FEZ Mini Robot analog port problem

When testing a wheel rotation sensor, it was useful to the wheel speed with the hand to control. I used the ZX-110 in which the FEZ Ultimate kit is included. The connector fits exactly on the FEZ Mini Robot A6 pin. The driver is POT.cs order to get the value from 0 to 100 can control. This went on for a few days and test results were good. However, after a small adjustment of the wheel rotation sensors program did not work anymore.
It was found that the analog value of no more up to 100, but only up to a value of 2 to be able to regulate. I did not understand this and have another pin connector A7 chosen that the program is working now.
Conclusion pin A6 is defective, this may be due to the ZX-110?
I assumed that the pins are 5 volt resistant and that this pot I could join.
If the defect was caused by a hardware or firmware error?
The last question is the processor to be repaired?

Hi Hinnie,

Analog input pins are NOT 5v tolerant. That’s what your problem is. You must ALWAYS keep the voltage on analog in pins below 3v3. So it’s likely that your processor is damaged. That the other analog in pins still work is lucky, since internally the ADC is single-pin and there is a multiplexer that switches in whatever pin you’re sampling. Seems like you might have just affected the multiplexer rather than the ADC itself. Sounds like it’s time to buy a new one - you could of course replace the processor. Good luck !

Hi Brett,
Thanks for your response, I meant to have understood that only an analog pin to 3.3 volts could measure but was resistant to 5 volts.
Unfortunately for me so gone.

Still a comment: At a later version of the FEZ Ultimate kit is another POT module delivered, what struck me is that the module now contains a resistor as the photo shows. Perhaps that added to the current limit but I have no electrical drawing.
See the appendix for the differences of the modules

I believe that resistor in the diagram is variable – as in it’s the knob/shaft being turned. The microcontroller can “detect” the position by looking at the voltage returning to the board, by seeing where the voltage is between 0V and 3.3V.

**edit just noticed you diagram says 5V.


I do not understand your comment.
It is clear to me that the analog pin can measure only up to 3.3 volts.

Of the red version of the POT I have no electrical drawing
But the picture is the resistance clearly see, my suspicion is that this resistance is added to limit the current.
Attached a photo of both versions of the POTH
If that really is the reason I want my green copy to give an update.
Can someone explain the resistance?

Essentially the dial is like a resistor that is able to change it’s value.

Turn it all the way one way, and there’s no resistance. Whatever voltage is put in, comes out at essentially the level off the output.

Turn it the all the way the other way, and the resistance is at 100%, and essentially no voltage comes out on the output.

At least that’s the way I understand it.

So here’s the thing. The uC is 5v tolerant on DIGITAL INPUT, but the multiplexer in the chip is not.

[quote]Although the pins are 5V tolerant, the ADC multiplexing is not and this can cause wrong
readings on the affected pin or other analog pins. Please consult LPC23xx user manual for
more details.[/quote]
that’s from the user manual - http://www.ghielectronics.com/downloads/USBizi/USBizi_User_Manual.pdf
So if you apply more than 3v3 you risk damaging the multiplexer, in your case you look like you’ve damaged the input side of the multiplexer which means you’ve only lost the use of one pin, but you could have damaged the output side which then could have meant you lost all ADC access.

On your question about the different Potentiometers. The green INEX brand one is a general pot, obviously used in arduino and other microcontrollers, not just a 3v3 tolerant device.

The simplest way to guarantee that you don’t over-stress the input is to connect the other end of the connection to the 3v3 power line, not the 5v power line. What did you have your potentiometer connected to? I’m pretty sure on the original Domino shield that had those connectors on it was explicitly wired to 3v3 not 5v.

Unfortunately, the FEZ Mini Robot has only a 5-volt connection to the 3 pin connectors. I want to explore some of the 3 pin connectors of the FEZ Mini Robot is also switchable to as the FEZ Connect shield is realized. In order to make this possible, there must be a 3.3 voltage regulator to be added to the control board of the robot. That seems a simpler solution than the output of the modules to suit max 3.3 volts. This is also better for controlling the motor controller TB6612FNG on the “power circuit board” of the FEZ Mini Robot.

A question for GHI is to explain why the extra resistance on the red POTH is added?

you create a voltage drop over a resistor. So you can influence the voltage by adding more resistance - but you’re going to consume current doing so, and generate heat. So if you want to adjust the power down, an additional resistor could help.

The analog port of the FEZ Mini is defective because the POTH gone to maximum was (= 5 volts) while the power was switched.

The POTH is one of the components of the FEZ Ultimate Kit which you can properly use the FEZ Connect shield you since the supply voltage for the connector can choose between 3v3 and 5v

Because there is the FEZ Mini Robot Kit has the same connecter I have POTH connected thereto for testing the wheel rotation sensors.
However, with this kit you can not switch the supply voltage and has a fixed value of 5Volts

The POTH was first delivered in a green version but was later replaced by the red version. In my FEZ Ultimate Kit is the green copy delivered.
On the pictures is clearly perceptible that the red version has an additional resistor.

Can GHI me explain why that resistor is added?
Is this resistor a voltage limiter so you POTH also on the FEZ Mini Robot can use?


It is a form of protection so pin is never shorted high or low.

@ Gus

The sensor ZX-03 reflector like the FEZ Mini Robot comes also has a 5 volt problem.
If this sensor is connected a white analog measurement value than other analog channels are affected.
In my case, the ZX-03 as a floor sensor is installed to detect lines. A second type of sensor GP2D12 for measuring distances.
The GP2D12 distance gives the wrong values ​​as the result of the floor sensor is more than 3.3 V voltage on the analog offers.
I solved this by the ZX-03 also with an extra “protection” add resistance.

@ Hinnie

Hi, i actually have the 5v problem with ZX-03.
I connected the GP2D12 for measuring distance and the ZX-03 to detect lines.
Can you explain me where you added the resistance on the ZX-03? And what for a value of the resistance?

Thank you very much for your help,


@ Dragonsshout

There is a 4k7 resistor between the transistor collector and the +5 volt placed the TCRT5000, I’d rather have a voltage divider made ​​with the 10k resistor but that was mechanically difficult.
In the picture of the robot bottom you can see that the extra resistor does not cause problems during assembly.

Note: TCRT5000 is also used as a simple wheel rotation sensor for determining the distance driven. But more important for me was that the robot really can drive straight, this is because the interrupt handling of the wheel sensors are integrated in the motor driver.

@ Hinnie

Thank you for your answer, i just saw it now.
In the mean time i did a voltage divider with adding a 5,6k Ohm resistor between the output of the transistor and the cable connector and changed the 10k Ohm resistor with a 3,3k Ohm. It works great!

Thanks for your note it can be very helpfull.

But can somebody say me why GHI have this design problem? Maybe we have to report them this issue…

My thread: http://www.tinyclr.com/forum/topic?id=8913&page=2#msg89941

Sorry i inverted the 3,3k Ohm and the 5,6k Ohm…