Spartan-3 1.2V FPGA Family: Functional Description
R
DCM
180˚ 0˚
FDDR
D1
Q1
CLK1
clamp diodes are always connected to the pad, regardless
of the signal standard selected. The presence of diodes lim-
its the ability of Spartan-3 I/Os to tolerate high signal volt-
ages. The V
IN
absolute maximum rating in
in
Module 3:
specifies the
voltage range that I/Os can tolerate.
Slew Rate Control and Drive Strength
Two options, FAST and SLOW, control the output slew rate.
The FAST option supports output switching at a high rate.
The SLOW option reduces bus transients. These options are
only available when using one of the LVCMOS or LVTTL
standards, which also provide up to seven different levels of
current drive strength: 2, 4, 6, 8, 12, 16, and 24 mA. Choos-
ing the appropriate drive strength level is yet another means
to minimize bus transients.
shows the drive strengths that the LVCMOS and
LVTTL standards support. The Fast option is indicated by
appending an "F" attribute after the output buffer symbol
OBUF or the bidirectional buffer symbol IOBUF. The Slow
option appends an "S" attribute. The drive strength in milliam-
peres follows the slew rate attribute. For example,
OBUF_LVCMOS18_S_6 or IOBUF_LVCMOS25_F_16.
Table 3:
Programmable Output Drive Current
Signal
Standard
LVCMOS12
LVCMOS15
LVCMOS18
LVCMOS25
LVCMOS33
LVTTL
Current Drive (mA)
2
4
6
8
-
12
-
16
-
-
24
-
-
-
DDR MUX
Q
D2
Q2
CLK2
DS099-2_02_070303
Figure 2:
Clocking the DDR Register
Pull-Up and Pull-Down Resistors
The optional pull-up and pull-down resistors are intended to
establish High and Low levels, respectively, at unused I/Os.
The weak pull-up resistor optionally connects each IOB pad
to V
CCO
. A weak pull-down resistor optionally connects
each pad to GND. These resistors are placed in a design
using the PULLUP and PULLDOWN symbols in a sche-
matic, respectively. They can also be instantiated as com-
ponents, set as constraints or passed as attributes in HDL
code. These resistors can also be selected for all unused
I/O using the Bitstream Generator (BitGen) option Unused-
Pin. A Low logic level on HSWAP_EN activates the pull-up
resistors on all I/Os during configuration.
Boundary-Scan Capability
All Spartan-3 IOBs support boundary-scan testing compat-
ible with IEEE 1149.1 standards. See
for more information.
Weak-Keeper Circuit
Each I/O has an optional weak-keeper circuit that retains
the last logic level on a line after all drivers have been turned
off. This is useful to keep bus lines from floating when all
connected drivers are in a high-impedance state. This func-
tion is placed in a design using the KEEPER symbol.
Pull-up and pull-down resistors override the weak-keeper
circuit.
SelectIO Signal Standards
The IOBs support 17 different single-ended signal stan-
dards, as listed in
Furthermore, the majority of
IOBs can be used in specific pairs supporting any of six dif-
ferential signal standards, as shown in
The desired
standard is selected by placing the appropriate I/O library
symbol or component into the FPGA design. For example,
the symbol named IOBUF_LVCMOS15_F_8 represents a
bidirectional I/O to which the 1.5V LVCMOS signal standard
has been assigned. The slew rate and current drive are set
to Fast and 8 mA, respectively.
Together with placing the appropriate I/O symbol, two exter-
nally applied voltage levels, V
CCO
and V
REF
select the
desired signal standard. The V
CCO
lines provide current to
the output driver. The voltage on these lines determines the
DS099-2 (v1.2) July 11, 2003
Advance Product Specification
ESD Protection
Clamp diodes protect all device pads against damage from
Electro-Static Discharge (ESD) as well as excessive voltage
transients. Each I/O has two clamp diodes: One diode
extends P-to-N from the pad to V
CCO
and a second diode
extends N-to-P from the pad to GND. During operation,
these diodes are normally biased in the off state. These
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