DERATING CURVES
This section describes how to use the derating curves
on the following pages to determine potential specified
timing variations based on system capacitive loading.
The Pin State Tables beginning on page 42 in this doc-
ument have a column named “Max Load.” This column
describes the specification load presented to the spe-
cific pin when testing was performed to generate the
timing specification documented in the AC Characteris-
tics section of this data sheet.
Referring to Figure 20, 3.3-V I/O Drive Type D Rise
Time, on page 90, a time value of approximately 8.1 ns
corresponds to a capacitive load of 70 pF.
Also referring to Figure 20, a time value of approxi-
mately 10 ns corresponds to a capacitive load of 90 pF.
Subtracting 8.1 ns from the 10 ns, it can be seen that
the rise time on the MWE signal will increase by 1.9 ns.
Therefore, the MWE hold from CAS Low (min) param-
eter will increase from 30 ns to 31.9 ns (30 ns +1.9 ns).
For example, to find out the effect of capacitive loading
on a DRAM specification such as MWE hold from CAS
Low, first find the specification load for MWE from
Table 7 on page 44. The value here is 70 pF. Note the
output drive type is programmable to C, D, or E. For
this example, assume a drive strength of D, a system
DRAM interface of 3.3 V, and a system load on the mi-
crocontroller’s MWE pin of 90 pF.
If the capacitive load on MWE had been less than 70
pF, the time given in the derating curve for the load
would be subtracted from the time given for the speci-
fication load. This difference can then be subtracted
from the MWE hold from CAS Low (min) parameter to
determine the derated AC timing parameter.
100
90
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70
60
50
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10
100
90
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70
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Load (pF)
100 120 140 160
20
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80
Load (pF)
100 120 140 160
Figure 14. 3.3-V I/O Drive Type A Rise Time
Figure 15. 3.3-V I/O Drive Type A Fall Time
50
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35
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5
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100 120 140 160
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100 120 140 160
Load (pF)
Load (pF)
Figure 16. 3.3-V I/O Drive Type B Rise Time
Figure 17. 3.3-V I/O Drive Type B Fall Time
Élan™SC400 and ÉlanSC410 Microcontrollers Data Sheet
89
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