AN2897 Freescale Semiconductor / Motorola, AN2897 Datasheet - Page 15

• Manufacturer Part Number: AN2897
• Description: Using the eTPU Angle Clock
• Manufacturer: Freescale Semiconductor / Motorola

3.3.3
Once the gap has been found and verified, the application knows the position of the wheel at the time of
the last tooth edge. The Angle Clock can now be initialized with this angle. There are two considerations
for the systems engineer at this point. First, if the application is synchronizing to a 720 degree cycle, there
may or may not be sufficient information to determine the correct half of the engine cycle. Additional
information might be required to completely synchronize the Angle Clock to the wheel. Second, there is
typically some time lost between the detection of the last edge and the software conclusion that the wheel
angle is verified. If the Angle Clock is arbitrarily started at this time, there may be some error in the
indicated angle.
If the Angle Clock is required to track a 720 degree cycle, then it is possible that the information on which
half of the cycle the gap was detected is not yet available to the application. While full synchronization
may not have been achieved, the application can declare “Half_Sync.” This means that the crank shaft
position is known but the cam shaft position may subsequently require the shaft angle to be corrected by
360 degrees. In an automotive application, Half_Sync is sufficient to operate the engine using a wasted
spark and split fueling strategy. If the system is operated in a half sync mode until full sync is possible, it
is important to consider the effect on the system of the Angle Clock getting a sudden 360 degree correction
if this should be required.
Various cam shaft detection strategies may be used to effectively eliminate the cam uncertainty at the time
the crank shaft is synchronized. If such a strategy is available, Half_Sync is not required.
When synchronization is verified, the angle at the last tooth edge is known. The software should
immediately write that angle to TCR2, write the TRR value derived from the last period, and start the
Angle Clock. The first series of ticks will be delayed by the latency of the function to this point, but if
necessary the Angle Clock will go into High Rate mode at the next tooth edge to catch up.
3.4
There are two ways to minimize the effects of anomalies that can cause loss of synchronization in the eTPU
Angle Clock: prevent as many as possible, and provide a means to detect and correct those that cannot be
prevented. The prevention is typically done by the eTPU hardware system, properly setup by the software.
However, once an anomaly is detected and processed by the system either as a superfluous or a missing
Freescale Semiconductor
Subtracting adjacent tooth times yields the following periods:
Period[1] = 0x196E6
Period[2] = 0x2F0C3
Period[3] = 0x16612
We can use a “Gap Ratio” algorithm to test if this represents a gap detection.
Use Gap_Ratio = 0.7
Is_Gap = if ((Period[2] * Gap_Ratio > Period[1]) && (Period[3] < Gap_Ratio * Period[2])
Period[2] * Gap_Ratio = 0x203BB
This is larger than Period[1] and Period[3]...
...therefore the Is_Gap condition is satisfied.
Anomalies
Starting the Angle Clock
Using the eTPU Angle Clock, Rev. 0
System Design
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