2.3.2.1 5V Tolerance

There are two schemes to achieve 5V tolerance.

3.3V PCI and 3.3V PCI-X are the only I/O standards that directly allow 5V tolerance. To implement this, an internal clamp diode between the input pad and the VCCI pad is enabled so that the voltage at the input pin is clamped, as shown in the following equation:

V_input = VCCI + V_diode = 3.3V + 0.7V = 4.0V

The internal VCCI clamp diode is only enabled while the device is powered on, so the voltage at the input will not be clamped if the VCCI or VCCA are powered off. An external series resistor (~100 ?) is required between the input pin and the 5V signal source to limit the current to less than 20 mA (see the following figure). The 100 ? resistor was chosen to meet the input Tr/Tf requirement (see Table 2-24). The GND clamp diode is available for all I/O standards and always enabled.

Figure 2-3. Use of an External Resistor for 5V Tolerance

5V tolerance can also be achieved with 3.3V I/O standards (3.3V PCI, 3.3V PCI-X, and LVTTL) using a bus-switch product (for example, IDTQS32X2384). This will convert the 5V signal to a 3.3V signal with minimum delay (see the following figure).

Figure 2-4. Bus Switch IDTQS32X2384