Ambit BuildGates Synthesis User Guide

Setting Constraints

set_fanout_load_limit load_limit port_list

where:

■load_limit is the fanout load limit on the specified ports.

■port_list is the list of ports with this fanout load limit.

External Sources and Sinks

The external sources and sinks at a port specify the number of sources and sinks to the net which is connected to the ports that are external to the design. If you set the number of external sources or sinks on a port, then the port capacitance for such a port should include only the external port capacitances. The total wire capacitance of a net is automatically computed using the asserted external sources and sinks and results in an accurate delay computation. The set_num_external_ commands may be omitted if the port capacitance includes internal wire capacitances.

The syntax for the commands is as follows:

set_num_external_sources num port_list

set_num_external_sinks num port_list

where:

■num is the non-negative integer number of sources or sinks.

■port_list is the list of ports with the specified number of sources or sinks.

The source and sink numbers are factored into the wire capacitance and the wire resistance estimation done for the port nets using the wire load models. The value does not add towards the total fanout count for Design Rule Violation (DRV).

Wire Capacitance

After the physical design is complete, you can back-annotate capacitance on each net and re-analyze the netlist for timing (see Figure 6-18). The syntax for setting the wire capacitance command is as follows:

set_wire_capacitance float net_names [-pvt {min, typ, max}]

Ambit BuildGates Synthesis User Guide

Setting Constraints

where:

■float is the wire capacitance.

■net_names is a list of nets with this capacitance.

Figure 6-18 Wire Capacitance

Wire Resistance

Similar to the capacitance specification, you can also specify resistance for each net after the physical design is done. The syntax for setting the wire resistance command is as follows:

set_wire_resistance float net_names [-pvt {min, typ, max}]

where:

■float is resistance value.

■net_names is a list of nets with this resistance.