MC705P6ACPE Datasheet, PDF (53/98 Pages) Freescale Semiconductor, Inc – M68HC05 Microcontrollers

Chapter 9
Analog Subsystem
9.1 Introduction
The MC68HC705P6A includes a 4-channel, multiplexed input, 8-bit, successive approximation
analog-to-digital (A/D) converter. The A/D subsystem shares its inputs with port C pins PC3–PC7.
9.2 Analog Section
The following paragraphs describe the operation and performance of analog modules within the analog
subsystem.
9.2.1 Ratiometric Conversion
The A/D converter is ratiometric, with pin V_REFH supplying the high reference voltage. Applying an input
voltage equal to V_REFH produces a conversion result of $FF (full scale). Applying an input voltage equal
to V_SS produces a conversion result of $00. An input voltage greater than V_REFH will convert to $FF with
no overflow indication. For ratiometric conversions, V_REFH should be at the same potential as the supply
voltage being used by the analog signal being measured and referenced to V_SS.
9.2.2 Reference Voltage (V_REFH)
The reference supply for the A/D converter shares pin PC7 with port C. The low reference is tied to the
V_SS pin internally. V_REFH can be any voltage between V_SS and V_DD; however, the accuracy of
conversions is tested and guaranteed only for V_REFH = V_DD.
9.2.3 Accuracy and Precision
The 8-bit conversion result is accurate to within ±1 1/2 LSB, including quantization; however, the accuracy
of conversions is tested and guaranteed only with external oscillator operation.
9.3 Conversion Process
The A/D reference inputs are applied to a precision digital-to-analog converter. Control logic drives the
D/A and the analog output is successively compared to the selected analog input which was sampled at
the beginning of the conversion cycle. The conversion process is monotonic and has no missing codes.
9.4 Digital Section
The following paragraphs describe the operation and performance of digital modules within the analog
subsystem.
MC68HC705P6A Advance Information Data Sheet, Rev. 2.1
Freescale Semiconductor
53