Shmoo Plots

The hunt for electrical bugs begins with the Shmoo plot. The Shmoo plot provides useful information on operating margins relative to specifications for a device under test (DUT) and is frequently used as a trouble shooter or a quality measure.

The Shmoo plot illustrates the region of correct operation of the DUT under various combinations of operating conditions resulting from the combination of test variables such as voltage, frequency, timing or temperature.

A functional test (a set of test patterns) is applied repeatedly while sweeping each test variable over a range. Each functional test result is aggregated into Pass or Fail, i.e. whether the DUT passes or fails the entire test pattern set under the given combination of test variables. Specific combinations might violate test constraints or not performed for other reasons; In this case the test result could be No test.

The Shmoo plot will be illustrated by measuring the effect of power supply reduction on performance:
Nominally, a DUT operates at 1 Volt Vdd with 25 ns delay. A set of test patterns is applied repeatedly to the DUT while sweeping:

• the output delay measured using the compare strobe:
  sweeping variable Delay 1 from 20 ns to 30 ns (x-axis)
• the supply voltage Vdd:
  sweeping variable DUT Power 1 from 0.8 Volts to 1.2 Volts (y-axis)

Click to set up the Shmoo plot