Output Enable Group logic option

A logic option that assigns an output enable group number for the specified node. Turning on this option directs the Fitter to view the specified nodes as an output enable group so that the design does not violate the requirements for the maximum number of pins driving out of a VREF group when a voltage-referenced input pin or bidirectional pin is present. For bidirectional pins, the Fitter determines all possible pins that may potentially drive out when any bidirectional pin is driving in by looking at the output enable of all the bidirectional pins in the VREF group. This behavior can result in the VREF group exceeding the maximum number of outputs and the Fitter is unable to fit the design.

Turning on the Output Enable Group option allows you to specify an output enable group for specific pins, thus allowing you to specify which pins in the design are driving in and out at the same time. The Fitter only considers pins as potential outputs when they are in separate output enable groups or when they are not in an output enable group. By specifying an output enable group, you can lower the total number of outputs in the VREF group when any pin is driving in. As a result, the Fitter does not count all of the potential outputs of the bidirectional pins and the number of outputs in the VREF group remains within the legal range. You should turn on this option when the Fitter cannot detect the output enable group of the pins in the VREF group, for example when the output enable comes from a state machine or complex logic.

This option is useful for specifying the output enable group number of DQ I/O and DQS I/O pins, and DM pins if they are only used during a write operation, for use with the dedicated DDR SDRAM interface. Each individual controller should have a different output enable group number.

For detailed information on the number of outputs supported by a VREF group, refer to the device family data sheet available from the Literature section of the Altera website .

This option must be assigned to a pin or it is ignored.

Scripting Information

Keyword: output_enable_group

Settings: <integer>