Datasheet

P(n)
P(n-1)
P(n-1)
K
K counts
HET[2]
ext. ref.
signal
APCNT
period
counter
SCNT
step
counter
ACNT
angle
generator
P(n-1)
K
Angle Functions www.ti.com
23.3 Angle Functions
Engine management systems require an angle-referenced time base to synchronize signals to the engine
toothed wheel. The N2HET has a method to provide such a time base for low-end engine systems. The
reference is created by the N2HET using three dedicated instructions with fractional angle steps equal to
/8, /16, /32, /64.
23.3.1 Software Angle Generator
The N2HET provides three specialized count instructions to generate an angle referenced time base
synchronized to an external reference signal (the toothed wheel signal) that defines angular reference
points.
The time base is used to generate fractional angle steps between the reference points. The step width K
(= 8, 16, 32, or 64) programmed by the user defines the angle accuracy of the time base. These fractional
steps are then accumulated in an angle counter to form the absolute angle value.
The first counter, APCNT, incremented on each loop resolution clock measures the periods P(n) of the
external signal. The second counter SCNT counts by step K up to the previous period value P(n-1),
measured by APCNT, and then recycles. The resulting period of SCNT is the fraction P(n-1) / K. The third
counter ACNT accumulates the fractions generated by SCNT.
Figure 23-30 illustrates the basic operation of APCNT, SCNT, and ACNT.
A N2HET timer program can only have one angle generator.
Figure 23-30. Operation of N2HET Count Instructions
Due to stepping, the final count of SCNT does not usually exactly match the target value P(n-1).
Figure 23-31 illustrates how SCNT compensates for this feature by starting each cycle with the remainder
(final count - target) of the previous cycle.
964
High-End Timer (N2HET) Module SPNU562–May 2014
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