Forum Discussion

Altera_Forum's avatar
Altera_Forum
Icon for Honored Contributor rankHonored Contributor
14 years ago

What's the maximum input current of 3.3V LVTTL for Cyclone4?

Hi, AN447 mentions "The PCI-clamp diode in Cyclone III and Cyclone IV devices can support a maximum of 10-mA DC current" at page 3, but Table 3 tell us the Maximum Allowed Current from driver ...
Altera_Forum's avatar
Altera_Forum
Icon for Honored Contributor rankHonored Contributor
14 years ago

The 10 mA rating is a DC value, not the overshoot peak current. The point is discussed in AN447:

--- Quote Start ---

10-mA DC current limit refers to the current that the diode sinks and not the drive strength of the driver. This limit is only applicable when the PCI-clamp diode is enabled and when the 2.5-V Cyclone III and Cyclone IV device receiver interfaces with 3.0-V or 3.3-V LVTTL/LVCMOS I/O systems.

--- Quote End ---

In my opinion, you should preferably adjust FPGA driver strengths accordingly and use series termination with other chips to avoid overshoots instead of relying on the PCI diode clamping the input voltage.