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Topic: DUE didn't die after months of 5V (Read 3882 times)previous topic - next topic

dgelman

Jan 25, 2013, 06:49 pm
Hello,

For a couple of months now, I have been using the DUE to read rotary encoders powered at 5V.  The inputs from these encoders are 5V into my DIO. I have just realized this NOW!... It looks like my board is just fine, but it is interesting.

What interests me is that it is still working just fine after reasoning upon thousands of pulses at 5V > is this a fluke? or can the board tolerate high frequency pulses at 5V?

Daniel

Dialup is the future.

Grumpy_Mike

#1
Jan 26, 2013, 10:44 pm
Quote
is this a fluke?

Well let's start by assuming the it did receive 5V signals and the encoder did not have an open collector output.
Most likely your encoder connects to zero and goes to 5V through a pull up resistor. This means that the 5V has a high impedance, that of the pull up resistor, maybe say 10K.
Assume that the input clamping diodes start to conduct at 0.6V above 3V3. So the voltage drop across that resistor will be 5 - 3.9 = 1.1V, for a 10K resistor that will give a current of 0.11mA.

It is likely that the processor could stand this, especially it is only two pins. But no doubt it was stressed.

dgelman

#2
Feb 28, 2013, 04:42 am
Hello Grumpy_Mike,

I know it is a bit late to reply,

I spoke with a colleage of mine, and his experience with CMOS level micros and TTL level encoders came to a conclusion that as the pulses go by they do not draw enough current to cause any damage.  I appreciate your explanation and simple logic but I want to share with you that on the Arduino DUE site,

Quote

Input and Output
Digital I/O: pins from 0 to 53
Each of the 54 digital pins on the Due can be used as an input or output, using pinMode(), digitalWrite(), and digitalRead() functions. They operate at 3.3 volts. Each pin can provide (source) a current of 3 mA or 15 mA, depending on the pin, or receive (sink) a current of 6 mA or 9 mA, depending on the pin. They also have an internal pull-up resistor (disconnected by default) of 100 KOhm. In addition, some pins have specialized functions:

Usually pull-up resistors are 10K, so if that is the case, the 0.11mA current is insignificant to the 6-9mA sink.  What are your thoughts?  As it stands, my board still receives the 5V pulses and the control seems to be just fine.  My concern is what happens in the long run.

Daniel
Dialup is the future.

Grumpy_Mike

#3
Feb 28, 2013, 09:41 am
Current is only one aspect of damage, over voltage is another. Too much voltage can punch through insulation on the chip because it is designed for only 3V3. With chips operating slightly above there limit it is not a go / no go situation but an increase in stress. Without stressing a large sample for a long time it is impossible to say what the life shortning will be. Reliability is a tricky subject but basically to maximise it you run the system at below maximum ratings not above it.

dgelman

#4
Mar 01, 2013, 05:35 pm
Grumpy,

Okay, I definitely agree with you.  I guess I should redesign my shield to implement a voltage level translator for Channels A and Channels B for the encoders, perhaps in future designs - but it seems like the controller is doing just fine, after months of these pulses.

Ill let you know if something happens.

Daniel
Dialup is the future.

retrolefty

#5
Mar 01, 2013, 05:43 pm

Hello Grumpy_Mike,

I know it is a bit late to reply,

I spoke with a colleage of mine, and his experience with CMOS level micros and TTL level encoders came to a conclusion that as the pulses go by they do not draw enough current to cause any damage.  I appreciate your explanation and simple logic but I want to share with you that on the Arduino DUE site,

Quote

Input and Output
Digital I/O: pins from 0 to 53
Each of the 54 digital pins on the Due can be used as an input or output, using pinMode(), digitalWrite(), and digitalRead() functions. They operate at 3.3 volts. Each pin can provide (source) a current of 3 mA or 15 mA, depending on the pin, or receive (sink) a current of 6 mA or 9 mA, depending on the pin. They also have an internal pull-up resistor (disconnected by default) of 100 KOhm. In addition, some pins have specialized functions:

Usually pull-up resistors are 10K, so if that is the case, the 0.11mA current is insignificant to the 6-9mA sink.  What are your thoughts?  As it stands, my board still receives the 5V pulses and the control seems to be just fine.  My concern is what happens in the long run.

Daniel

You are mixing up the pin output current limits when a Due pin is in output mode and can source or sink a specific amount of current. The maximum voltage input specifications for when a pin is in input mode is based on if the voltage is higher or lower then input voltage ratings, input clamping protection diodes start conducting and these diodes tend to have very low maximum current ratings on the order of 1 ma or so. So if your external applied voltage to an input pin is current limited to a value is equal or less then a conduction clamping diode can handle then you are ok with applying that out of spec input voltage, but if not damage to the clamping diode can result.

Lefty

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