By Diego Hammerschlag
Sr. Technical Leader
The “Orbiter” was a spacecraft on a mission to study the planet Mars. Unfortunately, Lockheed Martin and NASA had a mix up using Imperial units (pounds, miles, etc.) and Metric units (kilometers, kilograms, etc.) Bad things happen to spacecraft when such mix ups occur. The Orbiter was no exception to this and after entering orbit at 57 km, instead of the 140 km it was supposed to, it quickly disintegrated due to the material stress induced. This “simple” mistake only cost US$ 327.6 Million, not to mention, countless hour of work on all the researchers whose work was on the spacecraft.
You may wonder, what does this have to do with your latest advanced node project? As technologies keep shrinking, libraries have been slowly but surely migrating to different units. Years ago it was the norm to use nanoseconds and picofarads for units of timing and capacitance respectively but this is no longer the case. In the case of timing, some collateral uses nanoseconds whereas other uses picoseconds and old legacy collateral may still be using microseconds. Thus far I have only mentioned timing and capacitance units, but this trend is true for all units used in the flow. The good news is that these issues should not affect the way EDA tools interpret the various libraries that are used since the units they require are specified in the files themselves. The bad news is that flows require other files that do not specify the units to be used and those files put your design at risk.
The most common example of a file without unit specification but implicitly using them is the SDC file that mainly contains the design’s timing intent. A command in an SDC file may be specified as
> set_load 3.5 ...
> set_input_delay –clock clk 4.5 [get_ports ...]
Note that neither of the examples shown specifies the unit. The unit is inferred from some rule based on the unit definitions of the library collateral or other mechanisms. The fact is that different EDA tools follow different unit inference rules. This is not only true across different vendors but, even within the same vendor, there are no consistent rules to resolve unit-conflicts across libraries. Synthesis tools frequently rely on the units used in the first library read. This means that, depending on the ordering of your libraries, the units will be interpreted differently. This is neither intuitive nor safe and need to be handled carefully.
To make matters worse, the advent of additional languages to specify additional information further exacerbates the problem. Take, for example, the Si2 Common Power Format (CPF). CPF is used to capture power intent and has its own commands set_power_unit and set_timing_unit for setting power and timing units respectively. As a result, the question comes up of whether or not the commands mentioned take precedence over the library collateral and / or native commands? Is the scope of such commands only in the CPF file? What should be the default if the commands are not used in the CPF file?
Finally, as if there were not enough unit challenges already, there is a question of what units should be used when generating output files. Should it be the same as the units used on the input files? Should a given tool always generate output files using the same units? Should it only be controlled by native commands?
Hopefully this article has helped illustrate the challenges ahead of us due to the shift in the units we use to do our job. In part II of this article, I will discuss in more detail specific cases of how tools defer in determining their default units and several ways in which such differences could be addressed.
Don’t let your project become another “Mars Orbiter”!!!
For more information on the Mars Obrbiter refer to: