Due to an issue in some Cyclone® IV GX devices, excessive overshoot and ringing may be observed on signal rising edges when RS OCT with calibration is enabled for output pins. This is because the pull-up resistance in the I/O buffer is smaller than the intended 25 Ohms or 50 Ohms. This issue only affects the following Cyclone IV GX device variants :
- EP4CGX15
- EP4CGX22
- EP4CGX30
If you observe excessive overshoot on signals driven from I/O pins which have RS OCT with calibration enabled in the affected devices, as a workaround, use RS OCT without calibration instead.