AN2577 datasheet(28/34 Pages) STMICROELECTRONICS

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eTPU software design

AN2577

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Example

case Apparent_Gap:

ToothPeriodLast = ToothPeriod;

ToothPeriod= TempToothPeriod;

ToothTime = EdgeCaptureTime;

ToothCount = ToothCount + 1;

if (ToothPeriod >= ToothPeriodLast*GapRatio)//verify

{

CrankStatus = Half_Sync;

SetChannelInterrupt();//Inform the host that the crank

status is updated

ToothCount = 1;

/* More */

Last_State = GapVerified;

}

else

Last_State = PossibleGapVerifying;

/* More */

break;

4.2.7

Gap verified

Once the gap is verified, the system can go to Half_Sync or even Full_Sync mode. The

tooth position is now known, and subsequent gaps can be anticipated. An angle can be

written to the TCR2, although if the mode is Half_Sync, the angle could be off by 360

degrees.

4.2.8

Counting

Once wheel sync has been achieved, the angle clock system executes repeated cycles of

tooth detection and period timing, driving the TCR2 in wheel synchronization. The rate of

the ticks following a tooth are extrapolated from the previous period by the software when it

programs the Tick Rate register.

The Tick Rate register is organized as a 15-bit whole number and a 9-bit fraction and is

programmed in terms of TCR1 counts. The eTPU can automatically capture the TCR1 time

of each tooth edge, so the period is trivial to determine. The new edge time is then stored for

the next period calculation. To determine the tick period, the software need only divide the

measured tooth period by the number of tick counts per tooth. This division can present

some problems for the designer depending on the selected resolution of TCR1 and the

speed range of the wheel. See Section 3.1.3: Angle resolution on page 13. Even when the

TCR1 is properly selected, the software engineer needs to ensure that the intermediate

result of the fixed point calculation does not overrun a register or truncate needed resolution

of the tick rate.

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