Apparatus for testing and measuring electronic device and method of calibrating its timing and voltage level

The present invention is intended to provide a testing and measuring apparatus for accurately and quickly calibrating the input and output timing of a plurality of test signal patterns and voltage levels. The invention also offers a method used for the calibration. The apparatus is equipped with a plurality of units (timing vector generators) each having a timing-generating circuit (a capture timing generator) and an external common reference timing circuit (a golden edge generator) outside the units. Each unit comprises: (1) a timing comparator circuit (a capture comparator) for comparing the timing each of the timing-generating circuits with the timing of the reference timing circuit to determine whether the former timing leads or lags the latter timing; and (2) a counter circuit which counts a number of comparisons made by the comparator circuit until their sequential relation has been reversed.

FIELD OF THE INVENTION 
The present invention relates to an apparatus (e.g., an IC tester) for 
measurement of signals for testing and measuring an electronic device and 
to a method of calibrating its timing and voltage level. In particular, 
the invention relates to the above-described circuit that is capable of 
accurately and quickly calibrating input and output timing of plural test 
signal patterns and voltage levels. 
BACKGROUND OF THE INVENTION 
For example, a testing apparatus such as an IC tester for a digital circuit 
generates various waveforms at any user-determined timing and detects the 
voltage level of the waveforms. High accuracy in timing is needed. 
However, the necessary accuracy varies among an operating speed of a 
device to be tested. For high-speed devices, accuracy may be often higher 
than hundreds of picoseconds. To achieve such strict timing accuracy, it 
is essential to calibrate the timing of the testing apparatus. 
In recent years, IC devices tend to have a large number of input/output 
pins, for example 256 pins. With this trend, IC testers which examine 
these devices also tend to have a larger number of pins. With an 
increasing number of pins, units of a per-pin structure such as in timing 
vector generators and pin-electronics are increased. Thus, it is 
inevitable that the number of pieces of hardware which needs calibration 
for timing and voltage levels is also increased. 
A conventional method involves serially calibrating timing of pins with 
respect to a reference pin or to an external reference. In this method, 
timing is calibrated for each pin. The results of each measurement are 
stored in a storage device such as a capture memory. Upon the completion 
of the measurements of all the pins, a main CPU analyzes the contents of 
the capture memory to obtain data for the calibration. However, since 
measurements for the calibration of each pin must be performed 
successively, an exorbitant amount of time is needed for this method. An 
amount of measured data is also exorbitant. Thus, the transfer and 
calculation time by the CPU is undesirably long. 
Another conventional method which alleviates these problems is disclosed in 
Japanese Patent Laid-Open No. 41875/1989. A control CPU is provided for 
each per-pin resource, and calibration is processed in parallel. This 
method shortens the time required for the measurements of timing in the 
above-described serial calibration method. However, since a control CPU 
must be provided for each per-pin resource, this increases the cost of the 
whole measuring apparatus. 
SUMMARY OF THE INVENTION 
The present invention has been proposed to solve the above discussed 
problems. It is an object of the invention to provide a testing and 
measuring apparatus which accurately and quickly calibrates the input and 
output timing of a plurality of test signal patterns and voltage levels. 
It is another object of the invention to improve a method of calibrating 
the timing and the voltage levels. 
An apparatus for testing and measuring an electronic device in accordance 
with the present invention is provided with a plurality of units each 
having a timing-generating circuit. The apparatus has an external common 
reference timing circuit, a common reference timing circuit inside one of 
the units, or different timing circuits. The apparatus is characterized in 
that each of the units comprises: a timing comparator circuit which 
compares the timing of each timing-generating circuit with the timing of 
the reference timing circuit to determine whether the former timing leads 
the latter timing; and a counter circuit which counts the number of 
comparisons made by the comparator circuit until the time relation between 
the two circuits changes. 
A method of calibrating timing in accordance with the present invention 
comprises the steps of: varying timing of each timing generating circuit 
or of the reference timing circuit in a stepwise fashion in such a way 
that the timing of each timing-generating circuit outpaces, or is outpaced 
by the timing of the reference timing circuit; counting the number of 
variations in the phase until the lead of the timing of each 
timing-generating circuit relative to the timing of the reference-timing 
circuit changes into a lag or vice versa; and determining the relation 
between the timing of the reference timing circuit and the timing of each 
timing generating circuit to calibrate the timing of each timing 
generating circuit. 
An apparatus for testing and measuring an electronic device according to 
the invention further comprises a plurality of units each having a 
voltage-generating circuit. The apparatus has an external common reference 
voltage level circuit, a common reference voltage circuit inside one of 
the units, or independent reference voltage level circuits on the units. 
Each of the units further comprises: a comparator circuit which compares 
the voltage level of the voltage-generating circuit with the voltage level 
of the reference voltage level circuit to determine whether the former 
voltage level is higher or lower than the latter voltage level; and a 
counter circuit which counts the number of comparisons made by the 
comparator circuit and stops the counting when the two voltage levels are 
reversed in magnitude. 
A method of calibrating voltage levels in accordance with the present 
invention further comprises the steps of: varying the voltage level of 
each voltage-generating circuit or of the reference voltage level circuit 
in a stepwise fashion so that the voltage level of each voltage-generating 
circuit becomes higher or lower than the voltage level of the reference 
voltage level circuit; counting the number of different voltage levels 
until the relation between the voltage level of each voltage-generating 
circuit and the voltage level of the reference voltage level circuit is 
reversed in their magnitude; and determining the relation between the 
voltage level of the reference voltage level circuit and the voltage level 
of each voltage-generating circuit to calibrate the voltage level of each 
voltage-generating circuit. 
The apparatus for testing and measuring an electronic device is not limited 
to such devices IC testers. For example, the invention embraces an 
apparatus for testing and measuring IC verifiers or the like which need 
calibration of timing between plural units and calibration of voltage 
levels. One example in applying the current invention is timing vector 
generators of per-pin structure and per-pin boards of pin-electronics in 
the field of IC testers.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
FIG. 1 is a fragmentary diagram which schematically shows an IC tester for 
illustrating an embodiment of the invention. The fundamental structure and 
operation of the illustrated IC tester will be briefly described. In FIG. 
1, n pin-electronics PE2's of a per-pin structure correspond to n timing 
vector generators TVG1 of the per-pin structure. The timing vector 
generators TVG1 furnish test signals to a DUT (not shown) or receive 
response signals from the DUT via the pin-electronics. The n TVG1's and 
PE2's form a unit according to the invention. 
A control CPU 3 controls the operation of the whole IC tester including 
stopping and starting of a sequencer 4. The sequencer 4 controls the 
TVG1's etc. on a real-time basis. The sequencer 4 can supply addresses to 
a vector memory (VMem) 11 and performs operations including a conditional 
jump and a stopping test depending on whether the output signal from an 
EXOR gate 17 indicates a matched condition or an unmatched condition. 
Information such as a pattern, a timing, a format (drive data), and other 
data to be generated by the TVG1 is stored in the VMem 11 in the TVG1. 
When a test signal is supplied to the DUT, a driving timing generator 
(DTG) 12 generates a timing edge signal at a particular resolution 
(preferably 20 to 100 psec) according to the information stored in the 
VMem 11 and delivers the timing edge signal to a formatter (FMTR) 13. 
The FMTR 13 creates a driving waveform according to the timing edge from 
the DTG 12 and information on the pattern and format from the VMem 11. A 
pin driver 21 installed in the PE2 drives this driving waveform up to the 
voltage level desired by the user and delivers it to the DUT via switches 
SW1 and SW2. A D/A converter (DAC) 22 applies a voltage to the pin driver 
21 for setting a desirable pin drive level. Usually, a preferred range of 
this voltage is approximately from +10 to -4 V. A sufficient resolution is 
about 14 bits (0.855 mV). 
When the response signal from the DUT is received by TVG1, the pin output 
voltage from the DUT is applied to one input terminal of a pin comparator 
(PCMP) 23 via the SW1. A reference voltage is supplied to the other input 
terminal of the PCMP 23 from a DAC 24 for setting the pin comparator 
level. Usually, a preferred range of this voltage is from +10 to -4 V and 
a preferred resolution is about 14 bits as in the DAC 22. The comparator 
level of the DAC 24 is supplied from a data counter 25. The counter 25 is 
incremented by an input signal from the sequencer 4 and reset by an input 
reset signal from the control CPU 3. 
An output signal (hereinafter referred to as the "capture data") from the 
PCMP 23 is fed into a capturing comparator (CCMP) 15. The capture data is 
then fed to the EXOR gate 17 in response to a timing signal generated by a 
capturing timing generator (CTG) 14. The timing generated by the CTG 14 is 
determined based upon the timing information from the VMem 11. A preferred 
resolution of this timing is about 20 to 100 psec as in the resolution of 
the timing of the DTG 12 as described above. 
The EXOR gate 17 determines whether the expected data from the VMem 11 
agrees with the capture data from the CCMP 15. The results are given to 
the sequencer 4. The sequencer 4 receives a timing signal from the CTG 14 
via a delay circuit 16 that acts to generate a strobe signal. In this way, 
the results of the decision by the EXOR gate 17 is received by the 
sequencer 4 at an appropriate timing. 
FIG. 1 also shows a reference timing circuit such as a golden edge 
generator (GEG) 5, operated under the control of the sequencer 4 and a 
reference voltage level circuit such as a reference voltage generator 6 
operated under the control of the control CPU 3. The GEG 5 further 
comprises a timing generator (TG) 51 for generating a timing signal for 
the golden edge generator 51 and a golden edge driver (GE driver) 52. The 
reference voltage generator 6 comprises at least a voltage source 61 
producing a reference voltage used for calibration. 
The output signals from the GEG 5 and from the reference voltage generator 
6 are delivered to a counter circuit 7 via SW3 or SW4, the pin comparator 
23 installed in the PE2, the comparator 15, and the EXOR gate 17. Each 
TVG1 is equipped with the counter circuit 7. The counter circuit 7 
comprises an unmatched logging counter or UL counter 71, an AND gate 72, 
and a RS latch 73. The RS latch 73 is set when the signal from the control 
CPU 3 is applied to its S terminal. The RS latch 73 is reset when the 
output signal from the EXOR gate 17, which indicates whether the capture 
data is the same as the expected data, is applied to its R terminal. The 
AND gate 72 receives the outputs from the RS latch 73 and the output from 
the delay circuit 16 and send a signal to the counter circuit 7. The UL 
counter 71 also receives a signal from the control CPU 3 at its S terminal 
in such a way that the counter 71 is reset simultaneously with the RS 
latch 73. The value of the UL counter 71 is read by the control CPU 3. 
Embodiments of the calibration method according to the invention are 
described hereinafter. 
(1) Method of Calibrating Timing of CTG 14 
Since the timing of the timing-generating circuits successively varies, 
timing of the timing generator circuit CTG 14, for example, gets ahead of 
that of the reference timing circuit GEG 5. In this case, it is assumed 
that the SW1, SW2 and SW3 as shown in FIG. 1 are opened and that only SW4 
is closed. If the resolution of the CTG 14 is 20 psec and the worst timing 
error prior to calibration is +10 nsec, the error corresponds to about 2 m 
of the electrical conductor in length on the printed-wiring board which 
has a dielectric constant of 5 based upon a sufficiently large estimated 
error value. It is also assumed that the data length of the UL counter 71 
is 10 bits. 
Timing data and expected data about the CTG 14 are successively written at 
addresses ranging from N to N+1023 in the VMem 11. The timing data are 
regularly shifted by T/2.sup.10 at intervals of T. It is assumed that all 
the expected data are zeros. In this example, the expected signal is low 
level. Thus, exactly the same data items consisting of the same timing 
data item and the same expected data item are written to the VMem 11 of 
each TVG 1. Therefore, all the arrays of the pins can be written in 
parallel. 
Assuming that the data length of address is 16 bits, only 16.times.1024/8=2 
k bytes of data need to be written. However, according to the present 
invention, a counter is used instead of the VMem 11, and this transfer is 
unnecessary. This further shortens the required time for calibration. The 
GEG 51 is set in such a way that if the timing error is 0, the timing is 
expected to agree with the timing stored at the data address N+512 for 
each pin. The GE driver 52 delivers a rectangular wave which varies from a 
low level (or 0) to a high level (or 1) based on this timing. 
In the present method of calibration, the UL counter 71 is reset. At the 
same time, the RS latch 73 is reset. Upon the completion of resetting, the 
sequencer 4 starts from address N. The GE driver 52 produces a timing edge 
(golden edge) at every address. Every edge is captured by the TVG1. The 
EXOR 17 which acts as a timing comparator circuit decides whether they are 
matched. 
As shown in FIG. 2, at early addresses of the VMem 11, the capture timing 
as indicated by Ct.sub.k and Ct.sub.k+1 precedes the golden edge at 
P.sub.k, P.sub.k+1 i.e., the timing at which a transition is made from low 
level to high level. The CCMP 15 captures low level (or 0 level). Since 
the expected data is set to 0, a matched condition is obtained. Therefore, 
the EXOR 17 does not reset the RS latch 73. Accordingly, whenever the 
delay circuit 16 produces a strobe signal such as s.sub.k and s.sub.k+1, 
the UL counter 71 counts as indicated by signals such as c.sub.k', 
c.sub.k+1. 
As the address of the VMem 11 progresses, the timing of the CTG 14 lags the 
timing of the golden edge. Finally, the CCMP 15 captures high level (or 1 
level). Also in this case, the expected data is set to 0. As indicated by 
the capture timing Ct.sub.k+2 of FIG. 2, the expected data has an 
unmatched relation to the capture data. The RS latch 73 is reset at 
t.sub.1 and t.sub.2. When the RS latch 73 is reset and the output becomes 
0, the strobe signals such as s.sub.k+2 and s.sub.k+3 do not arrive at the 
UL counter 71. At this time, counter data B.sub.n which is the value of 
the UL counter 71 of the n-th TVG1 is stored in the UL counter 71. When 
the expected data has an unmatched relation to the capture data for every 
TVG1, the operation of the sequencer 4 is stopped. 
The value of the UL counter 71 of the TVG1 is read by the control CPU 3. 
B.sub.n read by the control CPU 3 shows that the timing data written to 
N+Bn-1 agrees with the timing of the golden edge with an accuracy of 
T/2.sup.10 at the n-th TVG1, where T is the interval. If the same results 
are stored using the VMem 11 and they are transferred to the desired 
processor, it is necessary to transfer data of 1024 bits per pin, or 1024 
bits.times.256/8=32 k bytes for a 256 pin system. On the other hand, in 
the present example, the read data is only 10 bytes at most per TVG1. Even 
for a system comprising 256 pins, the total amount of transfer is only 320 
bytes (=10.times.256 bits). The timing can be measured with a high 
accuracy for such a small amount of data transfer. 
In the above measurement, it is assumed that the timing value of the golden 
edge signal approximately agrees with the timing value written at the 
address N+512. If necessary, the above-described step is repeated for the 
address N+X (X.noteq.512) to determine the offset, the gain error, and the 
linearity of each CTG 14. In this way, the timing of each CTG 14 is 
calibrated accurately. 
(2) Method of Calibrating Timing of DTG 12 
By this method, timing of the timing-generating circuits is successively 
varied on a regular basis in such a way that the timing of each reference 
timing circuit such as the CTG 14 which has been calibrated precedes the 
timing of each timing-generating circuit such as the DTG 12. Accordingly, 
the timing of the timing-generating circuit will be calibrated. 
In FIG. 1, the SW2, SW3, and SW4 are open, and only the SW1 is closed to 
accomplish the calibration of DTG 12. As described above, the CTG 14 has 
been calibrated as the "independent reference timing circuit" in the 
present embodiment. It is assumed that the resolution of the DTG 12 is 20 
psec, and that the worst timing error prior to calibration is +10 nsec. 
The data written at addresses N to N+1023 of the VMem 11 are the same as 
ones used for the method of calibrating timing of CTG 14. Thus, the timing 
output from the CTG 14 is also the same as that used for the method of 
calibrating CTG 14. Such timing is preset in the DTG 12 of each TVG1. If 
the DTG 12 has no error, the timing value agrees with the timing value 
written at the address N+512 of the VMem 11. The FMTR 13 sends a 
rectangular wave to the CCMP 15 via the pin driver 21, the SW1, and the 
PCMP 23. The wave changes from a low level to a high level at the 
above-described address. Also, the UL counter 71 is reset and, at the same 
time, the RS latch 73 is set. The sequencer 4 is started upon the onset of 
the value comparison from address N. The FMTR 13 produces one timing edge 
which is to be calibrated at each address. The CCMP 15 captures every 
timing edge and determines if a matched relation is obtained. 
At early addresses of the VMem 11, since the timing (capture timing) of the 
CTG 14 precedes that of the FMTR 13, the CCMP 15 captures a low level. The 
UL counter 71 increments whenever the delay circuit 16 produces a strobe 
signal. Since the operation of the counter circuit 7 is previously 
described, it is briefly described. If the timing of the CTG 14 lags the 
timing of the FMTR 13, the CCMP 15 captures a high level. The RS latch 73 
is reset. The counter data C.sub.n (the value of the UL counter 71 of the 
n-th TVG1) obtained at this time is stored in the UL counter 71. When the 
expected data has an unmatched relation to the capture data for every 
TVG1, the operation of the sequencer 4 is stopped. The control CPU 3 reads 
the value of the UL counter 71 of the TVG1. The C.sub.n value shows that 
the timing of the FMTR 13 matches the timing of the CTG 14 written at 
N+C.sub.n -1 at the n-th TVG1. Thus, an accurate measurement of timing is 
performed with a quite small amount of transfer. 
If the need arises, the step described above is repeated at the address N+X 
(X 512). Thus, the offset, the gain error, and the linearity of each DTG 
12 are determined. In this way, the timing of each DTG 12 can be 
calibrated accurately. 
(3) Method of Calibrating Voltage Level of PCMP 23 
In this method, the voltage level of the above-described voltage-generating 
circuit such as DAC 24 is varied in a stepwise fashion so that the PCMP 23 
is calibrated according to the voltage level of the reference voltage 
level circuit such as a reference voltage generator 6. Since the method is 
implemented substantially the same way as those of the methods (1) and 
(2), the voltage calibration method will be briefly described. 
First, SW1, SW2, and SW4 are open, and only the SW3 is closed. The UL 
counter 71 has been previously reset. The RS latch 73 is set. After the 
control CPU 3 resets the counter 25, the sequencer 4 increments the data 
counter 25, and the voltage level of the DAC 24 is varied in a stepwise 
fashion. The PCMP 23 which operates as a voltage comparator circuit 
compares the voltage level of the DAC 24 with the voltage from a 
calibrated voltage source 61 of the reference voltage generator 6. 
Initially, the voltage level of the DAC 24 is lower than the voltage level 
of the calibrated voltage source 61. Thus, the PCMP 23 delivers a low 
level. If the voltage level of the DAC 24 is in excess of a given 
reference voltage, the PCMP 23 produces a high level. 
The output from the PCMP 23 is delivered to the CCMP 15 and then to the 
EXOR 17 at the timing of the CTG 14. It is to be noted that the timing of 
the CTG 14 is not required to be as accurate as in the calibration methods 
of (1) and (2) of timing described above. The EXOR 17 compares 0 output 
from the VMem 11 with the capture data from the CCMP 15. If the capture 
data is 0, 0 is sent to the RS latch 73. By the same token, if the capture 
data is 1, 1 is sent to the RS latch 73. Therefore, if the capture data is 
0, the output from the AND gate 72 is 0. The UL counter 71 is not 
incremented. The counter data Dn is preserved. 
When the expected data has an unmatched relation to the capture data for 
every TVG, the operation of the sequencer 4 is stopped. The control CPU 3 
reads the value of the UL counter 71 for the nth TVG1. Consequently, the 
threshold voltage for the DAC 24 can be determined at an accuracy equal to 
the resolution of the counter. The voltage level of the DAC 24 is thus 
calibrated. 
A measurement can be made in the same way as the foregoing by modifying the 
voltage level of the reference voltage generator 6. In this way, the 
linearity and other factors of the DAC 24 are measured. 
(4) Method of Calibrating Voltage Level of Pin Driver 21 
The voltage level of the above-described reference voltage level circuit 
such as DAC 22 is varied in a stepwise fashion so that the pin driver 21 
is calibrated. The PCMP 23 has been previously calibrated. 
In this case, in FIG. 1, the SW2, SW3, and SW4 are open, and only the SW1 
is closed. The voltage level of the pin driver 21 is calibrated based on 
the voltage level of the DAC 24, which varies in a stepwise fashion. The 
phase of the CTG 14 is also varied in a stepwise fashion instead of the 
VMem 11. The counter is incremented according to the trigger from the 
sequencer 4. The timing edge is made to shift in a stepwise fashion with a 
phase difference. 
The operation of the present invention is summarized for (A) the 
calibration of timing-generating circuits and (B) the calibration of 
voltage-generating circuits. 
(A) Calibration of Timing-Generating Circuits 
In a test apparatus such as an IC tester, a timing vector generator is 
equipped with a timing generating circuit. Usually, a common reference 
timing circuit exists outside the unit. In general, the timing of the 
timing generating circuits is calibrated based upon the reference timing 
circuit. 
In a test apparatus, each unit is often equipped with two or more 
timing-generating circuits. According to one method of the current 
invention, the timing-generating circuit which has been already calibrated 
is taken as an independent reference timing circuit. Other 
timing-generating circuits may be calibrated based upon the independent 
reference timing circuit. 
In accordance with the present invention, timing of timing-generating 
circuits among units is calibrated in the manner described below. For 
convenience of illustration, it is assumed that a common reference timing 
circuit is located outside the units. First, a timing comparator circuit 
incorporated in each unit determines whether a timing-generating circuit 
to be calibrated leads or lags timing of a reference timing circuit. To 
determine whether the former timing leads or lags the latter timing, the 
reference timing circuit delivers a periodic H/L signal (rectangular wave) 
at intervals of T. Meanwhile, the timing-generating circuits deliver 
triggers whose phases successively vary. For example, the phase of each 
trigger increases successively within a range from T to 2T. In a preferred 
embodiment, the phase increases an increment of T/210. The phase decreases 
also successively within a range from T to 0 in steps of T/210. At this 
time, the first trigger may be set so that the first trigger is produced 
when the periodic H/L signal is at low level or high level. The timing of 
the timing-generating circuit outpaces or sometimes is outpaced by the 
timing of the reference timing circuit by increasing or reducing the 
phase. Thus, the timing comparator circuit of each unit determines whether 
a timing relation between the reference and timing-generating circuit of a 
corresponding unit has been reversed (i.e. a leading and lagging circuits 
are reversed). 
A counter circuit counts the number of the agreements or disagreements 
detected by the timing comparator circuit. The counting is continued until 
a leading circuit changes to a lagging circuit or vice versa. The change 
does not always simultaneously occur for every unit. Therefore, the timing 
error among the timing generating circuits can be determined based upon a 
total count for a given accuracy. This accuracy depends on the phase 
described above. If the phase of each timing-generating circuit changes 
completely linearly, and if the phase changes in an increment of 
T/2.sup.l0, the accuracy is also T/2.sup.10. 
The measurement described above is carried out several times by varying the 
timing of the periodic H/L signal from the reference timing circuit 
relative to the first trigger from the timing-generating circuit so as to 
accurately determine the relation. Specifically, the offset, the 
linearity, the gain error, etc. of each timing circuit as well as the 
timing error among the timing-generating circuits are accurately 
determined. This permits accurate calibration of the timing generating 
circuits. 
(B) Calibrating of Voltage-Generating Circuits 
A test apparatus such as an IC tester normally has an external common 
reference voltage circuit outside a unit such as pin-electronics. The 
voltage level of a voltage-generating circuit in the unit is calibrated 
based on a voltage level of a reference voltage circuit. 
As mentioned above, the reference voltage circuit is usually located 
outside a unit but it may be located in the same unit. In this embodiment, 
the voltage-generating circuit is used as a "reference voltage level 
circuit." In a test apparatus, each unit is often equipped with two or 
more voltage-generating circuits. In this case, the calibrated 
voltage-generating circuit may be used as an independent reference voltage 
level circuit. The timing of the other voltage-generating circuits in the 
same unit may be calibrated based upon the independent reference voltage 
level circuit. 
In the present example, the voltage level among the units is calibrated in 
the manner described below. For convenience of illustration, it is assumed 
that a common voltage-generating circuit is present outside the units. 
First, the voltage level comparator circuits in different units 
simultaneously determine whether a voltage level of a voltage-generating 
circuit to be calibrated is higher than that of the above-described 
reference voltage level circuit. The reference voltage level circuit 
delivers a voltage signal for a given level. On the other hand, the 
voltage-generating circuit delivers a stepwise voltage signal within a 
rang from 0 to V or vice versa, for example, in steps of V/210. Thus, the 
voltage level of the voltage-generating circuit becomes higher or lower 
than that of the reference voltage level circuit. 
The counter circuit counts the number of comparisons made by the voltage 
level comparator circuit. The counting is continued until the magnitude 
relation is reversed. The counter circuit used herein may be the same as 
one used for the timing calibration method. The magnitude relation is not 
always reversed simultaneously among the units. Therefore, the error in 
the voltage level between the voltage-generating circuit is determined 
based upon the count value for a given accuracy. This accuracy depends on 
the resolution of the steps by which the voltage is varied. If the voltage 
of the voltage-generating circuit is varied completely linearly, and if 
the voltage varies in steps of V/210, the accuracy is also V/210. 
The measurements described above may be taken several times while varying 
the reference voltage level. This enables an accurate determination of the 
relation between the reference voltage level circuit and the voltage level 
of each voltage-generating circuit. That is, the offset, the linearity, 
etc. of each voltage-generating circuit as well as the error between the 
voltage-generating circuits are accurately determined. Hence, the 
voltage-generating circuit is calibrated with a high accuracy. 
In the present invention, a counter circuit for calibration further 
comprises a counter of about 10 bits and a simple gate device. Since the 
counter is based on parallel calibration measurements, the calibration 
method according to the current invention is enhanced. At the same time, 
the cost of the apparatus is reduced. 
When a calibration is made using a capture memory, as much as 32 k bytes of 
measurement data are required to be transferred or calculated per 
measurement point for a system having 256 pins. On the other hand, in the 
present invention, calibration measurement data are on the order of 2 k 
bytes at most per measurement point for a system having 256 pins. The time 
required to transfer the data to the control CPU is also substantially 
shorter. The amount of calculation is also substantially smaller. If a 
counter is used instead of a memory to generate timing data for 
calibration, the amount of transfer may be kept minimal, e.g., about 320 
bytes. The voltage level as well as timing are calibrated by the same 
counter circuit.