80C196EA Datasheet, PDF (36/46 Pages) Intel Corporation – CHMOS 16-BIT MICROCONTROLLER

8xC196EA — AUTOMOTIVE
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 specifica-
tions 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.
SAM = ^T----S----^A---^M---8--^×-----f---^–----2--
C O NV = -T---^C----^O----^N---^V----^×-----f---^–-----3-- – 1
2×B
where:
SAM
CONV
T_SAM
T_CONV
f
B
equals a number, 1 to 7
equals a number, 2 to 31
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)
At 40 Mhz, to meet T_SAM and T_CONV minimum specifications:
10-bit mode: SAM= [ 5, 6, 7] ⇒ T_SAM ≥ 1µs
CONV= [ 18, 19, 20, …, 31] ⇒ T_CONV ≥ 10µs
8-bit mode:
SAM= [ 5, 6, 7] ⇒ T_SAM ≥ 1µs
CONV= [ 23, 24, …, 31] ⇒ T_CONV ≥ 10µs
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