
M68HC11E Family Data Sheet, Rev. 5.1
Freescale Semiconductor
57
Chapter 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,
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 VDD 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.
3.2.2 Analog Converter
Conversion of an analog input selected by the multiplexer occurs in this block. It 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.
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
s before the converter can be used.
The charge pump is enabled by the ADPU bit in the OPTION register.