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ST90158 - EIGHT-CHANNEL ANALOG TO DIGITAL CONVERTER (A/D)
9.8 EIGHT-CHANNEL ANALOG TO DIGITAL CONVERTER (A/D)
9.8.1 Introduction
The 8-Channel Analog to Digital Converter (A/D)
comprises an input multiplex channel selector
feeding a successive approximation converter.
Conversion requires 138 INTCLK cycles (of which
84 are required for sampling), conversion time is
thus a function of the INTCLK frequency; for in-
stance, for a 20MHz clock rate, conversion of the
selected channel requires 6.9
μ
s. This time in-
cludes the 4.2
μ
s required by the built-in Sample
and Hold circuitry, which minimizes the need for
external components and allows quick sampling of
the signal to minimise warping and conversion er-
ror. Conversion resolution is 8 bits, with ±1 LSB
maximum error in the input range between V
SS
and the analog V
DD
reference.
The converter uses a fully differential analog input
configuration for the best noise immunity and pre-
cision performance. Two separate supply refer-
ences are provided to ensure the best possible
supply noise rejection. In fact, the converted digital
value, is referred to the analog reference voltage
which determines the full scale converted value.
Naturally
,
Analog and Digital V
SS
MUST be com-
mon. If analog supplies are not present, input ref-
erence voltages are referred to the digital ground
and supply.
Up to 8 multiplexed Analog Inputs are available,
depending on the specific device type. A group of
signals can be converted sequentially by simply
programming the starting address of the first ana-
log channel to be converted and with the AUTO-
SCAN feature.
Two Analog Watchdogs are provided, allowing
continuous hardware monitoring of two input chan-
nels. An Interrupt request is generated whenever
the converted value of either of these two analog
inputs is outside the upper or lower programmed
threshold values. The comparison result is stored
in a dedicated register.
Figure 87. Block Diagram
n
INTERRUPT UNIT
INT. VECTOR POINTER
INT. CONTROL REGISTER
COMPARE RESULT REGISTER
THRESHOLD REGISTER
THRESHOLD REGISTER
THRESHOLD REGISTER
THRESHOLD REGISTER
7U
7L
6U
6L
COMPARE LOGIC
DATA REGISTER 7
DATA REGISTER 6
DATA REGISTER 5
DATA REGISTER 4
DATA REGISTER 3
DATA REGISTER 2
DATA REGISTER 1
DATA REGISTER 0
SUCCESSIVE APPROXIMATION
A/D CONVERTER
ANALOG
MUX
AIN 7
AIN 6
AIN 5
AIN 4
AIN 3
AIN 2
AIN 1
AIN 0
CONVERSION
RESULT
AUTOSCAN LOGIC
CONTROL REG.
CONTROL
LOGIC
INTERNAL
TRIGGER
EXTERNAL
TRIGGER
VA00223
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