TRISTATE BUFFER

The Tristate Buffer module synthesizes internal non-inverting tristate buffers.

Tristate buffers are not supported for the XC9500XL family.

Figure 4.37 The Tristate Buffer Module

Inputs

The Input data port has the same width as the Output port.

Connections: The Input pin is always specified.

If the OE Phase attribute is Active Low, then when the Output Enable input is Low, the Input data passes to the Output. When the Output Enable is High, the Output is in high-impedance. If the OE Phase attribute is Active High, then when the Output Enable input is High, the Input data passes to the Output. When the Output Enable is Low, the Output is in high-impedance.

The Output Enable input must be driven. OE Phase can be Active High for the XC9500 and XC9500XL device families. To synthesize an active-High Output Enable for other device families, add an inverter to this line. This inverter is implemented within the internal tristate buffer block.

Connections: The Output Enable pin is always specified.

Note: In the Unified Libraries this pin is labeled T (for Tristate) rather than OE. Also, the bubble is present when OE Phase is Active Low and is absent otherwise.

Outputs

The Output port reflects the state of the Input port when the Output Enable is Low. When the Output Enable is High, the Output is in high-impedance. Although Tristate Buffer output ports can be tied together, only one port at a time can be active.

Connections: The Output pin is always specified.

Attributes

You can use this attribute to add pull-up resistors to Tristate Buffer outputs that are connected to on-chip buses.

This attribute is not supported for the XC9500 family.

Usage: This is an optional parameter for this module. Use this attribute to specify the number of pull-ups: Single or Double. If this attribute is not specified, no pull-up is added. The double pull-up resistor draws more power than a single resistor but supports faster transition times. See The Programmable Logic Data Book for timing details.

This attribute determines whether the Input data passes to the Output port when the Output Enable is High or Low.

Usage: This attribute can only be set by the user for the XC9500XL device family. The attribute is set to Active High for the XC9500 family and to Active Low for all other families by default.