Analog-to-Digital Converter (ADC)
PXN20 Microcontroller Reference Manual, Rev. 1
Freescale Semiconductor 34-41
34.4 Functional Description
34.4.1 Analog Channel Conversion
Three conversion modes are available within the ADC module:
• Normal conversion
• Injected conversion
• Triggered injected conversion using CTU
34.4.1.1 Normal Conversion
Normal conversion is programmed by configuring the normal conversion mask registers (NCMRn) in
which each channel can be individually enabled by setting the corresponding CHn bit. Mask registers must
be programmed before starting the conversion and cannot be changed until the conversion of all the
selected channels ends.
34.4.1.2 Start of Normal Conversion
A normal conversion can be started in one of two ways:
• By software (TRGEN = 0):
— If the external trigger enable bit is reset, the conversion chain starts when the NSTART bit in
MCR is set.
• By trigger (TRGEN = 1):
— An external trigger/enable signal is detected to start the conversion. The enable pin is checked
only when the conversion is started. The end and the restart of the conversion must be checked
by software. This feature is enabled setting the TRGEN bit in the MCR register. Two options
are available:
– If EDGLEV (edge/level selection) bit in MCR is reset then a rising/falling edge (depending
on the EDGE bit in MCR) detected in an external pin set the NSTART bit in MCR and starts
the programmed conversion. EDGE = 0 means falling, EDGE = 1 means rising edge.
– If EDGLEV (edge/level selection) bit in MCR is set, the conversion is started if and only if
the NSTART bit in MCR is set and the programmed level on the external trigger pin is
detected. The level is selected using the EDGE bit in MCR. EDGE = 0 means that the start
of conversion is enabled if the external pin is low. If EDGE = 1, the start of conversion is
enabled when the external pin is high.
The NSTART status bit of MCR is automatically set when a normal conversion starts. At the same time
NSTART bit of MCR 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.
If the contents of all the normal conversion mask registers is zero (i.e., no channel is selected), the
conversion operation is considered completed and the interrupt ECH is immediately issued after the start
of conversion.
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