AN2577 View Datasheet(PDF) - STMicroelectronics
Part Name
Description
MFG CO.
'AN2577' PDF : 34 Pages View PDF
AN2577
System design
3.2.1
speed of the wheel, the accuracy of the ticks is determined by the accuracy of the TRR
value plus a possible error due to software latency. The systems engineer needs to
understand the maximum acceleration expected in the wheel and design adequate
compensation in the software.
Acceleration
Acceleration is manifested in the angle clock by a tooth edge arriving earlier than expected
whenever the period of the last tooth separation is less than the pervious period. When the
previous tooth edge was detected, the TRR was written with a value that assumed evenly
spaced ticks between that tooth and the next. Acceleration of the wheel means that the
latest tooth is detected before the tick count is completed. When this occurs, the angle clock
switches automatically into High Rate mode. The balance of the ticks are counted at a rate
equal to the MCU system clock (not TCR1) divided by eight. Once the tick count is
complete, one additional count is added for the newly arrived tooth. The angle clock then
reverts to the Normal mode.
Typically, the detection of the tooth edge causes the Channel 0 logic to request service from
the eTPU engine. This service request is granted by a scheduler in a manner determined by
the following:
1. If the eTPU is idle and no other channel is requesting service, the scheduler will grant
the Channel 0 request immediately. The eTPU will require 6 system clocks to start
executing the service thread.
2. If more than one channel is requesting service, the scheduler will arbitrate the requests
according to the priorities and recent history of the scheduler. See the Reference
Manual for a detailed treatment of the scheduler process.
3. If another channel is being serviced, the eTPU will complete the thread being serviced,
then revert to 2.
Presumably, the service thread will determine the previous period and calculate a new TRR
value. Then, at some point in the thread, the TRR will be written. However, depending on
latency, the new tick stream may have started counting before the updated TRR was written.
In that event, all ticks that have been timed during the latency period will presumable be
slow and, after the TRR is corrected, the current period may run late as well.This effect is
actually rather small in real automotive systems, except perhaps during the very first firing
while cranking.
Example
A 36-1 (35 ten-degree teeth with one missing tooth) tooth wheels is running at 720
RPM when the engine is accelerated at a rate of 5000 RPM per second. If the angle
clock was inserting 100 ticks per tooth, and the latency for the tooth service was 10
microseconds, what is the tick distribution in the first two gaps after acceleration starts?
(system clock = 100 MHz)
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