Virtex-II Platform FPGAs: Functional Description
R
DS031-2 (v3.5) November 5, 2007
Module 2 of 4
Product Specification
5
Figure 6 shows the SSTL2, SSTL3, and HSTL configura-
tions. HSTL can sink current up to 48 mA. (HSTL IV)
All pads are protected against damage from electrostatic
discharge (ESD) and from over-voltage transients. Virtex-II
uses two memory cells to control the configuration of an I/O
as an input. This is to reduce the probability of an I/O con-
figured as an input from flipping to an output when sub-
jected to a single event upset (SEU) in space applications.
Prior to configuration, all outputs not involved in configura-
tion are forced into their high-impedance state. The
pull-down resistors and the weak-keeper circuits are inac-
tive. The dedicated pin HSWAP_EN controls the pull-up
resistors prior to configuration. By default, HSWAP_EN is
set high, which disables the pull-up resistors on user I/O
pins. When HSWAP_EN is set low, the pull-up resistors are
activated on user I/O pins.
All Virtex-II IOBs support IEEE 1149.1 compatible Bound-
ary-Scan testing.
Input Path
The Virtex-II IOB input path routes input signals directly to
internal logic and / or through an optional input flip-flop or
latch, or through the DDR input registers. An optional delay
element at the D-input of the storage element eliminates
pad-to-pad hold time. The delay is matched to the internal
clock-distribution delay of the Virtex-II device, and when
used, assures that the pad-to-pad hold time is zero.
Each input buffer can be configured to conform to any of the
low-voltage signaling standards supported. In some of
these standards the input buffer utilizes a user-supplied
threshold voltage, VREF. The need to supply VREF imposes
constraints on which standards can be used in the same
bank. See I/O banking description.
Output Path
The output path includes a 3-state output buffer that drives
the output signal onto the pad. The output and / or the
3-state signal can be routed to the buffer directly from the
internal logic or through an output / 3-state flip-flop or latch,
or through the DDR output / 3-state registers.
Each output driver can be individually programmed for a
wide range of low-voltage signaling standards. In most sig-
naling standards, the output High voltage depends on an
externally supplied VCCO voltage. The need to supply VCCO
imposes constraints on which standards can be used in the
same bank. See I/O banking description.
I/O Banking
Some of the I/O standards described above require VCCO
and VREF voltages. These voltages are externally supplied
and connected to device pins that serve groups of IOB
blocks, called banks. Consequently, restrictions exist about
which I/O standards can be combined within a given bank.
Eight I/O banks result from dividing each edge of the FPGA
bank has multiple VCCO pins, all of which must be con-
nected to the same voltage. This voltage is determined by
the output standards in use.
Table 4: LVTTL and LVCMOS Programmable Currents (Sink and Source)
SelectI/O-Ultra
Programmable Current (Worst-Case Guaranteed Minimum)
LVTTL
2 mA
4 mA
6 mA
8 mA
12 mA
16 mA
24 mA
LVCMOS33
2 mA
4 mA
6 mA
8 mA
12 mA
16 mA
24 mA
LVCMOS25
2 mA
4 mA
6 mA
8 mA
12 mA
16 mA
24 mA
LVCMOS18
2 mA
4 mA
6 mA
8 mA
12 mA
16 mA
n/a
LVCMOS15
2 mA
4 mA
6 mA
8 mA
12 mA
16 mA
n/a
Figure 6: SSTL or HSTL SelectI/O-Ultra Standards
VCCO
OBUF
VREF
Clamp
Diode
PAD
VCCAUX = 3.3V
VCCINT = 1.5V
DS031_24_100900