原创 cyclone 系列IO口问题--PCI clamp diode

（转自）http://www.altera.com.cn/support/devices/io/features/io-features.html
1. PCI钳位二极管
到底能不能与5V接口呢？通过限流电阻R2
R1=5V设备的输出电阻，Iih是输入最大的电流，或者DIODE的最大限流
R2=（5V-3.3V）/（Iih）-R1  就是最小的限流电阻

PCI-Clamp Diode

When to Use

• A PCI diode-enabled pin only affects signal voltage about 0.7 V above V_CCIO level. Higher voltage levels are clipped, effectively reducing voltage level at pin to about V_CCIO + 0.7-V level. 0.7 V is an approximation of the on-chip diode turn-on voltage.
• Use when the voltage overshoot seen at the FPGA pin exceeds
  acceptable maximum level. FPGAs in the Cyclone family have maximum DC
  input and maximum overshoot (AC) voltage specifications.
• Use when interfacing a Cyclone II FPGA  or a Cyclone FPGA with a
  5.0-V LVTTL device to clamp voltage at the FPGA pin to an acceptable
  maximum level.
• For all cases of usage, determine if series resistor is needed to
  reduce DC current to acceptable limit through the on-chip diode.

How to Use

• In the Assignment Editor, set the PCI I/O assignment to ON to enable the on-chip clamp diode for the pin.

Feature Availability

• All banks of user I/O pins for Cyclone III FPGAs.
• Only with side bank user I/O pins for Cyclone II and Cyclone FPGAs.

Feature Limitations

• Not supported in dedicated input clock and configuration pins.
• There is a maximum of 10-mA DC current through the on-chip clamp
  diode for Cyclone III FPGAs and 25- mA for Cyclone II and Cyclone FPGAs.
• Not available in dual-purpose configuration pins that are used during configuration for FPGAs in the Cyclone III family.