eldo_user

Optimization
Setup Time Computation
Related Topics
Optimization Options
Specifying Design Variables
Scaling Design Objectives
Setup Time Computation
The goal here is to use Eldo to compute the setup time of a flip-flop. The definition of setup
time is: time between input and the clock (TIC) so that propagation time between clock and
output (TCO) is 10% above nominal value (computed when there is a large time between input
and the clock).
If the nominal value of TCO is already known, this gives:
TCO_TARGET= 1.1 * TCO_NOMINAL
A very general method, available even with older versions of Eldo, would be to sweep TIC,
measure TCO, adjusting the value of TIC so that TCO=TCO_TARGET.
.PARAM TIC=200n
.STEP PARAM TIC 205N 195N 0.1N
.EXTRACT LABEL=TCO
+ (XUP(V(Q),0.48,200n,300n,1) - XUP(V(CP),0.48,200n,300n,1))
.EXTRACT SWEEP xycond(XAXIS, meas(TCO)=1.1*TCO_NOMINAL)
Simulation time can be improved with the use of .OPTION AUTOSTOP=2. Accuracy is
proportional to the parameter sweep increment, and simulation time is proportional to the
number of steps in the swept interval.
The PASSFAIL optimization method is useful for this example. It computes the maximum
value for which an extract can be measured. This is not exactly the definition of set-up,
however, it can be used as a first step, this would give:
.OPTIMIZE METHOD=PASSFAIL RESULTS=TCO
.PARAMOPT TIC=(201N,199N,205N)
The DICHOTOMY method is also useful. It finds the value of an optimization variable so that
one extract yields a target value, provided that the extracted values for the Min and Max values
of the optimization variable bracket the target.
In this example, the Min value of the optimization parameter TIC has been computed with the
PASSFAIL method, this will give:
.EXTRACT TRAN LABEL=DCP
+ (XUP(V(Q),0.48,200N,300N,1) - XUP(V(CP),0.48,200N,300N,1))
+ GOAL={1.1*TCO_NOMINAL}
.PARAMOPT TIC=(201N,200.07N,205N) !MIN provided by PASSFAIL
Eldo® User's Manual, 15.3 681