Intel 8XC196NT, User Manual

Page: 276 / 597

11-5
ANALOG-TO-DIGITAL CONVERTER
11.4.1 Programming the A/D Test Register
The AD_TEST register (Figure 11-2) selects either an analog input or a test voltage (ANGND or
V
REF
) for conversion and specifies an offset voltage to be applied to the resistor ladder. To use the
zero-offset adjustment, first perform two conversions, one on ANGND and one on V
REF
. With the
results of these conversions, use a software routine to calculate the zero-offset error. Specify the
zero-offset adjustment by writing the appropriate value to AD_TEST. This offset voltage is added
to the resistor ladder and applies to all input channels. “Understanding A/D Conversion Errors”
on page 11-14 describes zero-offset and other errors inherent in A/D conversions.
AD_TEST
Address:
Reset State:
1FAEH
C0H
The A/D test (AD_TEST) register enables conversions on ANGND and V
REF
and specifies
adjustments for DC offset errors. Its functions allow you to perform two conversions, one on ANGND
and one on V
REF
. With these results, a software routine can calculate the offset and gain errors.
7 0
— — — — OFF1 OFF0 TV TE
Bit
Number
Bit
Mnemonic
Function
7:4 — Reserved; for compatibility with future devices, write zeros to these bits.
3:2 OFF1:0 Offset
These bits allows you to set the zero-offset point.
OFF1 OFF0
0 0 no adjustment
0 1 add 2.5 mV
1 0 subtract 2.5 mV
1 1 subtract 5.0 mV
1 TV Test Voltage
This bit selects the test voltage for a test mode conversion. (The TE bit
must be set to enable test mode.)
1 = V
REF
0 = ANGND
0 TE Test Enable
This bit determines whether normal or test mode conversions will be
performed. A normal conversion converts the analog signal input on one
of the analog input channels. A test conversion allows you to perform a
conversion on ANGND or V
REF
, selected by the TV bit.
1 = test
0 = normal
Figure 11-2. A/D Test (AD_TEST) Register