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K24 input pin behavior

Question asked by FRED WEDEMEIER on Dec 10, 2017
Latest reply on Dec 12, 2017 by xiangjun.rong

Is there an explanation for what I'm seeing on some MK24FN1M0VLL12 pins? The attached waveforms show an example, pin 98, which is PTB19. Channel 1 is a signal generator output and channel 2 is pin 98. The circuit essentially has a 121K resistor between the sig generator and pin 98.

 

mux0,jpg shows waveforms with the the PORTB_PCR19 MUX set to zero. There's a bit more than the 121K resistor in the circuit which accounts for the small attenuation you can see in both the high and low levels on channel 2.

 

mux6.jpg shows waveforms with PORTB_PCR19 MUX set to six, which is what's needed for the application. The 221K can't drive the input. I also set MUX to one with the same result.

 

offset6.jpg shows waveforms with some negative offset in the sig generator input. With very fine tweaking on the sig generator, waveform 2 can show some metastability. The only thing that maybe makes sense is the effect of a Schmitt trigger on the chip inputs, but the only reference I see  to Schmitt triggering in the ref manual or data sheet regards the RTC.

 

So is the Schmitt guess correct or is there something else going on?

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