEN RCCK External RC 1
OSC32EN OSC32CK Low Freq. External Crystal 1
TOSKON TOSCK 32 kHz Timer 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