Axcelerator Family FPGAs
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User I/Os2
Introduction
The Axcelerator family features a flexible I/O structure, supporting a range of mixed voltages (1.5 V,
I/O standards.
Each I/O provides programmable slew rates, drive strengths, and weak pull-up and weak pull-down
circuits. The slew rate setting is effective for both rising and falling edges.
I/O standards, except 3.3 V PCI and 3.3 V PCI-X, are capable of hot insertion. 3.3 V PCI and 3.3 V PCI-
X are 5 V tolerant with the aid of an external resistor.
The input buffer has an optional user-configurable delay element. The element can reduce or eliminate
the hold time requirement for input signals registered within the I/O cell. The value for the delay is set on
a bank-wide basis. Note that the delay WILL be a function of process variations as well as temperature
and voltage changes.
Each I/O includes three registers: an input (InReg), an output (OutReg), and an enable register (EnReg).
I/Os are organized into banks, and there are eight banks per device—two per side (
Figure 2-6 onpage 2-18). Each I/O bank has a common VCCI, the supply voltage for its I/Os.
For voltage-referenced I/Os, each bank also has a common reference-voltage bus, VREF. While VREF
must have a common voltage for an entire I/O bank, its location is user-selectable. In other words, any
user I/O in the bank can be selected to be a VREF.
The location of the VREF pin should be selected according to the following rules:
Any pin that is assigned as a VREF can control a maximum of eight user I/O pad locations in each
direction (16 total maximum) within the same I/O bank.
I/O pad locations listed as no connects are counted as part of the 16 maximum. In many cases,
this leads to fewer than eight user I/O package pins in each direction being controlled by a VREF
pin.
Dedicated I/O pins such as GND and VCCI are counted as part of the 16.
The two user I/O pads immediately adjacent on each side of the VREF pin (four in total) may only
be used as inputs. The exception is when there is a VCCI/GND pair separating the VREF pin and
the user I/O pad location.
The user does not need to assign VREF pins for OUTBUF and TRIBUF. VREF pins are needed
only for input and bidirectional I/Os.
The differential amplifier supply voltage VCCDA should be connected to 3.3 V.
A user can gain access to the various I/O standards in three ways:
Instantiate specific library macros that represent the desired specific standard.
Use generic I/O macros and then use Designer’s PinEditor to specify the desired I/O standards
(please note that this is not applicable to differential standards).
A combination of the first two methods.
for more details.
2.
Do not use an external resister to pull the I/O above VCCI for a higher logic “1” voltage level. The desired higher logic “1”
voltage level will be degraded due to a small I/O current, which exists when the I/O is pulled up above VCCI.