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4.3 SCHMOO CURVE (MAIN STORAGE)
4. 3. 1,.
EXAMPLE OF SCHMOO
CURVE
PROCEDURE
Figure 4-3 shows an example of a typical
four-point schmoo procedure.
It is used
for plotting the optimum storage strobe and
voltage operating point.
A
no-failure
voltage and strobe setting is selected as a
starting point.
The voltage is varied over
its range up and dc:wn to a point of
failure., and varied from that failing point
into the no-failure range (1 in Figure
4-3).
The no-failure points are determined
at tour different strobe settings by
running the worst- case patterns (Section
4.3.4.1) with the diagnostic control switch
set at TEST PATTERN.
Switches A, B,
c,
D
must equal FFOO, OOFF, OlFE, or, FE01.
Once the no-failure voltages have been
plotted,, the optimum strobe setting for a
33
31
Strobe
25
Voltage-High 31
Voltage-Low
25.5
35
45
32.5
32.5
25.5
25.5
single ·BSM is a point where the difference
between the upper and lower BSM limits is
greater than 5.0 volts (2 in Figure 4-3).
At a strobe setting ± 10 ns from optimum,
the difference between the high and low
voltage setting must be greater than 3.
0
volts (3 in Figure 4-3).
For a double
BSM,
the difference between
the upper and lower BSM limits at optimum
strobe must be greater than 3.6 volts.
At
a strobe setting ±10 ns from optimum, the
worst-case diagnostic must still run
successfully.
In both cases the strobe
setting must be between 35 ns and 55
ns.
It should be the earliest setting that will
meet requirerrents, and 35
ns
should be used
if possible.
55
32
26
Optimum Strobe
35 ns
Optimum Voltage
29.0 volts
Cl
c::
·e
Cl)
en
~
0
29
>
l
-10ns~+
10ns
I
~t
I
l
l:!l
0
>
LO
27
l
25
25
35
45
55
Strobe in Nanoseconds
Figure 4-3.
Schmoo Curve Example (Single BSM)
2025 FEMM (7/69)
4-3
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