afs600 Actel Corporation, afs600 Datasheet - Page 141

• Manufacturer Part Number: afs600
• Description: Actel Fusion Programmable System Chips Mixed-signal Family With Optional Arm Support
• Manufacturer: Actel Corporation

For Fusion devices requiring Level 3 and/or Level 4
compliance, the board drivers connected to Fusion I/Os
need to have 10 kΩ (or lower) output drive resistance at
hot insertion, and 1 kΩ (or lower) output drive resistance
at hot removal. This is the resistance of the transmitter
sending a signal to the Fusion I/O, and no additional
resistance is needed on the board. If that cannot be
assured, three levels of staging can be used to meet Level
3 and/or Level 4 compliance. Cards with two levels of
staging should have the following sequence:
Cold-Sparing Support
Cold-sparing means that a subsystem with no power
applied (usually a circuit board) is electrically connected
to the system that is in operation. This means that all
input buffers of the subsystem must present very high
input impedance with no power applied so as not to
disturb the operating portion of the system.
Pro I/O banks and standard I/O banks fully support cold-
sparing.
For Pro I/O banks, standards such as PCI that require I/O
clamp diodes, can also achieve cold-sparing compliance,
since clamp diodes get disconnected internally when the
supplies are at 0 V.
For Advanced I/O banks, since the I/O clamp diode is
always active, cold-sparing can be accomplished either by
employing a bus switch to isolate the device I/Os from
the rest of the system or by driving each advanced I/O pin
to 0 V.
If Standard I/O banks are used in applications requiring
cold-sparing, a discharge path from the power supply to
ground should be provided. This can be done with a
discharge resistor or a switched resistor. This is necessary
because the standard I/O buffers do not have built-in I/O
clamp diodes.
If a resistor is chosen, the resistor value must be
calculated based on decoupling capacitance on a given
power supply on the board (this decoupling capacitor is
in parallel with the resistor). The RC time constant should
ensure full discharge of supplies before cold-sparing
functionality is required. The resistor is necessary to
ensure that the power pins are discharged to ground
every time there is an interruption of power to the
device.
I/O cold-sparing may add additional current if the pin is
configured with either a pull-up or pull down resistor
and driven in the opposite direction. A small static
current is induced on each IO pin when the pin is driven
to a voltage opposite to the weak pull resistor. The
current is equal to the voltage drop across the input pin
divided by the pull resistor. Please refer to
1. Grounds
2. Powers, I/Os, other pins
Table 2-92 on
A d v an c ed v1 . 4
page
page 2-157
corresponding I/O standard.
For example, assuming an LVTTL 3.3 V input pin is
configured with a weak Pull-up resistor, a current will
flow through the pull-up resistor if the input pin is
driven low. For an LVTTL 3.3 V, pull-up resistor is ~45 kΩ
and the resulting current is equal to 3.3 V / 45 kΩ = 73 µA
for the I/O pin. This is true also when a weak pull-down is
chosen and the input pin is driven high. Avoiding this
current can be done by driving the input low when a
weak pull-down resistor is used, and driving it high when
a weak pull-up resistor is used.
In Active and Static modes, this current draw can occur in
the following cases:
Electrostatic Discharge (ESD) Protection
Fusion devices are tested per JEDEC Standard JESD22-
A114-B.
Fusion devices contain clamp diodes at every I/O, global,
and power pad. Clamp diodes protect all device pads
against damage from ESD as well as from excessive
voltage transients.
Each I/O has two clamp diodes. One diode has its
positive (P) side connected to the pad and its negative
(N) side connected to V
side connected to GND and its N side connected to the
pad. During operation, these diodes are normally
biased in the Off state, except when transient voltage is
significantly above V
By selecting the appropriate I/O configuration, the diode
is turned on or off. Refer to
and
about I/O standards and the clamp diode.
The second diode is always connected to the pad,
regardless of the I/O configuration selected.
Table 2-73 on page 2-128
2-155,
– Input buffers with pull-up, driven low
– Input buffers with pull-down, driven high
– Bidirectional buffers with pull-up, driven low
– Bidirectional buffers with pull-down, driven
– Output buffers with pull-up, driven low
– Output buffers with pull-down, driven high
– Tristate buffers with pull-up, driven low
– Tristate buffers with pull-down, driven high
high
Table 2-93 on page
for the specific pull resistor value for the
Actel Fusion Programmable System Chips
CCI
or below GND levels.
CCI
. The second diode has its P
Table 2-72 on page 2-128
2-155, and
for more information
Table 2-94 on
2-127