
Functional Description
MC68HC908QL4  MC68HC908QL3  MC68HC908QL2 Data Sheet, Rev. 4
Freescale Semiconductor
45
Figure 3-2. ADC10 Block Diagram
The ADC10 can perform an analog-to-digital conversion on one of the software selectable channels. The 
output of the input multiplexer (ADVIN) is converted by a successive approximation algorithm into a 10-bit 
digital result. When the conversion is completed, the result is placed in the data registers (ADRH and 
ADRL). In 8-bit mode, the result is rounded to 8 bits and placed in ADRL. The conversion complete flag 
is then set and an interrupt is generated if the interrupt has been enabled.
3.3.1  Clock Select and Divide Circuit
The clock select and divide circuit selects one of three clock sources and divides it by a configurable value 
to generate the input clock to the converter (ADCK). The clock can be selected from one of the following 
sources:
The asynchronous clock source (ACLK) — This clock source is generated from a dedicated clock 
source which is enabled when the ADC10 is converting and the clock source is selected by setting 
the ACLKEN bit. When the ADLPC bit is clear, this clock operates from 1–2 MHz; when ADLPC is 
set it operates at 0.5–1 MHz. This clock is not disabled in STOP and allows conversions in stop 
mode for lower noise operation.
Alternate Clock Source — This clock source is equal to the external oscillator clock or a four times 
the bus clock. The alternate clock source is MCU specific, see 
3.1 Introduction
 to determine source 
and availability of this clock source option. This clock is selected when ADICLK and ACLKEN are 
both low.
The bus clock — This clock source is equal to the bus frequency. This clock is selected when 
ADICLK is high and ACLKEN is low. 
AD0
ADn
V
REFH
V
REFL
ADVIN
A
CONTROL SEQUENCER
I
S
C
T
A
ADCK
BUS CLOCK
ALTERNATE CLOCK SOURCE
A
A
ACLK
A
ADCSC
A
A
M
C
DATA REGISTERS ADRH:ADRL
SAR CONVERTER
A
C
INTERRUPT
AIEN
COCO
1
2
1
2
MCU STOP
ADHWT
ADCLK
ACLKEN
ASYNC
CLOCK
GENERATOR
CLOCK
DIVIDE