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Forum: µC & Digital Electronics connected DIN MIDI Out cable disturbs encoder


von Lutz (Guest)


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Hey,

i am currently working on a MIDI related project utilizing a STM32F103 
and a Hex Buffer (74HC365). I have the following problem: reading the 
encoder works fine (handled via interrupts) until i connect a midi cable 
to the midi out port, then the readings of the encoder get sloppy. I 
think it might be some interference problem, since everything works 
fine, when i attach a serial adapter instead of a midi cable to the 
output.

I attached a simplified schematic of my curcuit.

Do you have any idea what could cause the problem and how i can protect 
my circuit?

von stefanus (Guest)


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The internal Pull-Up resistors habe about 50kΩ, so they are very weak. 
The lines can easily receive radio signals.

You need stronger Pull-Up resistors, for example 2,2kΩ. Also your 
capacitors are on the wrong side. The encoder contacts will shorten the 
capacitors, which leads to short pulses of very high current.

Better:
1
        2.2kΩ
2
3
3,3V o--[===]--+    100Ω
4
               |   
5
µC o-----------+---[===]---o Encoder contact
6
               |
7
              === 
8
               |
9
GND o----------+-----------o

von Lutz (Guest)


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i just figured it i make the ground connection closer to the power 
supply the problems disappears. any idea what could be happening there?

von stefanus (Guest)


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Search for en explanation of "ground loop" and why GND connections shall 
form a star to work properly. The web is full of related documentation.

von Lutz (Guest)


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Hey Stefanus!

Thanks a lot. your circuit seems to work much better - meaning without 
any problems! i placed some pullup resistors, and you are right: the 
capacitors should be placed behing the 100 Ohm resistors.

von Lothar M. (lkmiller) (Moderator)


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Lutz wrote:
> any idea what could be happening there?
The usual EMC effects.

But because no one knows anything about the real hardware it results in 
guessing: lousy GND layout, missing or useless decoupling caps,...

von Lutz (Guest)


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im still working on a breadboard to test out all the components. so 
there is not really any propper GND layout yet. but it will follow your 
advices when i create the pcb layout.

von stefanus (Guest)


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The main related pyhsical effect is:

Every line has an inductance and a capacitance. So every line is more or 
less an antenna (more for higher frequencies) which sends and receives 
radio signals.

Also the purpose of GND is a unique common potential that all signals 
are derived from. If you have a few centimeters of GND wire, it will 
have already more than 0 Ohm. As higher the frequency, as higher the 
resistance. And resistance together with currency produces a voltage 
drop.

So GND is not the same GND everywhere, and that arises lots of different 
problems.

von stefanus (Guest)


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> im still working on a breadboard

Breadboard make it even worse because the usually have contacts and 
cables made from iron which has a significantly higher resistance than 
copper.

Consider to solder the power supply wires and plug only the signal wires 
which are less critical.

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