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Enhanced Time Processing Unit (eTPU2)
MPC5644A Microcontroller Reference Manual, Rev. 6
908
Freescale Semiconductor
In the first option the missing tooth is not counted on the angle measurement. For example, a flywheel with
59 physical teeth and one missing tooth can be considered as 58 identical teeth numbered (0-57) and tooth
number 58 has a double number of angle TICKS. In this case a 720 degrees engine cycle has 118 teeth.
TCR2 reflects the real angle, since it counts angle ticks continuously.
In the second option, the missing teeth are counted as “regular” teeth by automatic insertion of “dummy”
teeth. The microcode has to write a non-zero value to the MISSCNT field in TPR. This field is a 2-bit down
counter which affects the operation of the Counter Control logic.
For example, a flywheel with 59 physical teeth (0-58) and one missing tooth (59) can be considered as 60
teeth numbered (0-59), all having the same number of angle ticks. The microcode has to write “01” to the
MISSCNT bits during the period of tooth number 58 to indicate that next tooth (59) is missing.
When the Tooth Tick Counter reaches the TICKS value, TCR2 is incremented as if a physical tooth has
been detected. In addition, the MISSCNT value initializes a “dummy tooth counter” which is decremented
to indicate the number of left “dummy teeth” which still need to be generated. Because a dummy tooth was
counted, EAC does not enter Halt Mode and Tooth Tick Counter continues incrementing in the absence of
a physical tooth detection.
In case of extreme acceleration on very low RPM (cold start) there can be a situation that the first physical
tooth after one or two missing teeth appears even before the “dummy” tooth is generated. Due to the
acceleration the EAC switches to High Rate mode in order to run through all the valid angle values,
including the dummy teeth. When the Tooth Tick Counter reaches the TICKS value on High Rate mode,
and the “dummy tooth” down counter is not zero, the generated “dummy tooth” advances to the next tooth
and decrements the “dummy tooth” counter, but does not switch the EAC back to Normal mode. The last
“dummy tooth” decrements the counter to zero, indicating that no more dummy teeth are to be inserted,
and the next tooth is an estimated physical tooth. The EAC continues at High Rate mode until the Tooth
Tick Counter reaches the TICKS value again, then advances to the next tooth while switching back to
Normal mode. When in High Rate mode, the TPR does not reflect the MISSCNT downcounting; see
Section 24.5.7.12.1, TPR buffering
MISSCNT can be rewritten before it reaches 0, allowing it to count more than three missing teeth, as long
as no physical tooth arrives between the first MISSCNT write and the rewrite.
24.5.7.6.3
Combining missing teeth and last tooth
The Last Tooth indication takes effect when there are no more missing teeth to be generated, i.e the
“dummy tooth” counter value is zero. If, for example, the microcode sets the missing teeth counter to “10”
(two missing teeth) and sets the LAST flag, the first and the second dummy teeth will increment TCR2,
and the third estimated tooth, which correlates with the physical tooth (the first of the next cycle), will reset
TCR2, because LAST was set. This scheme enables the microcode to define one or more missing teeth to
be replaced by “dummy tooth” insertion, and the end of the engine cycle in one service request. It is
assumed that the two missing teeth must come together in the same engine cycle, and not split between
two engine cycles (either the missing teeth are both last in an engine cycle or both not last, but not last in
one engine cycle and first in the next).
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Страница 52: ...Introduction MPC5644A Microcontroller Reference Manual Rev 6 52 Freescale Semiconductor...
Страница 56: ...Memory Map MPC5644A Microcontroller Reference Manual Rev 6 56 Freescale Semiconductor...
Страница 130: ...Device Performance Optimization MPC5644A Microcontroller Reference Manual Rev 6 130 Freescale Semiconductor...
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