RyanScottLewis / thermoclock

An implementation of a clock display for Linux from hardware, to kernel SPI/character device drivers, to user space application.

Thermoclock

An implementation of a clock display for Linux from hardware, to kernel SPI/character device drivers, to user space application.

Hardware

The display is pulled from an old clock, It has 4 seven segment digits, 2 dot delimiter, each day of the week, AM indicator, and a few errata as outputs.

Using a multimeter in diode/LED mode, the IO was determined to use the first 2 pins as separate common-anode pins for two "banks" of segments, which are assigned in a strange manner (detailed in Pinout.md.)

I also discovered that the very first digit has no "F" segment. I think this is to save money on a single LED, since that digit on the clock probably only ever showed a 1 or 2, so the F segment was not needed.

To interface with this, I've used 3 74HC595 serial-in parallel-out (SIPO) shift registers connected in series. This is then connected to the MISO and MOSI lines of the SPI bus. The CLK and CE lines on all the shift registers are tied together and also connected to the SPI bus.

This way, to power a bank the shift register needs to contain a 1 or high signal at the bit offset where the common-anode pin for that bank is connected to. To power a segment on that bank, the shift register must contain a 0 or low signal at the bit offset the segment's cathode pin is connected to.

Driver

The Linux driver is a simple SPI driver, which then sets up a character device file.

At the moment, the segment's name/identifier can be written to the character device file and the corrent data (command) will be shifted into the registers to power that segment.

The syntax is:

0G   0th digit, segment G
3A   2rd digit, segment A
WED  segment for WED

The full list can be found in src/thermoclock.h.

Overlay

The driver can be mounted on another SPI bus or use another CE line by using parameters when adding the device to the device tree:

/boot/config.txt (on RaspberryPi)

# Use CE 1
dtoverlay=thermoclock:reg=1

# Use different SPI frequency (74HC595 max speed)
dtoverlay=thermoclock:spi-max-frequency=100000000

Application

Usage: thermoclock

The application parses the current time and generates a list of commands that will display the correct current time. It then cycles through this list of commands and sends them repeatedly, using persistance of vision to give the illusion that all of the segments are active at the same time.

The display can also be cleared, if needed with thermoclock --clear or thermoclock -c

SystemD Service

The application can be started with sudo systemctl start thermoclock and started at boot with sudo systemctl enable thermoclock.

TODO

• Build dependencies
  • rive-devel-tools is required!
• Pacman package
• Display temp
  • Add thermister module and support
  • NOAA/Wunderground APIs
• Eliminate flickers
  • Use binary format?
    • Send raw commands directly instead of translating within the char driver?
  • Send the list of commands to loop and have the driver loop them. This way, the driver is only parsing the list once, then simply storing the raw commands to loop over
  • See doc/External Timer Idea.md

Development

Linux

sudo pacman -S linux-headers

RaspberryPi

sudo pacman -S linux-raspberrypi-headers

Cross Compiling

TODO