Basic Operation
854
SPNU563A – March 2018
Copyright © 2018, Texas Instruments Incorporated
Analog To Digital Converter (ADC) Module
22.2.1.3 How to Set Up the Input Channel Acquisition Time
The signal acquisition time for each group is separately configurable using the ADG1SAMP[11:0],
ADG2SAMP[11:0], and ADEVSAMP[11:0] registers.
The acquisition time is specified in terms of ADCLK cycles and ranges from a minimum of 2 ADCLK
cycles to a maximum of 4098 ADCLK cycles.
For example, Group1 acquisition time, t
ACQG1
= G1SAMP[11:0] + 2, in ADCLK cycles.
The minimum acquisition time is specified in the device datasheet. This time also depends on the
impedance of the circuit connected to the analog input channel being converted. See the
ADC Source
Impedance for Hercules™ ARM
®
Safety MCUs Application Report
).
22.2.1.4 How to Select an Input Channel for Conversion
The ADC module needs to be enabled first before selecting an input channel for conversion. The ADC
module can be enabled by setting the ADC_EN bit in the ADC Operating Mode Control Register
(ADOPMODECR). Multiple input channels can be selected for conversion in each group. Only one input
channel is converted at a time. The channels to be converted are configured in one or more of the three
conversion groups’ channel selection registers. Channels to be converted in Group1 are configured in the
Group1 Channel-Select Register (ADG1SEL), those to be converted in Group2 are configured in the
Group2 Channel-Select Register (ADG2SEL), and those to be converted in the Event Group are
configured in the Event Group Channel-Select Register (ADEVSEL).
The description in this section only refers to the case when the enhanced channel selection mode is not
enabled. Input channel selection in the enhanced channel selection mode is defined in
22.2.1.5 How to Select Between Single Conversion Sequence or Continuous Conversions
Each group has its own mode control register. The MODE field of these control registers allow the
application to select between a single conversion sequence or continuous conversion mode.
NOTE:
Selecting continuous conversion mode for all three groups
All three conversion groups cannot be configured to be in a continuous conversion mode. If
the application configures the group mode control registers to enable continuous conversion
mode for all three groups, then the Group2 will be automatically be configured to be in a
single conversion sequence mode.
With conversions ongoing in continuous conversion mode, if the MODE field of a group is cleared, then
that group switches to the single conversion sequence mode. Conversions for this group will stop once all
channels selected for that group have been converted.
22.2.1.6 How to Start a Conversion
The conversion groups Group1 and Group2 are software-triggered by default. A conversion in these
groups can be started just by writing the desired channels to the respective Channel-Select Registers. For
example, in order to convert channels 0, 1, 2, and 3 in Group1 and channels 8, 9, 10, and 11 in Group2,
the application just has to write 0x0000000F to ADG1SEL and 0x00000F00 to ADG2SEL. The ADC
module will start by servicing the group that was triggered first, Group1 in this example.
The conversions for all groups are performed in ascending order of the channel number. For the Group1
the conversions will be performed in the order: channel 0 first, followed by channel 1, then channel 2, and
then channel 3. The Group2 conversions will be performed in the order: channels 8, 9, 10, and 11.
The Event Group is only hardware-triggered. There are up to eight hardware event trigger sources defined
for the ADC module. Check the device datasheet for a complete listing of these eight hardware trigger
options.
The trigger source to be used needs to be configured in the ADEVSRC register. Similar registers also
exist for the Group1 and Group2 as these can also be configured to be event-triggered.
The polarity of the event trigger is also configurable, with a falling edge being the default.
