MC68HC05P18 Datasheet, PDF (81/144 Pages) Freescale Semiconductor, Inc – Advance Information

Freescale Semiconductor, Inc.
Analog-to-Digital (A/D) Converter
Digital Section
9.4 Digital Section
This section describes the operation and performance of digital modules
within the analog subsystem.
9.4.1 Conversion Times
Each input conversion requires 32 PH2 clock cycles, which must be at a
frequency equal to or greater than 1 MHz.
9.4.2 Internal versus External Oscillator
If the MCU PH2 clock frequency is less than 1 MHz (2 MHz external
oscillator), the internal RC oscillator (approximately 1.5 MHz) must be
used for the A/D converter clock. The internal RC clock is selected by
setting the EERC bit in the EEPROM programming register (EEPROG).
NOTE: The RC oscillator is shared with the EEPROM module. The RC oscillator
is disabled while the MCU is in stop mode.
When the internal RC oscillator is being used, these limitations apply:
1. Since the internal RC oscillator is running asynchronously with
respect to the PH2 clock, the conversion complete bit (CC) in the
A/D status and control register must be used to determine when a
conversion sequence has been completed.
2. Electrical noise will slightly degrade the accuracy of the A/D
converter. The A/D converter is synchronized to read voltages
during the quiet period of the clock driving it. Since the internal and
external clocks are not synchronized, the A/D converter
occasionally will measure an input when the external clock is
making a transition.
3. If the PH2 clock is 1 MHz or greater (for instance, external
oscillator 2 MHz or greater), the internal RC oscillator should be
turned off and the external oscillator used as the conversion clock.
MC68HC(8)05P18 — Rev. 2.0
MOTOROLA
Analog-to-Digital (A/D) Converter
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Advance Information
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