Fusion Family of Mixed Signal FPGAs
Revision 4
2-147
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
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
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
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
R1 = 36
(±5%), P(r1)min = 0.069
R2 = 82
(±5%), P(r2)min = 0.158
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
R1 = 220
(±5%), P(r1)min = 0.018
R2 = 390
(±5%), P(r2)min = 0.032
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.