[Python-ideas] SI scale factors in Python

[Ken Kundert]

Here is a fairly typical example that illustrates the usefulness of supporting 
SI scale factors and units in Python.

This is simple simulation code that is used to test a current mirror driving an 
100kOhm resistor ...

Here is some simulation code that uses SI scale factors

    for delta in [-500nA, 0, 500nA]:
        input = 2.75uA + delta
        wait(1us)
        expected = 100kOhm*(2.75uA + delta)
        tolerance = 2.2mV
        fails = check_output(expected, tolerance)
        print('%s: I(in)=%rA, measured V(out)=%rV, expected V(out)=%rV, diff=%rV.' % (
            'FAIL' if fails else 'pass',
            input, get_output(), expected, get_output() - expected
        ))

with the output being:

    pass: I(in)=2.25uA, measured V(out)=226.7mV, expected V(out)=225mV, diff=1.7mV.
    pass: I(in)=2.75uA, measured V(out)=276.8mV, expected V(out)=275mV, diff=1.8mV.
    FAIL: I(in)=3.25uA, measured V(out)=327.4mV, expected V(out)=325mV, diff=2.4mV.

And the same code in Python today ...

    for delta in [-5e-7, 0, 5e-7]:
        input = 2.75e-6 + delta
        wait(1e-6)
        expected = 1e5*(2.75e-6 + delta)
        tolerance = 2.2e-3
        fails = check_output(expected, tolerance)
        print('%s: I(in)=%eA, measured V(out)=%eV, expected V(out)=%eV, diff=%eV.' % (
            'FAIL' if fails else 'pass',
            input, get_output(), expected, get_output() - expected
        ))

with the output being:

    pass: I(in)=2.25e-6A, measured V(out)=226.7e-3V, expected V(out)=225e-3V, diff=1.7e-3V.
    pass: I(in)=2.75e-6A, measured V(out)=276.8e-3V, expected V(out)=275e-3V, diff=1.8e-3V.
    FAIL: I(in)=3.25e-6A, measured V(out)=327.4e-3V, expected V(out)=325e-3V, diff=2.4e-3V.

There are two things to notice. In the first example the numbers are easier to
read: for example, 500nA is easier to read the 5e-7. Second, there is 
information in the units that provides provides useful information. One can 
easily see that the input signal is a current and that the output is a voltage.  
Furthermore, anybody can look at this code and do a simple sanity check on the 
expressions even if they don't understand the system being simulated. They can 
check the units on the input and output expressions to assure that they are all 
consistent.

-Ken