MC2 March Progress
Date: 1/4/2016
Tags: mc2
After when seems like forever I bring you an update on the MC2 project. The good news is that most of the hardware is now working, including MIDI Input/Output, the Neopixel chain, the I2C bus to the GPIO expander chips, which in turn connects to the buttons and encoders. In short I can now use it to change presets on the Axefx which is like the bare minimum functionality.

Some of the issues I've had to sort out:
  • Very slow compile times on the Raspberry Pi
    I'm only using a Pi B (version 1, rev 2) so it's got a single core running at 700MHz which frankly is molasses slow. So I decided that it was worth getting a cross compiler working on something with more grunt. It took days of mucking around, and 3 different builds, copying off almost the entire Pi's worth of headers and libraries, but it now works. More importantly it's fast. My main dev box (3.8ghz i5 /w 16GiB of ram) chews through the code very quickly, and then copies the binaries down to the Pi over the network.
  • PiTFT touch screen not working correctly
    The behaviour I was seeing was the cursor stuck in the top left, and the x & y axis' were swapped. I ended up building SDL from source and then inserting a pile of logging during the mouse driver setup only to discover that it was using the default mouse driver instead of the touch screen driver that I had configured in the environment. It turns out that as I'm running the code as root (sudo) the environment is different than my normal user account. Ok, so now I just set the right env vars in my C code before initializing SDL. Which forces it to do the right thing on any account.
  • I2C bus not working / unreliable
    This came down to adding 1000uF electrolytic over the 3.3v supply line, and also adding 0.15uF decoupling caps over the +V/GND pins of the GPIO expander chips. Also I had to change the pull up resistors connected to the SDA+SCL data lines from 4.7k to 10k.
  • Neopixel chain stopped working
    Eventually I found a cold solder joint on the ground pin output. Only after disassembling it completely, including the button harness and taking the plastic backing plane out.
  • MIDI input not working / garbled
    This has actually taken most of the week to sort out. The hardware for the input side seemed ok, and I even did see some incoming MIDI at one point. Initially it was garbled and I tweaked the baud rate with system settings. It seemed to work for a while then stopped again. I got very frustrated with not being able to see the logic signals in the circuit so I went and bought a Bus Pirate which can read both the UART and do logic analysis. I tested all the hardware I could in isolation. I went over the traces like 6 times. I tried the optical isolater chip in my MC1 and everything appeared to be good. I even pulled the diode out and tested it in isolation. I tested the Rx+Tx pins of the Pi by hooking it up to the Bus Pirate and had the Pi talking to the PC. Then I tried minicom and saw the incoming Tempo messages from the Axefx! Ah, so the hardware IS good, and thus I started looking at the software side again. It turns out that in more recently Linux kernels you can see custom baud rates in code. Most of the people getting MIDI working on the Pi are doing system config settings. But it's a hack. Using an API to set the speed to the exact right value is way better. Now the existing code snippets kinda get you there, but for me I had to merge the old code and new code to get something that works for me. This is the final result:
            int MidiHnd = open("/dev/ttyAMA0", O_RDWR | O_NOCTTY | O_NDELAY);
            if (MidiHnd > 0)
                struct termios2 tio;
                ioctl(MidiHnd, TCGETS2, &tio);
                tio.c_cflag = BOTHER | CS8 | CLOCAL | CREAD;
                tio.c_iflag = IGNPAR;
                tio.c_oflag = 0;
                tio.c_lflag = 0;
                tio.c_ispeed = 31250;
                tio.c_ospeed = 31250;
                ioctl(MidiHnd, TCSETS2, &tio);
    The MIDI output was an easy fix: just turn the 4011 logic chip the right way around. And boom: working! Hahaha... in my defense it has a little stamped circle at the other end of the chip. Which is sometimes used to mark the "1" pin. Grrrr.
Oh and here is some video showing it working:
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