AD5220 digipot goes to limit in 1 pulse

[case-sensitive]

good day,

I got a AD5220 digital poti and data sheet says, I only need so switch a push button to change the resistance. Off course also need to select up / down and pull CS low to enable. And get a pull up resistor happening (which is not in the manual, but you need it). But as soon as I pull the CLK input low, it just goes all the way, not just 1 step. I then tried all sorts of debouncing circuits, no joy. Nothing to see on the oscilloscope either, CLK goes from 5 to 0 V without any other noise, so no bounce there. Then hooked up a square wave generator, set to 1 HZ, first pulse and resistance went from 50% to 0.
Manual says it goes 1 step per pulse and for testing one can use a manual switch

Not here to discuss why I am not using SPI.

Thanks in advance for any help.

https://www.analog.com/media/en/technical-documentation/data-sheets/AD5220.pdf

 

[Nigel Goodwin]

I want to run it with a small PLC and use its digit al outputs. Unfortunately its mechanical relais. The resistance has to change pretty slow, so push button would do the trick. I almost feel its easier to use a servo motor and mechanical poti

A mechanical relay is bad news - it will bounce - a LOT.

So in order to do that you would need to provide debouncing - I would suggest figure 3 from the digikey link in post #10.

Either use a schmitt trigger IC (I originally used a CD40106), or use a PIC set to use schmitt inputs (I now use a PIC16F18857), and suitably programmed. Value wise, I use two 22K resistors, and 1uF capacitors (no diode) for my push buttons (which only need to be slow), and two 22K resistors and a 0.01uF capacitor for the relay input (which needs to be MUCH faster, with a 0.01uF it exceeds 1KHz).

 

[case-sensitive]

https://www.analog.com/en/resources...ns-control-regulated-switching-converter.html nice image there on the debounce resistors, 100k!
I am using a lab supply, would like to hope it has clean voltage output

 

[Diver300]

How are you measuring the resistance? The AD5220 is only meant to work with the potentiometer voltages between the supply voltage rails.

 

[case-sensitive]

measured between 5 and 6, but I guess need to use 3 to 5. I mean the multimeter has to push a voltage through as well. I need the digipot to pretend its a thermistor. Means it will be probed from some other device.

 

[Diver300]

Ohm meters can use a range of voltages. Also, an Ohm meter won't have it's voltage referenced to Vdd and Gnd of the digital potentiometer.

I suggest you start with connecting A and B to Vdd and Gnd, and measuring the voltage between W and Gnd.

If that works, make sure that in your design where the digital potentiometer is simulating a thermistor, you keep the voltages on A, B and W between Vdd and Gnd. That could be really tricky if you don't know what circuit will be used to measure the simulated thermistor, or even what polarity will be used.

 

[case-sensitive]

oh dear, that is not good. Its to a PLC which runs at 24V. I know the +and GND of where the thermistor is connected to, better measure the voltage.
Interestingly enough, until I triggered the digipot, it had 5kOm, suggesting wiper is half way.
Maybe try a DS1804 / DS1809 instead, problem is, only 64 steps. Finally its so much trouble, may as well go back to the initial plan. Use servo motor and mechanical poti.
Having said that, still waiting for the X9C103 to arrive, in the video he used a multimeter just like that....

 

[case-sensitive]

post 22 won't work, its 2.5 V then when the switch is closed, will not pull the CLK low enough. It actually goes to 0 Ohm and 9 kOhm at random. Each time I close the switch it goes to the opposite end

 

[Nigel Goodwin]

post 22 won't work, its 2.5 V then when the switch is closed, will not pull the CLK low enough. It actually goes to 0 Ohm and 9 kOhm at random. Each time I close the switch it goes to the opposite end

As I've said all along, it's a useless 'supposed' debouncer - and won't work at all if the chip doesn't have a schmitt trigger input.

As I've already mentioned multiple times, use figure 3 from the digikey link in post #10 - but again, while it's an actual working debouncer, it still relies on a schmitt input on the chip.