mc68hc908gr16 Freescale Semiconductor, Inc, mc68hc908gr16 Datasheet - Page 50

• Manufacturer Part Number: mc68hc908gr16
• Description: M68hc08 Microcontrollers Microcontroller
• Manufacturer: Freescale Semiconductor, Inc

Analog-to-Digital Converter (ADC)
3.3.3 Conversion Time
Conversion starts after a write to the ADC status and control register (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.
3.3.4 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 each conversion and
will stay set until 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.
When a conversion is in process and the ADSCR is written, the current conversion data should be
discarded to prevent an incorrect reading.
3.3.5 Accuracy and Precision
The conversion process is monotonic and has no missing codes.
3.3.6 Result Justification
The conversion result may be formatted in four different ways:
All four of these modes are controlled using MODE0 and MODE1 bits located in the ADC clock register
(ADCLK).
Left justification will place the eight most significant bits (MSB) in the corresponding ADC data register
high, ADRH. This may be useful if the result is to be treated as an 8-bit result where the two least
significant bits (LSB), located in the ADC data register low, ADRL, can be ignored. However, ADRL must
be read after ADRH or else the interlocking will prevent all new conversions from being stored.
Right justification will place only the two MSBs in the corresponding ADC data register high, ADRH, and
the eight LSBs in ADC data register low, ADRL. This mode of operation typically is used when a 10-bit
unsigned result is desired.
Left justified sign data mode is similar to left justified mode with one exception. The MSB of the 10-bit
result, AD9 located in ADRH, is complemented. This mode of operation is useful when a result,
represented as a signed magnitude from mid-scale, is needed. Finally, 8-bit truncation mode will place
the eight MSBs in the ADC data register low, ADRL. The two LSBs are dropped. This mode of operation
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1. Left justified
2. Right justified
3. Left Justified sign data mode
4. 8-bit truncation mode
Number of bus cycles = conversion time × bus frequency
Conversion time =
MC68HC908GR16 Data Sheet, Rev. 5.0
16 to 17 ADC cycles
ADC frequency
Freescale Semiconductor