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| 型號： | AN2933 |
| 英文描述： | Understanding the eTPU Channel Hardware |
| 中文描述： | 理解硬件頻道的eTPU |
| 文件頁數(shù)： | 30/40頁 |
| 文件大?。?/td> | 741K |
| 代理商： | AN2933 |
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Understanding the eTPU Channel Hardware, Rev. 0
Using the Channel Hardware
Freescale Semiconductor
30
Another important use for a double match is when two asynchronous timebases are used, such as time and
angle. A single pin action can be programmed into both OPACs and the first occurring will switch the pin.
The service request can be made on the first or the second that occurs, or on a specific match.
The two matches may not even be related. One could be used to switch the output pin, and the other as a
wake-up call for an unrelated function. Or one could be used to qualify a transition and the other for an
unrelated function.
Finally, there are cases where both matches have identical OPACs and are working with the same TCR.
The application may calculate the operations separately, and the separate action units would simply be a
means to simplify the software.
4.1.3
Using Double Transitions
The most obvious need for the DoubleTransition modes is to detect two transitions that occur too close
together for the latency of the system. For example, a pulse width can be measured to the accuracy of one
TCR count. However, there are other situations where the double transition circuitry in the eTPU channels
can provide significant functionality.
If a pulse edge is noisy, a single capture register will detect the first programmed transition of the pin and
record its time. If two transitions are programmed in a SingleTransition mode, the presence and timing of
a second edge can be recorded. This can be done by programing the second IPAC to the same or opposite
sense of IPACA.
Sometimes a pin transition needs to be recorded, but the edge need not be serviced until some later event.
In that situation, a DoubleTransition mode can be selected and IPACB can be programmed to NoDetect.
The transition will be recorded and the timebase captured, but the channel will not be serviced as a result.
A subsequent match, or even an event on another channel, can be used to service the detected transition.
Remember that even though the transition did not request service, its TDL will be used to resolve the
thread vector.
4.1.4
Combining Matches and Transitions
In the case of a noisy input edge, the first transition may be detected and processed and the channel may
be setup for the following transition much faster than the noise on the first transition settles out. The most
common way to handle this noise is to block out a subsequent transition for a selected time after the first
one is first detected. By using a MatchB (m2) mode, Transition A is blocked until Match A occurs. The
software may setup Match A to be a fixed time or a time derived from the expected period of the input
transitions.
The operation can be carried a step further, using the Ordered (_o) modes. In these modes, the matches
establish an acceptance window for the transition. In other modes, the second transition may not be
blocked, but service is requested on the first or second match.
In MCUs where the input and output pin are separately connected, one channel can indeed do an input
function and output function simultaneously. A particular case of this arises in some MCUs where the
analog to digital converter requires a time or angle trigger signal. It is possible to drive the ADC trigger
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