Image Processing Reference
In-Depth Information
used. Generally, frequency measurement is carried out by using a fast-Fourier-transform
(FFT) based digital sampling oscilloscope. Sampling frequency and memory capacity of the
oscilloscope are key for the FFT analysis.
First, the sampling frequency of the oscilloscope must be set in compliance with Shannon's
sampling theorem. To satisfy this requirement, the sampling frequency must be set to at
least double that of the VMONs. Second, the frequency resolution of the oscilloscope must
be determined in order to obtain the necessary voltage resolution. Basically, the frequency
resolution
f of an FFT is equal to the inverse of the measurement period T meas .fa
100-M word memory and a sampling speed of 40 GS/s are used, continuous measurement
during a maximum measurement period of 25 ms can be carried out. If the frequency of
the VMON output is several hundred megahertz and the coefficient of voltage-to-frequency
conversion is about several millivolts per megahertz, highly accurate voltage measurement of
the low-frequency LSD with an accuracy of about 1 mV can be achieved.
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2.3 Support of off-chip digital signal processing
The proposed scheme has several drawbacks due to the simplicity of the ring-oscillator probe.
One of the drawbacks is that the voltage-to-frequency dependence of the ring oscillator suffers
from process and temperature variation. However, we can calibrate it by measuring the
frequency-to-voltage dependence of each VMON before the in-situ measurement by setting
the chip in standby mode. We can also compensate for temperature variation by doing this
calibration frequently.
Figure 8 shows the measurement procedure of the proposed in-situ measurement scheme.
First, the chip must be preheated in order to set the same condition for in-situ measurement,
because the temperature is one of the key parameters for the measurement. This preheating
is carried out by running a measuring program in the same condition as for the in-situ
measurement. A test program is coded in order to execute an infinite loop because
multiple measurements are necessary for improving the measurement accuracy. Because the
measuring program is executed continuously, the temperature of the chip eventually reaches a
state of thermal equilibrium. After the chip has reached this state, the calibration for the target
VMON is executed just before the in-situ measurement. In the calibration, the frequency of the
VMON output of a selected VMON is measured by varying the supply voltage while the chip
is set in standby mode. Note that the calibration method can compensate for macroscopic
temperature fluctuations, but not for microscopic fluctuations that occur in a short period
of time that are much less than the calibration period. After the calibration, the in-situ
measurement is executed by resetting the supply voltage being measured. In measuring the
other VMONs continuously, the calibration step is repeated for each measurement. If other
measurement conditions such as supply voltage, clock frequency, and the program being
measured are changed, the chip must be preheated again.
Each VMON consumes a current of about 200
μ
A under the worst condition, and this current
flows to and from the measurement points. This current itself also causes an IR drop; however,
this current is almost constant, so the influence of this IR drop is also constant. In addition, the
effect of the IR drop is assumed to obey a superposition principle, so the IR drop caused by the
VMON can be separated from the IR drop caused by the chip operating current. Therefore,
the IR drop caused by the VMON can be compensated for by the calibration.
Another drawback of our measurement scheme is that the simple ring-oscillator probe does
not have any sample-and-hold circuits. This results in degradation of resolution. However,
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