K24 input pin behavior

fcw — Mon, 11 Dec 2017 00:54:35 GMT

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?

Re: K24 input pin behavior

xiangjun_rong — Wed, 13 Dec 2017 05:38:19 GMT

Hi, Fred,

I have checked the internal ticket system, there are schmitt trigger circuit for each input cell. I have checked the Input leakage current spec in data sheet of K24, it is 1uA, in other words, the impedance of input pin is 1M ohm.

I suppose that the quadrature module works if you remove the 120K ohm serial resistor, is it right?

Xiangjun rong

topic K24 input pin behavior in Kinetis Microcontrollers

K24 input pin behavior

Re: K24 input pin behavior