I/O Structures in IGLOO and ProASIC3 Devices
7- 20
v1.1
5 V Input and Output Tolerance
IGLOO and ProASIC3 devices are both 5 V-input– and 5 V–output–tolerant if certain I/O standards
3.3 V LVTTL/LVCMOS standards support 5 V output tolerance. Refer to the appropriate family
datasheet for the detailed description and configuration information.
This feature is not shown in the I/O Attribute Editor.
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
page 7-23 for details of board and macro 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 in the recommended operating conditions in the datasheet. This is a requirement to
ensure 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.
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
CCI –VOH)/IOH and
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.