XC2VP70 Xilinx, Inc., XC2VP70 Datasheet - Page 48

• Manufacturer Part Number: XC2VP70
• Description: Virtex-ii Pro Field Programmable Gate Array
• Manufacturer: Xilinx, Inc.

Global clocks are driven by dedicated clock buffers (BUFG),
which can also be used to gate the clock (BUFGCE) or to mul-
tiplex between two independent clock inputs (BUFGMUX).
The most common configuration option of this element is as
a buffer. A BUFG function in this (global buffer) mode, is
shown in
The Virtex-II Pro global clock buffer BUFG can also be con-
figured as a clock enable/disable circuit
as a two-input clock multiplexer
description of these two options is provided below. Each of
them can be used in either of two modes, selected by con-
figuration: rising clock edge or falling clock edge.
This section describes the rising clock edge option. For the
opposite option, falling clock edge, just change all "rising"
references to "falling" and all "High" references to "Low",
except for the description of the CE and S levels. The rising
clock edge option uses the BUFGCE and BUFGMUX prim-
itives. The falling clock edge option uses the BUFGCE_1
and BUFGMUX_1 primitives.
BUFGCE
If the CE input is active (High) prior to the incoming rising
clock edge, this Low-to-High-to-Low clock pulse passes
through the clock buffer. Any level change of CE during the
incoming clock High time has no effect.
If the CE input is inactive (Low) prior to the incoming rising
clock edge, the following clock pulse does not pass through
the clock buffer, and the output stays Low. Any level change
of CE during the incoming clock High time has no effect. CE
must not change during a short setup window just prior to
the rising clock edge on the BUFGCE input I. Violating this
setup time requirement can result in an undefined runt
pulse output.
BUFGMUX
BUFGMUX can switch between two unrelated, even asyn-
chronous clocks. Basically, a Low on S selects the I
a High on S selects the I
the other is done in such a way that the output High and Low
DS083-2 (v2.9) October 14, 2003
Advance Product Specification
Figure 50: Virtex-II Pro BUFGCE Function
Figure 49: Virtex-II Pro BUFG Function
Figure
R
49.
CE
I
I
BUFGCE
1
BUFG
input. Switching from one clock to
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DS031_62_101200
(Figure
O
O
(Figure
51). A functional
50), as well
0
input,
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1-800-255-7778
time is never shorter than the shortest High or Low time of
either input clock. As long as the presently selected clock is
High, any level change of S has no effect.
If the presently selected clock is Low while S changes, or if
it goes Low after S has changed, the output is kept Low until
the other ("to-be-selected") clock has made a transition
from High to Low. At that instant, the new clock starts driv-
ing the output.
The two clock inputs can be asynchronous with regard to
each other, and the S input can change at any time, except
for a short setup time prior to the rising edge of the presently
selected clock; that is, prior to the rising edge of the
BUFGMUX output O. Violating this setup time requirement
can result in an undefined runt pulse output.
All Virtex-II Pro devices have 16 global clock multiplexer
buffers.
Figure 52
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Virtex-II Pro™ Platform FPGAs: Functional Description
C L K 0
CLK1
Out
Figure 52: Clock Multiplexer Waveform Diagram
The current clock is CLK0.
S is activated High.
If CLK0 is currently High, the multiplexer waits for CLK0
to go Low.
Once CLK0 is Low, the multiplexer output stays Low
until CLK1 transitions High to Low.
When CLK1 transitions from High to Low, the output
switches to CLK1.
No glitches or short pulses can appear on the output.
S
Figure 51: Virtex-II Pro BUFGMUX Function
shows a switchover from CLK0 to CLK1.
Wait for Low
S
I
I
0
1
BUFGMUX
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Switch
O
DS083-2_46_121701
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