eCAP
C2000 Microcontroller Workshop - Control Peripherals 7 - 37
eCAP
Capture Module (eCAP)
The eCAP module timestamps transitions
on a capture input pin
Timer
Timestamp
Values
Trigger
pin
The capture units allow time-based logging of external TTL signal transitions on the capture input
pins. The C28x has up to six capture units.
Capture units can be configured to trigger an A/D conversion that is synchronized with an
external event. There are several potential advantages to using the capture for this function over
the ADCSOC pin associated with the ADC module. First, the ADCSOC pin is level triggered,
and therefore only low to high external signal transitions can start a conversion. The capture unit
does not suffer from this limitation since it is edge triggered and can be configured to start a
conversion on either rising edges or falling edges. Second, if the ADCSOC pin is held high
longer than one conversion period, a second conversion will be immediately initiated upon
completion of the first. This unwanted second conversion could still be in progress when a
desired conversion is needed. In addition, if the end-of-conversion ADC interrupt is enabled, this
second conversion will trigger an unwanted interrupt upon its completion. These two problems
are not a concern with the capture unit. Finally, the capture unit can send an interrupt request to
the CPU while it simultaneously initiates the A/D conversion. This can yield a time savings
when computations are driven by an external event since the interrupt allows preliminary
calculations to begin at the start-of-conversion, rather than at the end-of-conversion using the
ADC end-of-conversion interrupt. The ADCSOC pin does not offer a start-of-conversion
interrupt. Rather, polling of the ADCSOC bit in the control register would need to be performed
to trap the externally initiated start of conversion.
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