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May 14, 1999 (Version 1.6)
6-39
XC4000E and XC4000X Series Field Programmable Gate Arrays
6
The top and bottom Global Early buffers are about 1 ns
slower clock to out than the left and right Global Early buff-
ers.
The Global Early buffers can be driven by either semi-ded-
icated pads or internal logic. They share pads with the Glo-
bal Low-Skew buffers, so a single net can drive both global
buffers, as described above.
To use a Global Early buffer, place a BUFGE element in a
schematic or in HDL code. If desired, attach a LOC
attribute or property to direct placement to the designated
location. For example, attach a LOC=T attribute or property
to direct that a BUFGE be placed in one of the two Global
Early buffers on the top edge of the device, or a LOC=TR to
indicate the Global Early buffer on the top edge of the
device, on the right.
Power Distribution
Power for the FPGA is distributed through a grid to achieve
high noise immunity and isolation between logic and I/O.
Inside the FPGA, a dedicated Vcc and Ground ring sur-
rounding the logic array provides power to the I/O drivers,
as shown in
Figure 39
. An independent matrix of Vcc and
Ground lines supplies the interior logic of the device.
This power distribution grid provides a stable supply and
ground for all internal logic, providing the external package
power pins are all connected and appropriately de-coupled.
Typically, a 0.1
μ
F capacitor connected between each Vcc
pin and the board’s Ground plane will provide adequate
de-coupling.
Output buffers capable of driving/sinking the specified 12
mA loads under specified worst-case conditions may be
capable of driving/sinking up to 10 times as much current
under best case conditions.
Noise can be reduced by minimizing external load capaci-
tance and reducing simultaneous output transitions in the
same direction. It may also be beneficial to locate heavily
loaded output buffers near the Ground pads. The I/O Block
output buffers have a slew-rate limited mode (default) which
should be used where output rise and fall times are not
speed-critical.
Pin Descriptions
There are three types of pins in the XC4000 Series
devices:
Permanently dedicated pins
User I/O pins that can have special functions
Unrestricted user-programmable I/O pins.
Before and during configuration, all outputs not used for the
configuration process are 3-stated with a 50 k
- 100 k
pull-up resistor.
After configuration, if an IOB is unused it is configured as
an input with a 50 k
- 100 k
pull-up resistor.
XC4000 Series devices have no dedicated Reset input.
Any user I/O can be configured to drive the Global
Set/Reset net, GSR.
See
“Global Set/Reset” on page 11
for more information on GSR.
XC4000 Series devices have no Powerdown control input,
as
the
XC3000
and
XC2000
XC3000/XC2000 Powerdown control also 3-stated all of the
device
I/O pins. For XC4000 Series devices, use the global 3-state
net, GTS, instead. This net 3-states all outputs, but does
not place the device in low-power mode. See
“IOB Output
Signals” on page 23
for more information on GTS.
families
do.
The
Device pins for XC4000 Series devices are described in
Table 16
. Pin functions during configuration for each of the
seven configuration modes are summarized in
Table 22 on
page 58
, in the “Configuration Timing” section.
GND
Ground and
Vcc Ring for
I/O Drivers
Vcc
GND
Vcc
Logic
Power Grid
X5422
Figure 39: XC4000 Series Power Distribution