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AN2577

Application note Using the eTPU angle clock

STMicroelectronics 

eTPU angle clock hardware

2

eTPU angle clock hardware

AN2577

The eTPU angle clock provides a counter that can be synchronized to a periodic input signal

such as would be produced by a sensor detecting teeth on a wheel attached to a rotating

shaft. A typical toothed wheel used in automotive applications might have 35 teeth spaced

at 10 degree intervals with a missing tooth in the 36th position (Figure 1). The angle clock

hardware detects and times the teeth, compensates for the missing teeth, and inserts a

fixed number of additional counts (ticks) between each pair of teeth at a rate determined by

eTPU software. Typically, a system would insert 100 counts between each pair of teeth,

giving an angle resolution of 0.1 degree.

The angle clock is maintained in TCR2, one of two selectable timer counter registers in the

eTPU, which can be used by any channel to cause pin action and/or service requests on a

match, or which can be captured as a result of an input pin action. In addition, the eTPU can

export the angle clock to another eTPU or to an eMIOS device connected to a common

shared timer and counter (STAC) bus. The angle clock can be read in real time by the host.

The eTPU angle clock operates as a state machine, processing inputs from the input pin

and using parameters provided by the eTPU software. A simplified diagram of the eTPU

angle clock logic is shown in Figure 2.

Figure 2. Angle clock simplified state diagram

Tooth Edge

Hold

New Tooth

incr TCR2

tickctr = 0

New Edge

if LAST then TCR2 = 0

else incr TCR2

tickctr = 0

Miss Count

dec MISSCNT

wait TRR time

Tick Time

wait TRR time

tickctr > TICKS

tickctr = TICKS

MISSCNT > 0

tickctr = TICKS

MISSCNT = 0

TRR time-out

tickctr  TICKS

Tick Count

inc tickctr;

incr TCR2

Tooth Edge

Fast Count

inc tickctr;

incr TCR2

wait clock/8

tickctr  TICKS

The heart of the angle clock is a tick generator that inserts a selected number of ticks

between each pair of teeth. A block diagram of the tick generator is shown in Figure 3.

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