To compensate for these and other types of process variations,
we propose the process compensation unit shown in
Fig. 3. Process variations can be detected during postfabrication
testing. For example, each chip may be tested at a specific
temperature (e.g., 27 ◦C) with an expected ideal fosc. When
the measured output deviates from the expected fosc, the test
system will use the magnitude of this deviation to adjust the
length of the oscillator and compensate for the variation. The
length adjustment is a single-point calibration achieved using
the ProcessComp signal (shown in Fig. 3), which may be
stored onboard in a nonvolatile read-only memory or e-fuse,
and avoids other costly processes such as laser trimming. Alternative
calibration options include tuning of the oscillator supply
or body voltages. These other methods could require isolation
of the oscillator circuit and be more susceptible to voltage noise.
In contrast, the proposed calibration method does not require
circuit isolation, and the digital ProcessComp signal is less
sensitive to voltage noise than analog voltage bias adjustments.