MC68HC05V12 Datasheet, PDF (128/246 Pages) Freescale Semiconductor, Inc – HCMOS Microcontreller Unit

Freescale Semiconductor, Inc.
Analog-to-Digital (A/D) Converter
13.3 Analog Section
This subsection describes the analog section.
13.3.1 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.
13.3.2 V_REFH and V_REFL
The reference supply for the A/D is two dedicated pins rather than being
driven by the system power supply lines. The voltage drops in the
bonding wires of the heavily loaded system power pins would degrade
the accuracy of the A/D conversion. V_REFH and V_REFL can be any
voltage between V_SSA and V_CCA, as long asV_REFH > V_REFL;
however, the accuracy of conversions is tested and guaranteed only for
V_REFL = V_SSA and V_REFH = V_CCA.
13.3.3 Accuracy and Precision
The 8-bit conversions shall be accurate to within ± 1 least significant bit
(LSB) including quantization.
13.3.4 Conversion Process
The A/D reference inputs are applied to a precision internal digital-to-
analog (D/A) converter. Control logic drives this D/A and the analog
output is compared successively to the selected analog input which was
sampled at the beginning of the conversion time. The conversion
process is monotonic and has no missing codes.
Technical Data
Analog-to-Digital (A/D) Converter
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MC68HC05V12 — Rev. 2.0