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Old 04-30-2013, 05:27 PM
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John John is offline
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Default MIDI CPU Logic Input Voltage Level Reduction

P. writes:

Quote:
I've got a midi cpu being fed logic level signals generated by my analog modular sequencer. As they enter the midicpu, the signals are right at 5.9v.

Documentation states: max level for input signals may not exceed vREG, which is 5.05v in my case.

In your opinion:

Am I "reasonably close enough" on this one, being just .85 volt over vREG?

Or- does this constitute a serious danger to the microcontroller, and I absolutely have to drop those signals?
The manufacturer of the MIDI CPU microcontroller suggests that Vdd (thus, Vreg or 5.0V) is the safe limit. Therefore, I must recommend some sort of voltage limiting mechanism.

If you are confident that the logic signal maximum is no more that 5.9V, you can use simple resistor-based voltage division. For example:



To make the circuit more robust, the 4.7k resistor could be replaced with a 4.7V zener diode. This would allow for logic voltages higher than 5.9V. (Be sure to calculate currents and compare to the component specs!)

Last edited by John; 04-30-2013 at 05:30 PM.
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Old 05-06-2015, 02:26 PM
ernie ernie is offline
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Hello,

Hijacking an old thread, hope that's ok!

i'm quite surprised i couldn't find much information on the subject, but if there is.. please feel free to link a corresponding thread.

anyways, i like to build a gate to midi /and analog cv to midi (to control pitch bend cc's for instance) device.. this board seems most suitable.

1. I like to know how perky the MIDI CPU is on receiving the correct voltages, i would prefer using a voltage divider instead of having to deal with a whole separate circuit with analog switch ic's etc.


2. The thing that concerns me is that modular (eurorack) gate signal voltages all vary from manufacturers, some output something close to 0v - 12v and some put out 0v - 5v.. is there a way of using a zener diode in a voltage devider that would always output a perfectly save!!! gate signal for the MIDI CPU to sample?


Next on the list would be safely ''midifying'' envelopes and maybe even bipolar LFO signals but if the gate input could really be this simple.. i'm satisfied.


3. I also wondered about it's timing/tightness (possible offset in ms.. am i talking about sample frequency? hope you get what i mean) and the time it takes to make a midi note from a gate signal or button push.

Much thanks for your time!

All the best,

B
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Old 05-07-2015, 03:09 PM
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Hi Ernie, welcome to the forum.

Quote:
Originally Posted by ernie View Post
Hello,

Hijacking an old thread, hope that's ok!

i'm quite surprised i couldn't find much information on the subject, but if there is.. please feel free to link a corresponding thread.

anyways, i like to build a gate to midi /and analog cv to midi (to control pitch bend cc's for instance) device.. this board seems most suitable.

1. I like to know how perky the MIDI CPU is on receiving the correct voltages, i would prefer using a voltage divider instead of having to deal with a whole separate circuit with analog switch ic's etc.
You can get some results using only voltage dividers and zener diodes. The challenge is in the signal impedances. MIDI CPU CT #s 16-23 have fixed 10k pull-up resistors, which will have an impact on the voltage "seen" by the MIDI CPU if you use only a resistor network to scale the input voltage.

CTs 8-14 have pull-ups than can be configured to be disconnected. (See the firmware user manual.) That will help.

Ultimately, if you want the most robust outcome, a voltage scaling circuit with an op-amp is really the best way to go.

Quote:
2. The thing that concerns me is that modular (eurorack) gate signal voltages all vary from manufacturers, some output something close to 0v - 12v and some put out 0v - 5v.. is there a way of using a zener diode in a voltage devider that would always output a perfectly save!!! gate signal for the MIDI CPU to sample?
For trigger/gate signals, you can use an NPN transistor circuit instead of a voltage divider. It's really no more complicated. Just one transistor and one or two resistors per input. Check out the trigger input circuits for some of the Mutable Instruments Eurorack designs for examples.

Quote:
Next on the list would be safely ''midifying'' envelopes and maybe even bipolar LFO signals but if the gate input could really be this simple.. i'm satisfied.


3. I also wondered about it's timing/tightness (possible offset in ms.. am i talking about sample frequency? hope you get what i mean) and the time it takes to make a midi note from a gate signal or button push.
I don't publish sample rate or latency specs for the MIDI CPU because those numbers will vary quite a bit by MIDI CPU configuration. The best way to find out is to build a circuit and measure. In general, I think the MIDI CPU is fast enough to be useful for conversion of a typical LFO signal.
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Old 05-07-2015, 06:10 PM
ernie ernie is offline
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Quote:
Originally Posted by John View Post
Hi Ernie, welcome to the forum.

Thank you very much, appreciate the good support!!

There are of course some modules that do gate to midi, but for a better work flow and easy connection (no note dispatching in software for instance) i need to be able to program it to my liking.

About the timing specs secrets.. thats OK!!, but i would like to be convinced that it is just as fast/tight as some trigger to midi drum converters on the market, being that this board is not specifically made for converting gates.
I have had synth stuff with the poorest midi timing specs.. since then i like to hear what people think of the midi timing precision before i go for it .
Could you confirm a delay shorter than 1 ms ?, and with depending on MIDI CPU configuration do you mean how much code i use? not sure i get that.


Using NPN's is OK .. only drawback is that i need to build a regulator circuit, but i guess thats OK as well. probably need it for converting control signals anyways!

I will have another look at the manual, But again (sorry), i would love to know how precise it is.. I can't work with something that sometimes doesn't generate note off's (thus not recognising the falling edge of a gate) or sometimes puts out lacking/hanging timing bugginess.. what should have been a straight/tight beat.

Thanks

-B

Last edited by ernie; 05-08-2015 at 08:37 AM.
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Old 05-14-2015, 01:06 AM
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Quote:
Originally Posted by ernie View Post
About the timing specs secrets.. thats OK!!, but i would like to be convinced that it is just as fast/tight as some trigger to midi drum converters on the market, being that this board is not specifically made for converting gates.
I have had synth stuff with the poorest midi timing specs.. since then i like to hear what people think of the midi timing precision before i go for it .
Could you confirm a delay shorter than 1 ms ?, and with depending on MIDI CPU configuration do you mean how much code i use? not sure i get that.
Yes, for the default "factory" configuration, the latency should be well under 1 ms. Please keep in mind that the baud rate of the MIDI signal itself will add a small amount of latency. Around 0.5 ms or so.

Different configurations affect the latency because they can give the MIDI CPU "extra work" to do on every input polling cycle. The MIDI CPU will poll the inputs as fast as it can. It does not have a polling cycle with a fixed period.

Quote:
Using NPN's is OK .. only drawback is that i need to build a regulator circuit, but i guess thats OK as well. probably need it for converting control signals anyways!

I will have another look at the manual, But again (sorry), i would love to know how precise it is.. I can't work with something that sometimes doesn't generate note off's (thus not recognising the falling edge of a gate) or sometimes puts out lacking/hanging timing bugginess.. what should have been a straight/tight beat.
I understand. The MIDI CPU has a 30-day money-back guarantee. Please feel free to take advantage if you decide that it's not a good fit for your application.
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