R
XC3000 Series Field Programmable Gate Arrays
series resistor R2 may be included to add to the amplifier
output impedance when needed for phase-shift control,
crystal resistance matching, or to limit the amplifier input
swing to control clipping at large amplitudes. Excess feed-
back 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 fre-
quencies below 1 MHz may require individual characteriza-
tion with respect to a series resistance. Crystal oscillators
above 20 MHz generally require a crystal which operates in
a third overtone mode, where the fundamental frequency
must be suppressed by an inductor across C2, turning this
parallel resonant circuit to double the fundamental crystal
frequency, i.e., 2/3 of the desired third harmonic frequency
network. When the oscillator inverter is not used, these
IOBs and their package pins are available for general user
I/O.
Crystal Oscillator
Figure 18 also shows the location of an internal high speed
inverting amplifier that 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 oscilla-
tor is configured and connected as a signal source, two
special user IOBs are also configured to connect the oscil-
lator amplifier with external crystal oscillator components
as shown in Figure 19. A divide by two 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 com-
pletion of configuration. In Figure 19 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, pro-
duce the 360-degree phase shift of the Pierce oscillator. A
D
Q
Internal
External
Alternate
Clock Buffer
XTAL1
XTAL2
(IN)
R1
R2
Suggested Component Values
R1 0.5 – 1 MΩ
R2 0 – 1 kΩ
Y1
(may be required for low frequency, phase
shift and/or compensation level for crystal Q)
C1, C2 10 – 40 pF
Y1 1 – 20 MHz AT-cut parallel resonant
C1
C2
100 PIN
CQFP PQFP
164 PIN 175 PIN 176 PIN 208 PIN
44 PIN 68 PIN
84 PIN
132 PIN 160 PIN
CQFP
105
PGA
T14
P15
TQFP
91
PQFP
110
PLCC
30
PLCC PLCC PGA
PGA
P13
M13
PQFP
82
XTAL 1 (OUT)
XTAL 2 (IN)
67
61
82
76
47
43
57
53
J11
L11
26
99
85
100
76
X7064
Figure 19: Crystal Oscillator Inverter. When activated, and by selecting an output network for its buffer, the crystal
oscillator inverter uses two unconfigured package pins and external components to implement an oscillator. An optional
divide-by-two mode is available to assure symmetry.
7-18
November 9, 1998 (Version 3.1)
XILINX [ XILINX, INC ]
