
M68HC11E Family — Rev. 5
Data Sheet
MOTOROLA
Analog-to-Digital (A/D) Converter
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63
Data Sheet — M68HC11E Family
Section 3. Analog-to-Digital (A/D) Converter
3.1  Introduction
The analog-to-digital (A/D) system, a successive approximation converter, uses an 
all-capacitive charge redistribution technique to convert analog signals to digital 
values. 
3.2  Overview 
The A/D system is an 8-channel, 8-bit, multiplexed-input converter. The converter 
does not require external sample and hold circuits because of the type of charge 
redistribution technique used. A/D converter timing can be synchronized to the 
system E clock or to an internal resistor capacitor (RC) oscillator. 
The A/D converter system consists of four functional blocks: multiplexer, analog 
converter, digital control, and result storage. Refer to 
Figure 3-1
. 
3.2.1  Multiplexer 
The multiplexer selects one of 16 inputs for conversion. Input selection is controlled 
by the value of bits CD:CA in the ADCTL register. The eight port E pins are 
fixed-direction analog inputs to the multiplexer, and additional internal analog 
signal lines are routed to it. 
Port E pins also can be used as digital inputs. Digital reads of port E pins are not 
recommended during the sample portion of an A/D conversion cycle, when the 
gate signal to the N-channel input gate is on. Because no P-channel devices are 
directly connected to either input pins or reference voltage pins, voltages above 
V
DD
 do not cause a latchup problem, although current should be limited according 
to maximum ratings. Refer to 
Figure 3-2
, which is a functional diagram of an 
input pin. 
F
Freescale Semiconductor, Inc.
n
.