VHDL Programming

lengths that allow connections to adjacent logic areas. Sometimes, longer

connections are needed, and either a longer line must be used or shorter

lines must be connected together to form the connection. This is shown in

Figure 16-4.

The job of the place and route tool is to create the programming files

that will be used to specify the logic function of the logic macros in the logic

areas and the switch programming of the wires used to connect the macros

together. Too many switches on a routed signal can cause some negative

performance effects. Each switch adds capacitance and resistance to the

routed signal. After only a few connections, signals start to slow signifi-

cantly because of the capacitance and resistance of the line.

The place and route tool, therefore, must try to minimize long connections

and the number of switches for a particular signal to create designs with

the highest speed. To get the highest utilization, the place and route

tools need to pack as many of the logical functions into a logic area as

possible and then use as much local routing resources as possible to connect

these functions.

The place and route tools can make tradeoffs if the speed-critical signals

are known ahead of time and are implemented using the highest speed

interconnect signals. The placement algorithm also tries to place logical

gates on the critical path close to each other so that local interconnect can

be used to connect the gates. Local interconnect is usually very fast because

the wires are short. Short wires have less capacitance and resistance and,

therefore, can operate at much higher speeds.

372 Chapter Sixteen

1

10

2

11

9

18

8

17

3

12 13

4

14

5

15

6 7

Connects short

segments together

Figure 16-4
Vertical and Horizon-
tal Routing.