XC4000E and XC4000X Series Field Programmable Gate Arrays
4-26
March 30, 1998 (Version 1.5)
Output Multiplexer/2-Input Function Generator
(XC4000X only)
As shown in
XC4000X IOB contains an additional multiplexer not avail-
able in the XC4000E IOB. The multiplexer can also be con-
figured as a 2-input function generator, implementing a
pass-gate, AND-gate, OR-gate, or XOR-gate, with 0, 1, or
2 inverted inputs. The logic used to implement these func-
tions is shown in the upper gray area of
When configured as a multiplexer, this feature allows two
output signals to time-share the same output pad; effec-
tively doubling the number of device outputs without requir-
ing a larger, more expensive package.
When the MUX is configured as a 2-input function genera-
tor, logic can be implemented within the IOB itself. Com-
bined with a Global Early buffer, this arrangement allows
very high-speed gating of a single signal. For example, a
wide decoder can be implemented in CLBs, and its output
gated with a Read or Write Strobe Driven by a BUFGE
buffer, as shown in
. The critical-path pin-to-pin
delay of this circuit is less than 6 nanoseconds.
As shown in
, the IOB input pins Out, Output
Clock, and Clock Enable have different delays and different
flexibilities regarding polarity. Additionally, Output Clock
sources are more limited than the other inputs. Therefore,
the Xilinx software does not move logic into the IOB func-
tion generators unless explicitly directed to do so.
The user can specify that the IOB function generator be
used, by placing special library symbols beginning with the
letter “O.” For example, a 2-input AND-gate in the IOB func-
tion generator is called OAND2. Use the symbol input pin
labelled “F” for the signal on the critical path. This signal is
placed on the OK pin — the IOB input with the shortest
delay to the function generator. Two examples are shown in
Other IOB Options
There are a number of other programmable options in the
XC4000 Series IOB.
Pull-up and Pull-down Resistors
Programmable pull-up and pull-down resistors are useful
for tying unused pins to Vcc or Ground to minimize power
consumption and reduce noise sensitivity. The configurable
pull-up resistor is a p-channel transistor that pulls to Vcc.
The configurable pull-down resistor is an n-channel transis-
tor that pulls to Ground.
The value of these resistors is 50 k
Ω −
100 k
Ω
. This high
value makes them unsuitable as wired-AND pull-up resis-
tors.
The pull-up resistors for most user-programmable IOBs are
active during the configuration process. See
for a list of pins with pull-ups active before and
during configuration.
After configuration, voltage levels of unused pads, bonded
or unbonded, must be valid logic levels, to reduce noise
sensitivity and avoid excess current. Therefore, by default,
unused pads are configured with the internal pull-up resis-
tor active. Alternatively, they can be individually configured
with the pull-down resistor, or as a driven output, or to be
driven by an external source. To activate the internal pull-
up, attach the PULLUP library component to the net
attached to the pad. To activate the internal pull-down,
attach the PULLDOWN library component to the net
attached to the pad.
Independent Clocks
Separate clock signals are provided for the input and out-
put flip-flops. The clock can be independently inverted for
each flip-flop within the IOB, generating either falling-edge
or rising-edge triggered flip-flops. The clock inputs for each
IOB are independent, except that in the XC4000X, the Fast
Capture latch shares an IOB input with the output clock pin.
Early Clock for IOBs (XC4000X only)
Special early clocks are available for IOBs. These clocks
are sourced by the same sources as the Global Low-Skew
buffers, but are separately buffered. They have fewer loads
and therefore less delay. The early clock can drive either
the IOB output clock or the IOB input clock, or both. The
early clock allows fast capture of input data, and fast clock-
to-output on output data. The Global Early buffers that drive
these clocks are described in
.
Global Set/Reset
As with the CLB registers, the Global Set/Reset signal
(GSR) can be used to set or clear the input and output reg-
isters, depending on the value of the INIT attribute or prop-
erty. The two flip-flops can be individually configured to set
IPAD
F
OPAD
FAST
BUFGE
OAND2
from
internal
logic
X9019
Figure 20: Fast Pin-to-Pin Path in XC4000X
OAND2
F
X6598
D0
S0
D1
O
OMUX2
X6599
Figure 21: AND & MUX Symbols in XC4000X IOB
