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Enhanced Time Processing Unit (eTPU2)
MPC5644A Microcontroller Reference Manual, Rev. 6
Freescale Semiconductor
883
24.5.5.6
Enhanced Digital Filter – EDF
The EDF eliminates passing of signal transitions which are caused by noise. Its purpose is to eliminate
false transition service requests caused by noise pulses which are shorter than a programmed width.
The EDF has three modes of operations, selected by the CDFC field in the ETPU_ECR (see
Section 24.4.2.5, ETPU_ECR – eTPU Engine Configuration Register
). These modes offer selections of
trade-off between noise immunity and signal latency. CDFC also allows the filter to be bypassed.
gives an example of minimum detected signal pulse and maximum filtered noise pulse in the
three EDF operation modes. In Angle Mode, if AM = 01, the EDF in channel 0 is replaced with the digital
filter and synchronizer of the TCRCLK signal. In this mode, channel 0 works in combination with the
Angle Counter logic, and their operation is fully synchronized.
Following subsections provide the functional description of the eTPU channel digital filter.
24.5.5.6.1
Two-Sample Mode
In this mode the EDF works like the TPU2/3 digital filter. It uses the filter clock which is the system clock
divided by (2, 4, 8,.., 256) as a sampling clock. The filter clock is selected by the FPSCK field in the Engine
Configuration Register (ETPU_ECR) (see
Section 24.4.2.5, ETPU_ECR – eTPU Engine Configuration
). The EDF compares two consecutive samples. If both samples have the same value, the input
signal state is updated. Note that when the FPSCK field selects the system clock divided by two, the EDF
works like the TPU1 four-clock digital filter.
24.5.5.6.2
Three-Sample Mode
In this mode, like in the TPU2/3 mode, the EDF uses the filter clock as a sampling clock. The EDF
compares three consecutive samples. If all three samples have the same value, the input signal state is
updated.
The Three-Sample mode gives more signal latency than the Two-Sample mode, but also better noise
immunity and better ratio between minimum detected signal pulse to maximum filtered noise pulse. When
a certain filter clock frequency is selected for Two-sample mode, double filter clock frequency can be
selected to get better latency in Three-sample mode.
24.5.5.6.3
Continuous Mode
In this mode the EDF compares all the values sampled at the rate of system clock divided by two, between
two consecutive filter clock pulses. If the signal is continuously stable for the entire digital filter clock
period (i.e all the samples have the same signal value), the input signal state is updated.
This method gives the same latency and the same ratio between minimum detected signal pulse to
maximum filtered noise pulse, as the Two-Sample mode, as long as there is no noise. Each sampled noise
delays the signal transition detection by at least a whole digital filter clock period.
The Continuous mode gives the best noise immunity by comparing multiple samples of the noise. On the
other hand, when a short noise pulse appears in the middle of the filter clock period at the same time of a
real signal transition, the Continuous mode may reject a real signal transition and delay the response to the
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