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Lattice Semiconductor FPGA
Lattice Semiconductor
Successful Place and Route
16-13
Clock boosting is typically most useful in designs that are only missing timing on a few paths for one or two prefer-
ences. If the design is missing timing by over a few nanoseconds on any given path, clock boosting will not be able
to schedule skew in a way that will eliminate enough timing to make the critical preference. Clock boosting run
times can be shortened by using a preference file with only the failing preferences in it.
Figure 16-11. Clock Boosting Example
The example illustrated in
Figure 16-11 shows two register-to-register transfers that both need to meet the 10 ns
period constraint. By using delay cell DEL2 to delay the clock input on flip-flop FF_2, the first register transfer will
make its period constraint with a new minimum period of ~9.7 ns and the second register transfer will make its
period constraint by ~8.3 ns.
The D1, D2, and D3 delays shown in
Figure 16-11 are variable depending on the speed grade and Lattice Semi-
conductor FPGA device family. For complete timing information, reference the software generated timing data
sheet, included with ispLEVER, for the desired Lattice Semiconductor FPGA device family.
To Perform Clock Boosting in the Project Navigator
1. In the Project Navigator Sources window, select the target device.
2. In the Processes window, right-click the Clock Boosting under Place & Route Design process, and then
select Properties to open the Properties dialog box.
3. Select the Clock Boosting Output Filename property from the property list and type the name of the out-
put file name in the edit region (<file_name>.ncd).
4. Click Close to close the dialog box.
As shown in
Figure 16-12, the original .ncd and .prf files as well as the output .ncd file are typed into the corre-
sponding entries. Checking “Maximize Frequency” will push the tool to improve the frequency beyond the input
preference requirement. This is generally only useful for bench marking.
DEL1 ~= 0.7 ns
DEL2 ~= 1.3 ns
DEL3 ~= 2.0 ns
DEL
1
DEL
2
DEL
3
FF_1
Clock
Target Performance: 10 ns period (100 MHz)
FF_2
FF_3
7 ns
11 ns
Combinational
Logic