DS001 Datasheet, PDF (40/99 Pages) Xilinx, Inc – Spartan-II FPGA Family

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Spartan-II FPGA Family: Functional Description
LVTTL output buffers have selectable drive strengths.
The format for LVTTL OBUF primitive names is as follows.
OBUF_<slew_rate>_<drive_strength>
<slew_rate> is either F (Fast), or S (Slow) and
<drive_strength> is specified in milliamps (2, 4, 6, 8, 12, 16,
or 24). The default is slew rate limited with 12 mA drive.
OBUF placement restrictions require that within a given
V_CCO bank each OBUF share the same output source drive
voltage. Input buffers of any type and output buffers that do
not require V_CCO can be placed within any V_CCO bank.
Table 17 summarizes the output compatibility requirements.
The LOC property can specify a location for the OBUF.
Table 17: Output Standards Compatibility
Requirements
Rule 1 Only outputs with standards which share
compatible V_CCO may be used within the same
bank.
Rule 2
V_CCO
3.3
There are no placement restrictions for outputs
with standards that do not require a V_CCO.
Compatible Standards
LVTTL, SSTL3_I, SSTL3_II, CTT, AGP, GTL,
GTL+, PCI33_3, PCI66_3
2.5 SSTL2_I, SSTL2_II, LVCMOS2, GTL, GTL+
1.5 HSTL_I, HSTL_III, HSTL_IV, GTL, GTL+
OBUFT
The generic 3-state output buffer OBUFT, shown in
Figure 39, typically implements 3-state outputs or
bidirectional I/O.
With no extension or property specified for the generic
OBUFT primitive, the assumed standard is slew rate limited
LVTTL with 12 mA drive strength.
The LVTTL OBUFT can support one of two slew rate modes
to minimize bus transients. By default, the slew rate for each
output buffer is reduced to minimize power bus transients
when switching non-critical signals.
LVTTL 3-state output buffers have selectable drive
strengths.
The format for LVTTL OBUFT primitive names is as follows.
OBUFT_<slew_rate>_<drive_strength>
<slew_rate> can be either F (Fast), or S (Slow) and
<drive_strength> is specified in milliamps (2, 4, 6, 8, 12, 16,
or 24).
IOBUFT
T
I
IO
DS001_39_032300
Figure 39: 3-State Output Buffer Primitive (OBUFT
The Versatile I/O OBUFT placement restrictions require
that within a given V_CCO bank each OBUFT share the same
output source drive voltage. Input buffers of any type and
output buffers that do not require V_CCO can be placed within
the same V_CCO bank.
The LOC property can specify a location for the OBUFT.
3-state output buffers and bidirectional buffers can have
either a weak pull-up resistor, a weak pull-down resistor, or
a weak "keeper" circuit. Control this feature by adding the
appropriate primitive to the output net of the OBUFT
(PULLUP, PULLDOWN, or KEEPER).
The weak "keeper" circuit requires the input buffer within the
IOB to sample the I/O signal. So, OBUFTs programmed for
an I/O standard that requires a V_REF have automatic
placement of a V_REF in the bank with an OBUFT configured
with a weak "keeper" circuit. This restriction does not affect
most circuit design as applications using an OBUFT
configured with a weak "keeper" typically implement a
bidirectional I/O. In this case the IBUF (and the
corresponding V_REF) are explicitly placed.
The LOC property can specify a location for the OBUFT.
IOBUF
Use the IOBUF primitive for bidirectional signals that
require both an input buffer and a 3-state output buffer with
an active high 3-state pin. The generic input/output buffer
IOBUF appears in Figure 40.
With no extension or property specified for the generic
IOBUF primitive, the assumed standard is LVTTL input
buffer and slew rate limited LVTTL with 12 mA drive strength
for the output buffer.
The LVTTL IOBUF can support one of two slew rate modes
to minimize bus transients. By default, the slew rate for each
output buffer is reduced to minimize power bus transients
when switching non-critical signals.
LVTTL bidirectional buffers have selectable output drive
strengths.
The format for LVTTL IOBUF primitive names is as follows:
DS001-2 (v2.8) June 13, 2008
Product Specification
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