Using Adafruit Macropad as LinuxCNC Control Pendant
Update, 2021-09-25: For compatibility with CircuitPython 7.0.0
CircuitPython recently gained the power to have custom USB descriptors.
With these, we can define a USB HID device that will work with LinuxCNC's
hal_input.
For instance, the Adafruit Macropad has a (very coarse, just 20 detents/revolution) encoder, 12 keyswitch positions, and an OLED screen.
The two pieces of software below, when placed in the CIRCUITPY drive as boot.py and code.py configure it for use with hal_input, using a halcmd line similar to loadusr -W hal_input Macropad. I haven't actually done the work of hooking it all the way up to Touchy yet, but it causes all the buttons & the encoder to appear in halcmd show pin.
This is just the device I picked first; there's nothing to prevent you from hooking up more exotic things like voltage/temperature monitors through added CircuitPython code. Addition of output reports for status indicators is left for the motivated reader.
Pi Zero W USB Proxy
I'm not sure exactly what to call it, but here's a little something I set up this weekend.
On my Linux desktop, I have occasional problems where being stopped at the debugger prompt for a plugged-in USB device hoses the whole computer. The problem waxes and wanes but on a particularly frustrating day I decided that maybe a Pi was the answer to the problem.
Using screen I can access the USB-serial devices on the pi, and using sshfs I can access the files. If the whole pi freezes, I can just reboot it with essentially no harm done.
I selected a Pi Zero W with a Zero4U hub and Adafruit MiniPiTFT 1.14" attached. To a base raspbian lite system I added some software, including tio, udiskie, screen, and Adafruit Blinka; enabled ssh access and disk mounting by the pi user, and set up GNU screen and my custom script for the LCD which is (confusingly) also called screen.
The screen shows information about each of the 4 USB connectors. In brackets "S" is shown if there is a serial device; "D" is shown if there's a partitioned disk, "d" if there's an unpartitioned disk; and "M" is shown if it is mounted. After that, the device name is shown.
Automount can be toggled with the B button (silk screen 23) and any non-mounted disks can be mounted with the A button (silk screen 24)
So far I've only used it lightly, but if it prevents a single crash of my desktop, it will be worth it.
This isn't a detailed guide so a lot of the setup is omitted. However, here are the scripts that are the essential parts:
Quad CharliePlex FeatherWing hack
Adafruit makes these neat "CharlieWing" displays that allow you to control a 15x7 LED matrix using the I2C bus. I2C uses two signal wires (called SDA and SCL, for Serial DAta and Serial CLock), and can connect multiple devices as long as they have different addresses.
I noticed that the bigger brother of this device, with a whopping 144 LEDs, could be configured for 4 different I2C addresses, while this one could only be configured for 2.
Or could it?
Calibrating the DS3231 and PCF8523 RTCs
The DS3231 and PCF8523 real time clocks (RTCs) can both be calibrated by writing various register values. To follow the calibration procedures you'll need a frequency counter you trust, with at least 6 digits to calibrate the PCF8523 and 7 digits to calibrate the DS3231. (It also has to operate at the comparatively low frequency of 32.768kHz; a common inexpensive 8-digit frequency counter such as the "SANJIAN STUDIO" has a minimum of 100kHz so it's not usable for this purpose) I use an old HP 5315B universal counter that has been calibrated against GPS time.
Helpful Scripts for CircuitPython & Real Time Clocks (RTCs)
I have used two different RTCs in the Feather form factor. One has the PCF8523, and the other has the DS3231. The former has an SD card slot while the latter has higher precision including a temperature-compensated crystal oscillator.
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