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Analog to Digital Converter
General Release Specification
MC68HC(7)05H12
—
Rev. 1.0
144
Analog to Digital Converter
MOTOROLA
12.2 Introduction
The Analog to Digital converter system consists of a single 8-bit
successive approximation converter and a channel multiplexer. There is
one 8-bit result data register and one 8-bit status/control register.
The reference supply for the converter uses two dedicated pins rather
than being driven by the system power supply lines, because the voltage
drops in the bonding wires of such heavily loaded pins would decrease
the accuracy of the A/D conversion.
An internal RC type oscillator is activated by the ADRC bit in the A/D
status/control register. This RC oscillator is used to give sufficiently high
clock rate to the A/D when the bus speed is too low for the A/D to be
accurate.
Additionally, the ADON bit allows the user to disconnect the A/D when
not used, in order to save power. This is particularly useful to reduce
current consumption (by typically 100
μ
A) when going into the WAIT
mode.
The A/D is ratiometric and two dedicated pins supply the reference
voltage (VREFH and VREFL). An input voltage equal to or greater than
VREFH converts to $FF (full scale) with no overflow indication (if
greater). An input voltage equal to VREFL converts to $00. For
ratiometric conversions, the source of each analog input should use
VREFH as the supply voltage and be referenced to VREFL.
12.3 A/D Principle
The A/D reference inputs are applied to a precision internal digital to
analog converter. Control logic drives this D/A and the analog output is
successively compared to the selected analog input which was sampled
at the beginning of the conversion time. The conversion is monotonic
with no missing codes.
The 8-bit conversions are accurate to within
±
1.5 LSB including
quantization.