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ProASIC3/E Flash Family FPGAs
v2.1
2-39
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 and LVCMOS 2.5 V / 5.0 V
more details). There are four recommended solutions for
setups). 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 must ensure that the reflected
waveform at the pad does not exceed the limits provided
long-term reliability.
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.
Here 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 = (VCCI – VOH) / IOH, Rtx_out_low = VOL / IOL).
Example 1 (high speed, high current):
Rtx_out_high = Rtx_out_low = 10
Ω
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
tRISE = tFALL = 0.85 ns at C_pad_load = 10 pF (includes up
to 25% safety margin)
tRISE = tFALL = 4 ns at C_pad_load = 50 pF (includes up to
25% safety margin)
Example 2 (low–medium speed, medium current):
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
tRISE = tFALL = 4 ns at C_pad_load = 10 pF (includes up to
25% safety margin)
tRISE = tFALL = 20 ns at C_pad_load = 50 pF (includes up to
25% safety margin)
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
Table 3-4Figure 2-27 ProASIC3 Solution 1
Solution 1
5.5 V
3.3 V
Requires two board resistors,
LVCMOS 3.3 V I/Os
ProASIC3 I/O Input
Rext1
Rext2