Analog-to-Digital Converter (ADC)
MCF52235 ColdFire® Integrated Microcontroller Reference Manual, Rev. 6
28-35
Freescale Semiconductor
28.5.9.2
Description of Clock Operation
the conversion clock is the primary source for the ADC clock and is always
selected as the ADC clock when conversions are in process. The DIV value in the CTRL2 register should
be configured so the conversion clock frequency falls between 100 kHz and 5.0 MHz. Operating the ADC
at out-of-spec clock frequencies degrades conversion accuracy. Similarly, modifying the parameters affect
clock rates or power modes while the regulators are powered up (PD0=0 or PD1=0) also degrades
conversion accuracy.
The conversion clock ADC uses for sampling is calculated using the IPBus clock and the clock divisor bits
within the CTRL2 register. Please see
Section 28.4.1, “Control 1 Register (CTRL1)
.
The ADC clock is active 100% of the time while in loop modes, or if power
management is set to normal. It is also active during all ADC power-up for a period of time determined by
the PUDELAY field in the power (POWER) register. After the power-up delay times out, the ADC clock
continues until the completion of the ADC
n
scan when operating in auto standby or auto power-down
modes.
Figure 28-25. ADC Clock Generation
The oscillator clock feeds an 80:1 divider, generating the auto standby clock. The auto standby clock is
selected as the ADC clock during the auto standby power mode when both converters are idle. The auto
Table 28-22. ADC Clock Summary
Clock input
Source
Characteristics
Peripheral Clock
(=System Clock)
1/2 Core clock
Maximum rate is PLL output divided by 2 if PLL enabled. When PLL disabled,
max rate is oscillator clock divided by 2.
ADC 8MHz Clock
Relaxation
Oscillator (8MHz),
Crystal Oscillator
(1-16MHz), or
external Oscillator
Provides 8MHz for auto standby power saving mode.
0
0
1
CTRL2:DIV
(+2 x [DIV+1])
Peripheral (System) Clock
1/2 Core Frequency
0
1
Auto Power-Down
ADC Idle
Disabled (APD=0)
Auto Standby
Enabled (ASB=1)
Standby Current Mode Status
ADC Bit in SIM_PCE
(+80)
Oscillator Clock
(8MHz)
ADC Clock
Auto Standby Clock
(100kHz)
ADC
Conversion
Clock
DIV
Because
of
an
order
from
the
United
States
International
Trade
Commission,
BGA-packaged
product
lines
and
part
numbers
indicated
here
currently
are
not
available
from
Freescale
for
import
or
sale
in
the
United
States
prior
to
September
2010:MCF52234CVM60,
MCF52235CVM60
