R
XC4000E and XC4000X Series Field Programmable Gate Arrays
Any XC4000 Series 5-Volt device with its outputs config-
ured in TTL mode can drive the inputs of any typical
3.3-Volt device. (For a detailed discussion of how to inter-
face between 5 V and 3.3 V devices, see the 3V Products
section of The Programmable Logic Data Book.)
Power/Ground pin pairs are connected to special Power
and Ground planes within the packages, to reduce ground
bounce. Therefore, the maximum total capacitive load is
300 pF between each external Power/Ground pin pair.
Maximum loading may vary for the low-voltage devices.
Supported destinations for XC4000 Series device outputs
are shown in Table 12.
For slew-rate limited outputs this total is two times larger for
each device type: 400 pF for XC4000E devices and 600 pF
for XC4000X devices. This maximum capacitive load
should not be exceeded, as it can result in ground bounce
of greater than 1.5 V amplitude and more than 5 ns dura-
tion. This level of ground bounce may cause undesired
transient behavior on an output, or in the internal logic. This
restriction is common to all high-speed digital ICs, and is
not particular to Xilinx or the XC4000 Series.
An output can be configured as open-drain (open-collector)
by placing an OBUFT symbol in a schematic or HDL code,
then tying the 3-state pin (T) to the output signal, and the
input pin (I) to Ground. (See Figure 18.)
Table 12: Supported Destinations for XC4000 Series
Outputs
XC4000 Series devices have a feature called “Soft
Start-up,” designed to reduce ground bounce when all out-
puts are turned on simultaneously at the end of configura-
tion. When the configuration process is finished and the
device starts up, the first activation of the outputs is auto-
matically slew-rate limited. Immediately following the initial
activation of the I/O, the slew rate of the individual outputs
is determined by the individual configuration option for each
IOB.
XC4000 Series
Outputs
Destination
3.3 V,
5 V,
5 V,
CMOS TTL CMOS
Any typical device, Vcc = 3.3 V,
CMOS-threshold inputs
√
√
√
√
some1
Any device, Vcc = 5 V,
TTL-threshold inputs
√
√
Any device, Vcc = 5 V,
CMOS-threshold inputs
Unreliable
Data
Global Three-State
1. Only if destination device has 5-V tolerant inputs
A separate Global 3-State line (not shown in Figure 15 or
Figure 16) forces all FPGA outputs to the high-impedance
state, unless boundary scan is enabled and is executing an
EXTEST instruction. This global net (GTS) does not com-
pete with other routing resources; it uses a dedicated distri-
bution network.
OPAD
OBUFT
X6702
GTS can be driven from any user-programmable pin as a
global 3-state input. To use this global net, place an input
pad and input buffer in the schematic or HDL code, driving
the GTS pin of the STARTUP symbol. A specific pin loca-
tion can be assigned to this input using a LOC attribute or
property, just as with any other user-programmable pad. An
inverter can optionally be inserted after the input buffer to
invert the sense of the Global 3-State signal. Using GTS is
similar to GSR. See Figure 2 on page 11 for details.
Figure 18: Open-Drain Output
Output Slew Rate
The slew rate of each output buffer is, by default, reduced,
to minimize power bus transients when switching non-criti-
cal signals. For critical signals, attach a FAST attribute or
property to the output buffer or flip-flop.
For XC4000E devices, maximum total capacitive load for
simultaneous fast mode switching in the same direction is
200 pF for all package pins between each Power/Ground
pin pair. For XC4000X devices, additional internal
Alternatively, GTS can be driven from any internal node.
6-24
May 14, 1999 (Version 1.6)
XILINX [ XILINX, INC ]
