afs600 Actel Corporation, afs600 Datasheet - Page 143

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

5 V Input Tolerance
I/Os can support 5 V input tolerance when LVTTL 3.3 V,
LVCMOS 3.3 V, LVCMOS 2.5 V / 5 V, and LVCMOS 2.5 V
configurations are used (see
for more details). There are four recommended solutions
(see
of board and macro setups) to achieve 5 V receiver
tolerance. All the solutions meet a common requirement
of limiting the voltage at the input to 3.6 V or less. In
fact, the I/O absolute maximum voltage rating is 3.6 V,
and any voltage above 3.6 V may cause long-term gate
oxide failures.
Solution 1
The board-level design needs to ensure that the
reflected waveform at the pad does not exceed the limits
provided in
reliability requirement.
This scheme will also work for a 3.3 V PCI / PCI-X
configuration, but the internal diode should not be used
for clamping, and the voltage must be limited by the two
external resistors, as explained below. Relying on the
diode clamping would create an excessive pad DC
voltage of 3.3 V + 0.7 V = 4 V.
The following are some examples of possible resistor
values (based on a simplified simulation model with no line
effects and 10 Ω transmitter output resistance, where
Rtx_out_high = (V
Figure 2-95 • Solution 1
Figure 2-95
Table 3-4 on page
to
CCI
Figure 2-98 on page 2-132
– V
OH
) / I
OH
Table 2-74 on page 2-132
, Rtx_out_low = V
3-4. This is a long-term
5.5 V
Off-Chip
Rext1
for details
Rext2
Requires two board resistors,
OL
/ I
A d v an c ed v1 . 4
LVCMOS 3.3 V I/Os
OL
Solution 1
).
On-Chip
Fusion I/O Input
Example 1 (high speed, high current):
Rtx_out_high = Rtx_out_low = 10 Ω
Example 2 (low–medium speed, medium current):
Other values of resistors are also allowed as long as the
resistors are sized appropriately to limit the voltage at
the receiving end to 2.5 V < Vin(rx) < 3.6 V when the
transmitter sends a logic 1. This range of Vin_dc(rx) must
be assured for any combination of transmitter supply
(5 V ± 0.5 V), transmitter output resistance, and board
resistor tolerances.
Temporary overshoots are allowed according to
on page
R1 = 36 Ω (±5%), P(r1)min = 0.069 Ω
R2 = 82 Ω (±5%), P(r2)min = 0.158 Ω
Imax_tx = 5.5 V / (82 * 0.95 + 36 * 0.95 + 10) = 45.04 mA
t
up to 25% safety margin)
t
to 25% safety margin)
Rtx_out_high = Rtx_out_low = 10 Ω
R1 = 220 Ω (±5%), P(r1)min = 0.018 Ω
R2 = 390 Ω (±5%), P(r2)min = 0.032 Ω
Imax_tx = 5.5 V / (220 * 0.95 + 390 * 0.95 + 10) = 9.17 mA
t
to 25% safety margin)
t
up to 25% safety margin)
RISE
RISE
RISE
RISE
= t
= t
= t
= t
3-4.
FALL
FALL
FALL
FALL
3.3 V
= 0.85 ns at C_pad_load = 10 pF (includes
= 4 ns at C_pad_load = 50 pF (includes up
= 4 ns at C_pad_load = 10 pF (includes up
= 20 ns at C_pad_load = 50 pF (includes
Actel Fusion Programmable System Chips
Table 3-4
2-129