Datasheet

Analog-to-Digital Converter (ADC)
Technical Data MC68HC908AB32 — Rev. 1.1
232 Analog-to-Digital Converter (ADC) Freescale Semiconductor
14.4.1 ADC Port I/O Pins
PTB7/ATD7–PTB0/ATD0 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.
14.4.2 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.
14.4.3 Conversion Time
Conversion starts after a write to the ADSCR. One conversion will take
between 16 and 17 ADC clock cycles. The ADIVx and ADICLK bits
should be set to provide a 1-MHz ADC clock frequency.
14.4.4 Conversion
In continuous conversion mode, the ADC data register will be filled with
new data after each conversion. Data from the previous conversion will
be overwritten whether that data has been read or not. Conversions will
continue until the ADCO bit is cleared. The COCO bit is set after the first
conversion and will stay set until the next write of the ADC status and
control register or the next read of the ADC data register.
In single conversion mode, conversion begins with a write to the
ADSCR. Only one conversion occurs between writes to the ADSCR.
16 to 17 ADC cycles
ADC frequency
Conversion time =
Number of bus cycles = conversion time × bus frequency