How to set the input filter in MPC57xx eTimer

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How to set the input filter in MPC57xx eTimer

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richardsha
Contributor I

Hello,

MPC57xx etimer has input filter configuration by Channel n Input Filter Register (ETIMER_CHn_FILT), the latency is defined (((FILT_CNT + 3) * FILT_PER) + 2) peripheral clock periods.

The question to find the eTimer input filter calculation is not correclty, we set as :

FILT_CNT = 7;

FILT_PER =255;

MOTC_CLK = 16MHz;

the latency is (((7+3)*255)+2)*1/16MHz = 0,1595ms. but I tested result is about 0,42ms(2,38Khz), that mean  signal frequcye over 2,38KHz will be filtered, there are too much gap between 0.1595ms and 0.42ms. Could you help check my calculation if is correct? and how to set and calculate the lantency time period correctlly.

Thank you.

Richard

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PetrS
NXP TechSupport
NXP TechSupport

Hi Richard,

The calculation is correct. With that setting the filter’s latency is 0,1595ms. That means after signal edge the input level must be stable for at least 0,1595ms so it can propagate through filter. Thus the signal period must be greater then 0,319ms.

BR, Petr

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