I/O Structures in IGLOOe and ProASIC3E Devices

20

v1.4

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

are selected. Table 6 on page 6 shows the I/O standards that support 5 V input tolerance. Only 3.3 V

LVTTL/LVCMOS standards support 5 V output tolerance. Refer to the appropriate family datasheet

for 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

2.5 V / 5.0 V configurations are used (see Table 13 on page 19). There are four recommended

solutions for achieving 5 V receiver tolerance (see Figure 10 on page 21 to Figure 13 on page 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.

This solution requires two board resistors, as demonstrated in Figure 10 on page 21. 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

and

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

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

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 the overshoot and undershoot table in the

datasheet.