I/O Structures in IGLOO and ProASIC3 Devices
7- 24
v1.1
5 V Output Tolerance
IGLOO and ProASIC3 I/Os must be set to 3.3 V LVTTL or 3.3 V LVCMOS mode to reliably drive 5 V TTL
receivers. It is also critical that there be NO external I/O pull-up resistor to 5 V, since this resistor
would pull the I/O pad voltage beyond the 3.6 V absolute maximum value and consequently cause
damage to the I/O.
When set to 3.3 V LVTTL or 3.3 V LVCMOS mode, the I/Os can directly drive signals into 5 V TTL
receivers. In fact, VOL =0.4 V and VOH = 2.4 V in both 3.3 V LVTTL and 3.3 V LVCMOS modes exceeds
the VIL =0.8 V and VIH = 2 V level requirements of 5 V TTL receivers. Therefore, level 1 and level 0
will be recognized correctly by 5 V TTL receivers.
Schmitt Trigger
A Schmitt trigger is a buffer used to convert a slow or noisy input signal into a clean one before
passing it to the FPGA. Using Schmitt trigger buffers guarantees a fast, noise-free input signal to
the FPGA.
The Schmitt trigger is available for the LVTTL, LVCMOS, and 3.3 V PCI I/O standards.
This feature can be implemented by using a Physical Design Constraints (PDC) command
(Table 7-5on page 7-5) or by selecting a check box in the I/O Attribute Editor in Designer. The check box is
cleared by default.
Table 7-13 Comparison Table for 5 V–Compliant Receiver Solutions
Solution
Board Components
Speed
Current Limitations
1
Two resistors
Low to High1 Limited by transmitter's drive strength
2
Resistor and Zener 3.3 V
Medium
Limited by transmitter's drive strength
3
Bus switch
High
N/A
4
Minimum resistor value2,3,4,5
R = 47
Ω at T
J = 70°C
R = 150
Ω at T
J = 85°C
R = 420
Ω at T
J = 100°C
Medium
Maximum diode current at 100% duty cycle, signal
constantly at 1
52.7 mA at TJ = 70°C / 10-year lifetime
16.5 mA at TJ = 85°C / 10-year lifetime
5.9 mA at TJ = 100°C / 10-year lifetime
For duty cycles other than 100%, the currents can be
increased by a factor of 1 / (duty cycle).
Example: 20% duty cycle at 70°C
Maximum current = (1 / 0.2) × 52.7 mA = 5 × 52.7 mA =
263.5 mA
Notes:
1. Speed and current consumption increase as the board resistance values decrease.
2. Resistor values ensure I/O diode long-term reliability.
3. At 70°C, customers could still use 420
Ω on every I/O.
4. At 85°C, a 5 V solution on every other I/O is permitted, since the resistance is lower (150
Ω) and the current
is higher. Also, the designer can still use 420
Ω and use the solution on every I/O.
5. At 100°C, the 5 V solution on every I/O is permitted, since 420
Ω are used to limit the current to 5.9 mA.