Author Topic: 8 MIDI outs and hardware  (Read 147 times)

garcho

8 MIDI outs and hardware
« on: August 02, 2020, 03:24:17 PM »
I have a Teensy 4.1 MIDI master clock ready to be built but for the final few pieces of the puzzle.

If I have 8 UART outputs going to 8 MIDI jacks, should I buffer each with a pair of inverters? Or will a 47Ω resistor in series with each TX pin be sufficient protection for the Teensy? This is a one-off for my own use, so "good enough" is actually good enough. If the answer is "yes, you should buffer them", then I will follow up with a few more questions in a separate reply.

Along with the UARTs, there is the 3V3 47Ω pull-up resistor for each MIDI out. If I have 8 MIDI outputs, that's 7mA each, for a total of 56mA, right? That leaves roughly 200mA left for the convertor to comfortably provide, yes? Is there anyway to guesstimate how much current my sketch will use? Am I safe assuming a MIDI clock sketch with a few LED/button functions is less than 200mA? Is there a better way to proved the power for the 5mA current loop than from the Teensy itself? I plan on using a 5V wall wart.

There will be 8 LED-lit buttons triggered by Teensy GPIO pins but I have each going through a 4k7 resistor to the base of a 2N3904, so hopefully that's 0.7mA per pin and not something to be concerned with?

There is also a 3V 16x2 RGB-on-black LCD display, with an I2C "backpack" (AVR ATMega328p). I'm not sure how to go about measuring total current draw but I could blindly guess and hope it's something like 30-50mA total, including backlight. I'm basically only using the blue LEDs, so it won't be like powering a flashlight or something. It's this particular model: https://www.sparkfun.com/products/14073. Like I said, I plan on powering it (and the Teensy) from an external 5V wall wart so I don't think it matters anyway. As far as the Teensy is concerned, it's only 2 more GPIO pins, right?

Any thoughts, suggestions, answers, haikus?
Thanks for your time!

I posted this on the PJRC forum as well, I hope that's not bad form, or bad forum, if you will. Cheers y'all
"...and weird on top!"

Blackaddr

Re: 8 MIDI outs and hardware
« Reply #1 on: August 02, 2020, 06:29:28 PM »
I know this a one off but I would use a non-inverting buffer with a proper midi TX circuit for best compatibility with other MIDI devices.

There is a schematic for MIDI OUT in the TGA Pro manual here: (schematic pages are at the end, look for the MIDI IN / OUT stuff)
https://d3s5r33r268y59.cloudfront.net/datasheets/10448/2020-06-01-00-36-46/TGA_Pro_Users_Manual_-_v1.6.pdf

The TGA Pro is also Teensy based:
https://www.tindie.com/products/Blackaddr/arduino-teensy-guitar-audio-shield/
Blackaddr Audio
Digital Modelling Enthusiast
www.blackaddr.com

garcho

Re: 8 MIDI outs and hardware
« Reply #2 on: August 02, 2020, 06:36:53 PM »
I'm checking your website out now, great stuff! I got some similar advice about using an "octal buffer" 74HCT244, which appears to be as you're saying, a non-inverting buffer. Thanks for your time!
"...and weird on top!"