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XC4000 Series Field Programmable Gate Arrays
4-46
September 18, 1996 (Version 1.04)
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
. 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 decoupled.
Typically, a 0.1
µ
F capacitor connected near the Vcc and
Ground pins of the package will provide adequate decou-
pling.
Output buffers capable of driving/sinking the specified 12
mA (XC4000E) or 24 mA (XC4000EX) loads under speci-
fied worst-case conditions may be capable of driving/sink-
ing 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
Ω
- 100 k
Ω
pull-up resistor.
After configuration, if an IOB is unused it is configured as
an input with a 50 k
Ω
- 100 k
Ω
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
for
more information on GSR.
XC4000-Series devices have no Powerdown control input,
as the XC3000 and XC2000 families do. The 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
for more information on GTS.
Device pins for XC4000-Series devices are described in
. Pin functions during configuration for each of the
seven configuration modes are summarized in
, in the “Configuration Timing” section.
GND
Ground and
Vcc Ring for
I/O Drivers
Vcc
GND
Vcc
Logic
Power Grid
X5422
Figure 41: XC4000-Series Power Distribution