PXN20RM Datasheet, PDF (1122/1376 Pages) Freescale Semiconductor, Inc – PXN20 Microcontroller

Analog-to-Digital Converter (ADC)
If the START bit in MCR is reset while ADC is converting, the user needs to wait for the ADC to finish
the current chain conversion (i.e., ECH interrupt) before setting the START bit once again.
34.4.1.3 Normal Conversion Operating Modes
Two operating modes are available for the normal conversion:
• One Shot mode
• Scan mode
To enter one of these modes, it is necessary to program the MCR[MODE] bit. The first phase of the
conversion process involves sampling the analog channel and the next phase involves the conversion phase
when the sampled analog value is converted to digital as shown in Figure 34-39.
Sample B Convert B Sample C Convert C Sample D Convert D Sample E Convert E
Figure 34-39. Normal Conversion Flow
In One Shot mode (MODE = 0), a sequential conversion specified in the NCMRn mask register is
performed only once. At the end of each conversion, the digital result of the conversion is stored into the
corresponding data register.
For example: Channels A-B-C-D-E-F-G-H are present in the device where channels B-D-E are to be
converted in the One Shot mode. MODE = 0 is set for One Shot mode. Conversion starts from the channel
B followed by conversion of channels D-E. At the end of conversion of channel E the scanning of channels
stops.
The MSR[NSTART] status bit is automatically set when the normal conversion starts. At the same time
MCR[NSTART] is reset, allowing the software to program a new start of conversion. In that case the new
requested conversion starts after the running conversion is completed.
In Scan mode (MODE = 1), a sequential conversion of channels specified in the NCMRn register is
continuously performed. As in the previous case, at the end of each conversion the digital result of the
conversion is stored into the corresponding data register.
The MSR[NSTART] bit is automatically set when the normal conversion starts. Unlike One Shot mode,
the MCR[NSTART] is not reset. It can be reset by software when the user needs to stop scan mode. In that
case, the ADC completes the current scan conversion and after the last conversion resets MSR[NSTART]
as well.
For Example: Channels A-B-C-D-E-F-G-H are present in the device where channels B-D-E are to be
converted in the Scan mode. MODE = 1 is set for Scan mode. Conversion starts from the channel B
followed by conversion of the channels D – E. At the end of conversion of channel E the scanning of
channel B starts followed by conversion of the channels D – E. This sequence repeats itself until the
NSTART bit of MCR is reset by software.
If the conversion is started by an external trigger and EDGLEV = 0, MCR[NSTART] bit is not set. As a
consequence, once started the only way to stop scan mode conversion is to set MODE = 0.
34-42
PXN20 Microcontroller Reference Manual, Rev. 1
Freescale Semiconductor