R
Spartan and Spartan-XL Families Field Programmable Gate Arrays
Output Multiplexer/2-Input Function Generator
(Spartan-XL only)
By default, a 5V Spartan device output buffer pull-up struc-
ture is configured as a TTL-like totem-pole. The High driver
is an n-channel pull-up transistor, pulling to a voltage one
The output path in the Spartan-XL IOB contains an addi-
tional multiplexer not available in the Spartan IOB. The mul-
tiplexer can also be configured as a 2-input function
generator, implementing a pass gate, AND gate, OR gate,
or XOR gate, with 0, 1, or 2 inverted inputs.
transistor threshold below V . Alternatively, the outputs
CC
can be globally configured as CMOS drivers, with additional
p-channel pull-up transistors pulling to V . This option,
CC
applied using the bitstream generation software, applies to
all outputs on the device. It is not individually programma-
ble.
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. The select input is
the pin used for the output flip-flop clock, OK.
All Spartan-XL device outputs are configured as CMOS
drivers, therefore driving rail-to-rail. The Spartan-XL outputs
are individually programmable for 12 mA or 24 mA output
drive.
When the multiplexer is configured as a 2-input function
generator, logic can be implemented within the IOB itself.
Combined with a Global 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 global buffer.
Any 5V Spartan device with its outputs configured in TTL
mode can drive the inputs of any typical 3.3V device. Sup-
ported destinations for Spartan/XL device outputs are
shown in Table 7.
Three-State Register (Spartan-XL Only)
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
labeled "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
Figure 7.
Spartan-XL devices incorporate an optional register control-
ling the three-state enable in the IOBs. The use of the
three-state control register can significantly improve output
enable and disable time.
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.
OMUX2
D0
F
O
Spartan/XL devices have a feature called "Soft Start-up,"
designed to reduce ground bounce when all outputs are
turned on simultaneously at the end of configuration.
When the configuration process is finished and the device
starts up, the first activation of the outputs is automatically
slew-rate limited. Immediately following the initial activation
of the I/O, the slew rate of the individual outputs is deter-
mined by the individual configuration option for each IOB.
D1
OAND2
S0
DS060_07_081100
Figure 7: AND and MUX Symbols in Spartan-XL IOB
Output Buffer
An active High 3-state signal can be used to place the out-
put buffer in a high-impedance state, implementing 3-state
outputs or bidirectional I/O. Under configuration control, the
output (O) and output 3-state (T) signals can be inverted.
The polarity of these signals is independently configured for
each IOB (see Figure 6, page 7). An output can be config-
ured as open-drain (open-collector) by tying the 3-state pin
(T) to the output signal, and the input pin (I) to Ground.
Pull-up and Pull-down Network
Programmable pull-up and pull-down resistors are used for
tying unused pins to V or Ground to minimize power con-
CC
sumption and reduce noise sensitivity. The configurable
pull-up resistor is a p-channel transistor that pulls to V
The configurable pull-down resistor is an n-channel transis-
tor that pulls to Ground. The value of these resistors is typi-
cally 20 KΩ − 100 KΩ (See "Spartan DC Characteristics
.
CC
DS060 (v1.6) September 19, 2001
Product Specification
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