2-117
option is available to assure symmetry. The oscillator
circuit becomes active early in the configuration process to
allow the oscillator to stabilize. Actual internal connection
is delayed until completion of configuration. In Figure 17
the feedback resistor R1, between the output and input,
biases the amplifier at threshold. The inversion of the
amplifier, together with the R-C networks and an AT-cut
series resonant crystal, produce the 360-degree phase
shift of the Pierce oscillator. A series resistor R2 may be
included to add to the amplifier output impedance when
needed for phase-shift control, crystal resistance match-
ing, or to limit the amplifier input swing to control clipping
at large amplitudes. Excess feedback voltage may be
corrected by the ratio of C2/C1. The amplifier is designed
to be used from 1 MHz to about one-half the specified CLB
toggle frequency. Use at frequencies below 1 MHz may
require individual characterization with respect to a series
of conflicting levels on a common line. Each horizontal
Longline is also driven by a weak keeper circuit that
prevents undefined floating levels by maintaining the pre-
vious logic level when the line is not driven by an active
buffer or a pull-up resistor. Figure 16 shows 3-state buff-
ers, Longlines and pull-up resistors.
Crystal Oscillator
Figure 16 also shows the location of an internal high speed
inverting amplifier which may be used to implement an on-
chip crystal oscillator. It is associated with the auxiliary
buffer in the lower right corner of the die. When the
oscillator is configured by MakeBits and connected as a
signal source, two special user IOBs are also configured to
connect the oscillator amplifier with external crystal oscil-
lator components as shown in Figure 17. A divide by two
X1245
Figure 16. XACT Development System. An extra large view of possible interconnections in the lower right corner of the XC3020.
