I/O Structures in IGLOO and Pro ASIC3 Devices
v1.1
7 - 11
3.3 V PCI-X (Peripheral Component Interface Extended)
An enhanced version of the PCI specification, 3.3 V PCI-X can support higher average bandwidths;
it increases the speed that data can move within a computer from 66 MHz to 133 MHz. It is
backward-compatible, which means devices can operate at conventional PCI frequencies (33 MHz
and 66 MHz). PCI-X is more fault-tolerant than PCI. It also does not have programmable drive
strength.
Voltage-Referenced Standards
I/Os using these standards are referenced to an external reference voltage (VREF) and are supported
on E devices only.
HSTL Class I and II (High-Speed Transceiver Logic)
These are general-purpose, high-speed 1.5 V bus standards (EIA/JESD 8-6) for signaling between
integrated circuits. The signaling range is 0 V to 1.5 V, and signals can be either single-ended or
differential. HSTL requires a differential amplifier input buffer and a push-pull output buffer. The
reference voltage (VREF) is 0.75 V. These standards are used in the memory bus interface with data
switching capability of up to 400 MHz. The other advantages of these standards are low power and
fewer EMI concerns.
HSTL has four classes, of which low-power flash devices support Class I and II. These classes are
defined by standard EIA/JESD 8-6 from the Electronic Industries Alliance (EIA):
Class I – Unterminated or symmetrically parallel-terminated
Class II – Series-terminated
Class III – Asymmetrically parallel-terminated
Class IV – Asymmetrically double-parallel-terminated
SSTL2 Class I and II (Stub Series Terminated Logic 2.5 V)
These are general-purpose 2.5 V memory bus standards (JESD 8-9) for driving transmission lines,
designed specifically for driving the DDR SDRAM modules used in computer memory. SSTL2
requires a differential amplifier input buffer and a push-pull output buffer. The reference voltage
(VREF) is 1.25 V.
SSTL3 Class I and II (Stub Series Terminated Logic 3.3 V)
These are general-purpose 3.3 V memory bus standards (JESD 8-8) for driving transmission lines.
SSTL3 requires a differential amplifier input buffer and a push-pull output buffer. The reference
voltage (VREF) is 1.5 V.
Figure 7-6 SSTL and HSTL Topology
OUT
GND
IN
GND
VREF
VCCI
Device 1
Device 2