Rainbow Electronics Atmega169L - Manual (Page 238)

238
ATmega169V/L
2514A–AVR–08/02
Scanning the Clock Pins The AVR devices have many clock options selectable by fuses. These are: Internal RC
Oscillator, External Clock, (High Frequency) Crystal Oscillator, Low-frequency Crystal
Oscillator, and Ceramic Resonator.
Figure 111 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 111. 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 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 disconnecting the Oscillator pins from the scan path if not
provided.
Table 103. 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
OSC32EN OSC32CK Low Freq. External Crystal 1
TOSKON TOSCK 32kHz 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