Datasheet

MC68HC908GP32 Data Sheet, Rev. 10
Freescale Semiconductor 53
Chapter 4
Analog-to-Digital Converter (ADC)
4.1 Introduction
This section describes the 8-bit analog-to-digital converter (ADC).
4.2 Features
Features of the ADC module include:
• Eight channels with multiplexed input
• Linear successive approximation with monotonicity
• 8-bit resolution
• Single or continuous conversion
• Conversion complete flag or conversion complete interrupt
• Selectable ADC clock
4.3 Functional Description
The ADC provides eight pins for sampling external sources at pins PTB7/AD7–PTB0/AD0. An analog
multiplexer allows the single ADC converter to select one of eight ADC channels as ADC voltage in
(V
ADIN
). V
ADIN
is converted by the successive approximation register-based analog-to-digital converter.
When the conversion is completed, ADC places the result in the ADC data register and sets a flag or
generates an interrupt. (See Figure 4-1.)
4.3.1 ADC Port I/O Pins
PTB7/AD7–PTB0/AD0 are general-purpose I/O (input/output) pins that share with the ADC channels. The
channel select bits define which ADC channel/port pin will be used as the input signal. The ADC overrides
the port I/O logic by forcing that pin as input to the ADC. The remaining ADC channels/port pins are
controlled by the port I/O logic and can be used as general-purpose I/O. Writes to the port register or DDR
will not have any affect on the port pin that is selected by the ADC. Read of a port pin in use by the ADC
will return a logic 0.
4.3.2 Voltage Conversion
When the input voltage to the ADC equals V
REFH
, the ADC converts the signal to $FF (full scale). If the
input voltage equals V
REFL
, the ADC converts it to $00. Input voltages between V
REFH
and V
REFL
are a
straight-line linear conversion.
NOTE
Inside the ADC module, the reference voltages V
REFH
is connected to the
ADC analog power, V
DDAD
; and V
REFL
is connected to the ADC analog
ground, V
SSAD
. Therefore, the ADC input voltage should not exceed these
analog supply voltages.