R
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> 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
<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.
I
IO
OBUF placement restrictions require that within a given
VCCO bank each OBUF share the same output source drive
voltage. Input buffers of any type and output buffers that do
not require VCCO can be placed within any VCCO bank.
Table 17 summarizes the output compatibility requirements.
The LOC property can specify a location for the OBUF.
DS001_39_032300
Figure 39: 3-State Output Buffer Primitive (OBUFT
The Versatile I/O OBUFT placement restrictions require
that within a given VCCO bank each OBUFT share the same
output source drive voltage. Input buffers of any type and
output buffers that do not require VCCO can be placed within
the same VCCO bank.
Table 17: Output Standards Compatibility
Requirements
Rule 1 Only outputs with standards which share
compatible VCCO may be used within the same
bank.
The LOC property can specify a location for the OBUFT.
Rule 2 There are no placement restrictions for outputs
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).
with standards that do not require a VCCO
.
VCCO Compatible Standards
3.3
LVTTL, SSTL3_I, SSTL3_II, CTT, AGP, GTL,
GTL+, PCI33_3, PCI66_3
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 VREF have automatic
placement of a VREF 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
2.5
1.5
SSTL2_I, SSTL2_II, LVCMOS2, GTL, GTL+
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.
corresponding VREF) are explicitly placed.
The LOC property can specify a location for the OBUFT.
With no extension or property specified for the generic
OBUFT primitive, the assumed standard is slew rate limited
LVTTL with 12 mA drive strength.
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.
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.
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.
LVTTL 3-state output buffers have selectable drive
strengths.
The format for LVTTL OBUFT primitive names is as follows.
OBUFT_<slew_rate>_<drive_strength>
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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