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PWM IN MC43, Signal Conversion

Amir Darvish 3 weeks ago in Master modules / MC4x • updated by Gustav Widén (System support) 5 days ago • 2

Controller: IQAN-MC43, Connector C1, Pin 12 (IN_34, pull-up type frequency input)

Issue: I'm feeding a 200 Hz, ~60% duty cycle open-collector signal into pin 12, using the module's internal pull-up (per manual: 1.5 kΩ to internal 5V, 10 nF) with no external resistor. Instead of a clean square wave, the signal reads as a sawtooth that never reaches 0V - high sits around 4.6V, low only drops to ~4.3V.

I've confirmed pin 12 floats at 4.6V unloaded, and leakage from my circuit is negligible. Tried several pull-up configurations externally; sink current into the pin stays very low (~0.2-0.5 mA) regardless.


Questions:

1. Is 1.5 kΩ/10 nF correct for this input type?

2. What sink current is needed to reliably trigger a logic-low on this pin?

3. Any recommended interfacing for low-current-sinking sources on these inputs?


Thanks for any guidance.

Thomas Moberg (System support) 2 weeks ago

The 1.5 kΩ/10 nF is what internally is connected to that input.

The trigger levels for this input is voltage, not current.

Below 1 V is guaranteed low and above 4 is guaranteed high.

Are you sure this sensor is suppose to be on a pull up input and not a pull down?

What kind of sensor is it you have? 

If it is a NPN it should be connected to a pull up input and a PNP should be connected to a pull down.

Amir Darvish 2 weeks ago

Thanks for your reply.

The source of this PWM signal is an Arduino board (A000005: https://mm.digikey.com/Volume0/opasdata/d220001/medias/docus/8938/1050_A000005.pdf), which transfers the signal to the MC43 through an optocoupler, Vishay IL217AT (https://www.vishay.com/docs/83616/il215at.pdf). The optocoupler's output is an NPN phototransistor (open collector).

I've attached three scope captures:

1. The PWM signal at the optocoupler collector, before connecting to the MC43 - a clean, perfect square wave.

2. The same signal connected to pin 12 (pull-up input) - converts to a sawtooth that never reaches 0V.

3. The same signal connected to pin 29 (pull-down input) - stays a clean square wave, but at very low amplitude (~0.5V peak).

PWM_Before_Sending_to_MC43.jpg

PWM_to_MC43_PullUP.jpg

PWM_to_MC43_PullDown.jpg


Given it's an NPN open-collector output, I connected it to the pull-up input (pin 12) as you'd expect, but that's where the sawtooth distortion shows up. The pull-down input (pin 29) keeps the waveform shape clean but the amplitude is too low to be usable.


Please let me know your thoughts, or if you need any further clarification.

Thanks,

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