XQ2V1000 Datasheet, PDF (35/127 Pages) Xilinx, Inc – QPro Virtex-II 1.5V Military QML Platform FPGAs

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QPro Virtex-II 1.5V Military QML Platform FPGAs
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 or 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.
BUFGCE
I
O
CE
DS031_62_101200
Figure 43: Virtex-II BUFGCE Function
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 I0 input,
and a High on S selects the I1 input. Switching from one
clock to the other is done in such a way that the output High
and Low 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.
BUFGMUX
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 devices have 16 global clock multiplexer buffers.
Figure 45 shows a switchover from CLK0 to CLK1.
Wait for Low
S
CLK0
CLK1
OUT
Switch
DS031_46_112900
Figure 45: Clock Multiplexer Waveform Diagram
In Figure 45:
• 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.
I0
O
I1
S
DS031_63_112900
Figure 44: Virtex-II BUFGMUX Function
Local Clocking
In addition to global clocks, there are local clock resources
in the Virtex-II devices. There are more than 72 local clocks
in the Virtex-II family. These resources can be used for
many different applications, including but not limited to
memory interfaces. For example, even using only the left
and right I/O banks, Virtex-II FPGAs can support up to 50
local clocks for DDR SDRAM. These interfaces can operate
beyond 200 MHz on Virtex-II devices.
DS122 (v1.1) January 7, 2004
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