AN2577 datasheet(13/34 Pages) STMICROELECTRONICS

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AN2577

System design

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3.1.2

Free running angle clock

To avoid the step-over problem mentioned above, a designer might choose to use the

greater-or-equal comparator in the eTPU channel with a free running angle clock. In this

case, the trigger angle for any function must be offset by the angle at a reference point on

the wheel. For example, the software driver could maintain the top dead center (TDC) value

for the current cylinder. If the spark is set to 12 degrees before TDC, the driver need only

subtract the 12 degrees from the TDC angle to get an absolute firing point. Regardless

when the software is updated, the greater-or-equal compare will ensure that a event is not

missed. Then at some point in the cycle, for each driver the TDC angle must be updated for

the next cycle.

The disadvantage of using a free running angle clock is that a host read of TCR2 will not

directly yield the absolute position of the engine. The value will have to be reduced by a

reference angle, modulus 360/720 degrees, and the reference angle will have to be updated

every revolution.

Either the periodic or the free running angle clock can be applied successfully to an engine

control system.

3.1.3

Angle resolution

The eTPU angle clock can resolve the angle between the teeth into as many as 1024 ticks.

However the proper selection of the TICKS value in the TPR must take into account other

considerations. To understand the limits for TICKS in a given system, the engineer must

consider the configuration of the toothed wheel, the maximum and minimum speed that

must be processed, and the selected resolution of the TCR1 counter.

The TCR1 counter resolution is usually a compromise between the finest resolution required

by the eTPU channel drivers and the maximum pulse width that must be measured or

driven. In fact, arbitrarily long pulses can be processed regardless of the TCR1 resolution by

using software intervention, so the maximum time is seldom a great consideration. The

minimum TCR1 resolution is two times the period of the MCU system clock. However, many

systems engineers choose a convenient value derived from the clock. A typical TCR1

resolution might be 100 nanoseconds. The would enable the eTPU to time out pulses up to

about 0.8 seconds with a single write to a match register. (Using the greater-than-or-equal

comparison, the future equals half the range of the counter. Values larger than 0x800000

above the TCR value are considered in the past, and will match immediately using the

greater-than-or-equal comparison.)

Now consider the TRR. It is organized as a 1-bit integer and a 9-bit fraction.This means that

the largest value for a single tick would be 0X7FFF.1FF (32767.998) times the period of the

TCR. Since a zero value in the integer field is processed as 32768, the minimum TRR value

is actually 1. There are reasons to avoid both of these extremes, so the user should build in

a little margin when setting the limits.

The maximum number of ticks per tooth should ensure that there is more than one TCR1

time per tick at the maximum speed for the wheel. The minimum number of ticks per tooth

should ensure that the maximum value of TRR is not exceeded at the minimum wheel

speed.

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