88CO196EC Datasheet, PDF (38/44 Pages) Intel Corporation – CHMOS 16-Bit Microcontroller

Intel^® 88CO196EC
6.7
A/D Sample and Conversion Times
Two parameters, sample time and conversion time, control the time required for an A/D
conversion. The sample time is the length of time that the analog input voltage is actually
connected to the sample capacitor. If this time is too short, the sample capacitor will not charge
completely. If the sample time is too long, the input voltage may change and cause conversion
errors. The conversion time is the length of time required to convert the analog input voltage stored
on the sample capacitor to a digital value. The conversion time must be long enough for the
comparator and circuitry to settle and resolve the voltage. Excessively long conversion times allow
the sample capacitor to discharge, degrading accuracy.
The AD_TIME register programs the A/D sample and conversion times. Use the T_SAM and T_CONV
specifications in Table 12 and Table 14 to determine appropriate values for SAM and CONV;
otherwise, erroneous conversion results may occur.
When the SAM and CONV values are known, write them to the AD_TIME register. Do not write
to this register while a conversion is in progress; the results are unpredictable.
Use the following formulas to determine the SAM and CONV values.
T ×f–2
SAM = -----^S---^A---^M-------------------
8
T
×f–3
CONV = -----^C---^O----^N---^V------------------- – 1
2×B
where:
SAM
CONV
T_SAM
T_CONV
f
B
equals a number, 1 to 7, to be written to the AD_TIME register
equals a number, 2 to 31, to be written to the AD_TIME register
is the sample time, in µsec (Table 12 and Table 14)
is the conversion time, in µsec (Table 12 and Table 14)
is the operating frequency, in MHz
is the number of bits to be converted (8 or 10)
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Datasheet