X%
X%
Toothed
Input
Filter
Output
glitch during the window
glitch after the window
Angle Functions
840
SPNU503C – March 2018
Copyright © 2018, Texas Instruments Incorporated
High-End Timer (N2HET) Module
20.3.2.2.5 Input Noise Filtering
The toothed wheel input comes from an analog part and is sensitive to external noise. Due to this
sensitivity, the input needs to be filtered because of glitches in the signal.
The HWAG digitally filters the toothed wheel input signal before it is used inside the core. The filter blocks
the signal which negates the effect inside the HWAG. The HWAG provides two filter registers that filter the
same way.
The filters validate the input signal after n angle ticks. The n angle ticks are like X% of the tick counter.
The value of the remaining percentage of the tick counter (1- X%) need to be set because the tick counter
is a down counter. Calculate the value to put into the filter registers from the step width value (or angle
ticks value per tooth). The toothed wheel input is like a low pass filter with a cut-off frequency that
functions like a toothed-wheel speed, but without acceleration and decelerations side effects. For an
example of a windowing filter for a toothed wheel input on a falling active edge, see
NOTE:
At any time, the CPU can modify the filter values to fine tune with the application.
Figure 20-47. Windowing Filter for Toothed Wheel Input on Falling Active Edge
To calculate this number:
Step Width × (1 – X%) = Filter Register Value
If the step width value is equal to 512 and you want to filter 75% of the tooth, calculate the filter register as
follows:
512 × (1 – 0.75) = 128
When the tick counter reaches the filter register value, the toothed wheel input is unblocked.