Specifying Timing Points

There are situations where a particular point or set of points in your design needs to be flagged for reference in subsequent timing specifications. Timing points are used for these specifications.

There are two types of timing points.

• A TPSYNC timing point is used to allow a point to be used as the start or the end of timing path, even though the point may not apply to a flip-flop, latch, RAM or I/O pad.
  
  
• A TPTHRU timing point identifies an intermediate point on a path.
  
  

The following sections describe how these timing points are specified in a schematic. The syntax for specifying TPSYNC and TPTHRU constraints in a UCF or NCF constraints file is described in the “Attributes, Constraints, and Carry Logic” chapter of the Libraries Guide.

Using TPSYNC to Define Synchronous Points

There are cases where the timing of a design must be defined from or to a point in the design that is not a flip-flop, latch, RAM or I/O pad. For example, you might want to specify a point at the output of a latch defined using a function generator instead of a latch symbol. The TPSYNC timing point identifies one or a group of these points.

A TPSYNC attribute has the following syntax.

Schematic syntax

TPSYNC = identifier

UCF syntax

{NET | PIN | INST} TPSYNC= identifier;

identifier is a name that is used in timing specifications in the same way that groups are used. The same identifier can be used on several points which are then treated as a group from the point of view of timing analysis. The identifier must be different from any identifier used for a TNM attribute.

The way a TPSYNC timing point is used depends on the object to which it is attached.

• Attached to a net, TPSYNC identifies the source of the net as a potential source or destination for timing specifications.
  
  
  
  
• Attached to an output macro pin, TPSYNC identifies all of the sources inside the macro that drive the pin to which the attribute is attached as potential sources or destinations for timing specifications. In the following diagram. POINTY applies to the inverter.
  
  

  Figure 4.17 TPSYNCs Attached to Macro Pins

  If the macro pin is an input pin (that is, there are no sources for the pin in the macro), then all of the load pins in the macro are flagged as synchronous points. In the preceding figure POINTX applies to the input AND gate.
  
  
• Attached to a primitive pin, TPSYNC flags the primitive's pin as a potential source or destination for timing specifications; TPSYNC applies to the pin it is attached to.
  
  
• Attached to a primitive symbol, TPSYNC identifies the output(s) of that element as a potential source or destination for timing specifications. See the following figure.
  
  

The use of a TPSYNC timing point to define a synchronous point in a design implies that the flagged point cannot be merged into a function generator. For example, consider the following diagram.

In this example, because of the TPSYNC definition, the two gates cannot be merged into a single function generator.

Using TPTHRU to Define Through Points

The TPTHRU attribute defines an intermediate point in a path. A point or group defined with TPTHRU attributes is used in detailed timing specifications.

A TPTHRU attribute has the following syntax.

TPTHRU = identifier

identifier is a name that is used in timing specifications in the same way that groups are used. The same identifier can be used on several points which are then treated as a group from the point of view of timing analysis.

The identifier must be different from any identifier used for a TNM attribute or TPSYNC.

Timing specifications using TPTHRU groups are described in the “Specifying Time Delay in TS Attributes” section.