eldo_user

Examples
Example 24—Cell Characterization: Setup Time Extraction
.option mtfile
The above line forces the creation of a .mt0 file, where the final values of all measurements
(including ts) can be read (view extract_setup_time_passfail.mt0).
.plot tran v(in) v(ck) v(out)
The above line specifies that voltage/time plots should be performed of the inputs and output
voltages at nodes in, ck and out respectively.
.extract tran label=tp (xup(v(out), 'ps/2', 0n, 100n, 1) - xdown(v(ck),
'ps/2', 0n, 100n, 1))
The above line specifies to extract the tp variable, which is the delay between the clock edge
and the output signal edge.
.optimize method=passfail results=tp relin=0.001
.paramopt ts=(1n, -5n, 5n)
The above lines specify the .OPTIMIZE command is used with the passfail method. The goal is
the tp variable, and the variable to optimize is ts (the setup time)
The command is very compact and requires further explanation:
The parameter to optimize is defined with the .PARAMOPT statement: it is the value of the
setup time ts (see the VIN source definition). If the edge of the IN signal occurs early enough,
the flip-flop will be able to sample the input data. If it is too late (too close to the clock edge),
the flip-flop will not be able to sample the data and the output will not switch. The purpose of
the setup time extraction is to find the exact transition between these two behaviors, using a
dichotomy algorithm on the input optimization variable (tin in this example). A dichotomy
algorithm find a solution by dividing in half the search interval at each iteration.
As in any numerical root-finding algorithm, it is very important to feed the command with
initial values that 'bracket' the solution. Eldo will start anyway by verifying that the lowest value
corresponds to a 'pass', and the highest value corresponds to a 'fail' situation (you can easily see
this if you look into the standard output of the simulation). In this example, we use the interval
[-5n 5n] for the initial bracketing of 'ts'. If the edge happens at tck-5n, we are sure that the setup
time will be respected, and it will thus be a 'pass'. If it only happens at tck+5n (well after the
clock edge) we are sure that the setup time will be violated and thus the flip-flop will 'fail' (the
output will not toggle).
To minimize the number of iterations, it is of course desirable to use an initial bracketing
interval that is as narrow as possible. This requires a priori knowledge of the circuit, and needs
to be indicated by the user, Eldo cannot completely 'guess' it.
The iterations will find the value of the parameter to 'optimize', that is 'ts', which corresponds to
the transition from 'pass' to 'fail'. The 'ts' value that corresponds to this pass/fail transition is
nothing but the desired setup time.
Eldo® User's Manual, 15.3 1335