Why is excessive overshoot observed on signals when series on-chip termination (RS OCT) with calibration is enabled on some Cyclone IV GX devices?

Description

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

Resolution

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.