MC68HC705MC4 Datasheet, PDF (48/99 Pages) Motorola, Inc – Microcontrollers

MC68HC705MC4 Specification Rev. 1.5
SECTION 7
ANALOG SUBSYSTEM
The MC68HC705MC4 includes a 6-channel, 8-bit, multiplexed input, successive
approximation A/D converter, with six of the inputs available on external pins and four
additional internal channels.
7.1 ANALOG SECTION
7.1.1 RATIOMETRIC CONVERSION
The A/D is ratiometric, with two dedicated pins supplying the reference voltages (V_REFH
and V_REFL). An input voltage equal to V_REFH converts to $FF (full scale) and an input
voltage equal to V_REFL converts to $00. An input voltage greater than V_REFH will convert to
$FF with no overflow indication. For ratiometric conversions, the source of each analog
input should use V_REFH as the supply voltage and be referenced to V_REFL.
7.1.2 V_REFH and V_REFL
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 those
heavily loaded pins would degrade the accuracy of the A/D conversion. V_REFH and V_REFL
are internally wired to the analog supply voltages AV_DD and AV_SS. These pins are located
next to each other to permit optimal decoupling.
7.1.3 ACCURACY AND PRECISION
The 8-bit conversions shall be accurate to within ± 1¹/2 LSB including quantization.
7.2 CONVERSION PROCESS
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 that was sampled at the beginning of the conversion time. The
conversion process is monotonic and has no missing codes.
7.3 DIGITAL SECTION
7.3.1 CONVERSION TIMES
Each channel of conversion takes 32 clock cycles, which must be at a frequency equal to
or greater than 1 MHz.
Section 7: Analog Subsystem
MOTOROLA
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