M1AFS250-FGG256I Actel Corporation, M1AFS250-FGG256I Datasheet - Page 162

• Manufacturer Part Number: M1AFS250-FGG256I
• Description: Actel Fusion Family Of Mixed Signal Fpgas
• Manufacturer: Actel Corporation

Device Architecture
2- 14 6
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
LVCMOS 2.5 V configurations are used (see
recommended solutions (see
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
provided in
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 Ω transmitter output resistance, where Rtx_out_high = (V
Rtx_out_low = V
Example 1 (high speed, high current):
Rtx_out_high = Rtx_out_low = 10 Ω
Example 2 (low–medium speed, medium current):
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.
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
t
t
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
t
t
RISE
RISE
RISE
RISE
= t
= t
= t
= t
Table 3-4 on page
FALL
FALL
FALL
FALL
OL
= 0.85 ns at C_pad_load = 10 pF (includes up to 25% safety margin)
= 4 ns at C_pad_load = 50 pF (includes up to 25% safety margin)
= 4 ns at C_pad_load = 10 pF (includes up to 25% safety margin)
= 20 ns at C_pad_load = 50 pF (includes up to 25% safety margin)
/ I
OL
).
Figure 2-101
3-4. This is a long-term reliability requirement.
to
R e visio n 1
Table 2-74 on page 2-149
Figure 2-104 on page 2-148
for more details). There are four
for details of board and macro
CCI
– V
OH
) / I
OH
,