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8XC196MC, MD, MH USER’S MANUAL
12-6
12.4.3 Programming the A/D Time Register
Two parameters, sample time and conversion time, control the time required for an A/D conver-
sion. 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 con-
version 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 cir-
cuitry to settle and resolve the voltage. Excessively long conversion times allow the sample ca-
pacitor to discharge, degrading accuracy.
The AD_TIME register (Figure 12-4) specifies the A/D sample and conversion times. To avoid
erroneous conversion results, use the T
SAM
and T
CONV
specifications on the datasheet to determine
appropriate values.
AD_RESULT (Write)
Address:
Reset State (MC, MD):
Reset State (MH):
1FAAH
FFC0H
7FC0H
The high byte of the A/D result (AD_RESULT) register can be written to set the reference voltage for
the A/D threshold-detection modes.
15 8
REFV7 REFV6 REFV5 REFV4 REFV3 REFV2 REFV1 REFV0
7 0
— — — — — — — —
Bit
Number
Bit
Mnemonic
Function
15:8 REFV7:0 Reference Voltage
These bits specify the threshold value. This selects a reference voltage
that is compared with an analog input pin. When the voltage on the
analog input pin crosses over (detect high) or under (detect low) the
threshold value, the A/D conversion complete interrupt pending bit is set.
Use the following formula to determine the value to write to this register
for a given threshold voltage.
7:0 — Reserved; for compatibility with future devices, write zeros to these bits.
Figure 12-3. A/D Result (AD_RESULT) Register — Write Format
reference voltage
desired threshold voltage 256×
V
REF
ANGND–
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