
Analog-to-Digital (A/D) Converter
Overview
M68HC11E Family — Rev. 5
Data Sheet
MOTOROLA
Analog-to-Digital (A/D) Converter
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65
3.2.2  Analog Converter 
contains a digital-to-analog capacitor (DAC) array, a comparator, and a successive 
approximation register (SAR). Each conversion is a sequence of eight comparison 
operations, beginning with the most significant bit (MSB). Each comparison 
determines the value of a bit in the successive approximation register. 
The DAC array performs two functions. It acts as a sample and hold circuit during 
the entire conversion sequence and provides comparison voltage to the 
comparator during each successive comparison. 
The result of each successive comparison is stored in the SAR. When a conversion 
sequence is complete, the contents of the SAR are transferred to the appropriate 
result register. 
A charge pump provides switching voltage to the gates of analog switches in the 
multiplexer. Charge pump output must stabilize between 7 and 8 volts within up to 
100 
μ
s before the converter can be used. The charge pump is enabled by the 
ADPU bit in the OPTION register. 
3.2.3  Digital Control 
All A/D converter operations are controlled by bits in register ADCTL. In addition to 
selecting the analog input to be converted, ADCTL bits indicate conversion status 
and control whether single or continuous conversions are performed. Finally, the 
ADCTL bits determine whether conversions are performed on single or multiple 
channels. 
3.2.4  Result Registers 
Four 8-bit registers ADR[4:1] store conversion results. Each of these registers can 
be accessed by the processor in the CPU. The conversion complete flag (CCF) 
indicates when valid data is present in the result registers. The result registers are 
written during a portion of the system clock cycle when reads do not occur, so there 
is no conflict. 
3.2.5  A/D Converter Clocks 
The CSEL bit in the OPTION register selects whether the A/D converter uses the 
system E clock or an internal RC oscillator for synchronization. When E-clock 
frequency is below 750 kHz, charge leakage in the capacitor array can cause 
errors, and the internal oscillator should be used. When the RC clock is used, 
additional errors can occur because the comparator is sensitive to the additional 
system clock noise. 
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Freescale Semiconductor, Inc.
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