AN2897 Freescale Semiconductor / Motorola, AN2897 Datasheet - Page 16

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

System Design
tooth edge signal, the Angle Clock design can only help detect the problem, and pass the information on
to the host for corrective action.
3.4.1
The eTPU Angle Clock provides means to reject tooth edges detected outside of certain limits. This
rejection is done by an input pin hardware filter and by gating the tooth signal with a match registers in the
channel.
The TCRCLK input pin filter for the Angle Clock can use the input pin filters of the other eTPU channels,
or it can be programmed separately. These filters are designed to reject very short apparent pin transitions
such as might be caused by high frequency induced noise. In any case, this low pass filtering of the tooth
signal will introduce a time delay in the signal from the tooth. Since the filter time is generally in the order
of a few system clock times, the angle represented by this delay is very small even at high wheel speeds.
See the Reference Manual for details of the TCRCLK filter.
When Channel 0 is in one of the m2_ modes, the MatchA register can be used as a blocking timer for input
signals. In this mode, the eTPU software can provide a blanking time after the detection of a tooth edge,
during which time a (presumably spurious) additional edge will be blocked. This feature is particularly
useful when an imperfect tooth detection circuit presents a chopped or hashed edge to the eTPU, which
often happens at low speed. If the frequency of the edge noise is lower than the pin filter rejection, a noise
pulse could be detected and counted as a tooth edge signal unless this blanking is used. The blanking match
can be programmed as an absolute time or calculated by the eTPU as a percentage of the previous period.
The choice depends on the expected characteristics of the edge noise.
If a noisy edge is possible, then a noisy return edge might also be detected as an active transition. If
blanking is used to prevent noise from being processed as a tooth edge, then the blanking must be applied
to both edges of the tooth signal.
If a tooth signal is completely lost to the eTPU channel by some means, the hardware alone cannot
distinguish the long period from any other deceleration. However, a second match can be setup in Channel
0 to request service if a transition has not occurred within a specified time.
3.4.2
The tick counter in the eTPU Angle Clock ensures that there are exactly TICKS counts between physical
teeth. If the Angle Clock acquires an error less than one tooth count, the only possible reason is a software
error. Nevertheless, if even if the count acquires a small error, the count will rectify when the LAST tooth
is processed.
16
Error Prevention
Error Recovery
The most common cause of a small TCR2 angle error is writing the initial
angle to the angle clock when synchronization occurs. When gap
synchronization is first verified, if the tick rate timer expires just as the
TCR2 value is incremented by the tick counter, the tick counts for
subsequent teeth count may fall short by one tick until the last tooth
re-synchronizes the tick counter.
Using the eTPU Angle Clock, Rev. 0
NOTE
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