# Fomu: a beginner's guide

FPGAs are pretty cool pieces of hardware for tinkering with, and have become remarkably easy to approach as a hobbyist in recent years. Boards like the TinyFPGA BX don’t require any special hardware to use and can provide a simple platform for modestly-scoped projects or just for learning.

While historically the software tools for programming FPGAs are proprietary and provided by the hardware manufacturer, Symbiflow (enabled and probably inspired by earlier work like Project IceStorm) provides completely free and open-source tooling and documentation for programming some FPGAs, significantly lowering the cost of entry (most vendors provide some free version of their design software but limited to lower-end devices; a license for the non-free version of the software is well into the realm of “if you have to ask, you can’t afford it”) and appearing to yield better results in many cases.1

As somebody who finds it fun to learn new things and experiment with new kinds of creations, FPGAs are quite interesting to me- they’re quite complex devices that enable very powerful creations, with excellent depth for mastery. While I did some course lab work with Altera FPGAs in university (and a little bit of chip design/layout later), I’d call those mostly canned tasks with easily-understood requirements and problem-solving approaches; it was sufficient to familiarize myself with the systems, but not enough to be particularly useful.

The announcement of Fomu caught my interest because I was aware of the earlier Tomu but wasn’t sufficiently interested to try to acquire any hardware. With Fomu however, I’m rather more interested because it enables interesting capabilities for playing with hardware- others have already demonstrated small RISC-V CPUs running in that FPGA (despite its modest logic capacity), for instance.

Even more conveniently for being able to play with Fomu, I’ve been in contact with Mithro who is approximately half of the team behind Fomu and gotten access to a stockpile of “hacker edition” boards that have been hand-assembled but not programmed at all. With slightly early access to hardware, I’ve been able to do some exploration and re-familiarize myself with the world of digital logic design and figure out the hardware.

## Hardware

In summary, Fomu is a small (9.4 by 13 by 0.6 millimeters) circuit board with a Lattice ICE40UP5K-UWG30 FPGA, a 16-megabit SPI Flash for configuration (and other data) storage, a single RGB LED for blinkiness and a 48 MHz MEMS oscillator to provide a clock.

The whole thing is built so it can fit inside a standard USB port. Production boards are meant to ship with a USB bootloader that allows new configurations to be uploaded to the board only via that USB connection, but hacker boards are provided completely unprogrammed (and untested).

### Schematic

Before we can make the hardware do something, we’ll need to understand how everything is put together:

Unfortunately, this schematic leaves some things to be desired. While it does allow us to see what parts are actually on the board and generally how they’re connected, it fails to clearly mark the external connections- power and data lines on the USB connector, test points and utility I/O pads.

The USB connections are easy to figure out, however; it’s a standard pinout so we can easily identify which physical pads on the board correspond to VUSB (5V supply), ground and the two data lines (USBP, USBN). Rather trickier to work out is the function of each of the test points on the board, though there is a provided template for laser-cutting a programming jig which provides some hints:

This jig is meant to be built up by stacking four layers of material, engraving a small pocket in the bottom to hold the board to program and inserting pogo pins in the small holes to contact with the test points on the board. This template helps us in that it has labels for the test points, though! All of the test points are clearly identified, except it’s unclear what voltage is expected on the power supply.

By inspecting the board myself, I eventually determined that the test point for supplying power (marked VCC on the programming jig template) is downstream of the 3.3V regulator (not connected to the USB power supply pad) so it expects 3.3 Volts for programming.

### Convenient pinout diagram

By way of improving the schematic, here’s that same photo of the board with the signal names from the schematic pointed out on each of the pads, and the individual chips pointed out.

And the same in tabular form for easy searching:

Silkscreen Schematic Description
V +3V3 3.3V rail
S CS SPI chip select (active low)
C SCK SPI clock
I MISO SPI MISO
O MOSI SPI MOSI
R CRESET_B FPGA reset (active low)
G GND Ground
1 PIN1 User I/O 1
2 PIN2 User I/O 2
3 PIN3 User I/O 3
4 PIN4 User I/O 4

## Bootstrapping

My first task in attempting to bootstrap a board and load some configuration on it was building a programming jig. Given there was already a template for a laser-cut acrylic one and I have access to a benchtop laser cutter, this was easy:

It’s a little bit ugly because the pogo pins I had ready access too are too small to nicely fit in the laser-cut holes so I had to carefully glue them in place.

Actually programming a board can be done with the fomu-flash utility running on a Raspberry Pi. I conveniently had a Raspberry Pi 2 to hand, so a little wiring to the Pi’s GPIO header had a jig that should work. Unfortunately, it didn’t- all I got out when trying to make it identify the on-board flash chip was 1s:

fomu-flash -i Manufacturer ID: unknown (ff) Memory model: unknown (ff) Memory size: unknown (ff) Device ID: ff Serial number: ff ff ff ff Status 1: ff Status 2: ff Status 3: ff I gave up on that hardware after spending a while experimenting with it, and decided to design a custom programming jig that might be a little easier to ensure pin alignment is good. This is a little bit tricky because the minimum pitch of the test points is just 1.8 mm, which is not large enough for the 0.1-inch (2.54 mm) DuPont connectors commonly used for prototyping and desirable in this case because they’re very easy to connect to the Raspberry Pi’s GPIO header. ### A better jig Fortunately, I also have access to rather sophisticated prototyping tools and had some nice parts handy from other projects. In particular, a Form 2 stereolithographic 3D printer and some good pogo pins, Preci-dip 90155-AS. I computer-modeled a jig to be 3d-printed that should be both compact and robust, pictured below (see the end of this post for downloadable resources): ### Features The design takes great advantage of the flexibility of 3d printing for fabrication: it is easy to install the pogo pins off-vertical by making the (press-fit) holes at an arbitrary angle; this would be very difficult with conventional fabrication, but it allows the spacing of the pins at the top of the jig to be large enough that 2.54mm connectors can be used, despite the pad spacing on the board being only 1.8mm. On the bottom side, there are several narrow features that act as a shelf to support the board (which is 0.6mm thick) so its outside surface is flush with the bottom surface of the jig. A semicircular boss on one side mates with the cutout on the PCB to key the jig so it is obvious when the board is correctly oriented in the jig. A small cutout on one edge allows a tool to be inserted to pull the board out if needed, because the fit is close enough that it might stick sometimes (or a tool could be pushed through from the top). As a manufacturability consideration, the top surface has a slant between opposite corners. This improves the print quality on a Form 2- because dimensions on that side are not critical the part is designed to be printed with that side “down” (actually up, once in the printer) and supports attached to it on that end. By allowing the printer to gradually build up a slope rather than immediately build a plane, it can better produce the intended shape- an earlier version of the design with a flat top had a very rough finish because large and thin layers of material tend to warp until enough material is built up to be self-supporting. The choice of pogo pins in particular is key, since they’re made with a small shoulder and retaining barbs that allow them to be easily press-fit into a connector shell: The one downside of these pins is the short tail, intended for mounting to a circuit board. While the aforementioned DuPont connectors can be mated to the tail, they are not very secure and come off at the slightest force. A revised design choosing parts for their function and not just immediate availability might prefer to use a part like 90101-AS, which is intended for wire termination rather than board mounting- then wires can be securely attached to the pin rather than tenuously placed on it. My workaround that didn’t involve buying more parts was carefully gluing the wires in place, which seems to work okay. ## Programming Having built a jig that I could be confident would work correctly, we now return to the problem of actually programming the board. Connecting the new jig to my Raspberry Pi in the same way I did the first one, it failed in the same way- reading all 1s. At this point I was rather stumped, with a few possible explanations for the problems: 1. Both jigs are unreliable 2. I’m wiring the jigs up incorrectly 3. Software on my Pi is configured incorrectly 4. All of the Fomus I tried were faulty To discount the first two possibilities, I was able to borrow Mithro’s professionally-built jig that already had a Raspberry Pi 3 connected to it. I didn’t have any credentials to log in to that Pi and use it interactively however, so I was limited to checking its wiring and carefully ensuring I connected my Pi to the jig in the same way, then try programming again. This also failed. Having tried that I had to assume my Pi was somehow misconfigured, since it seemed increasingly unlikely that I was doing anything wrong and it seemed implausible that all of my boards were faulty. I eventually took the SD card out of the other jig’s Pi and inspected the software it would run by connecting it to another computer. This amounted to the same fomu-flash program I was using, so I inspected the system configuration in /boot/config.txt and found a variety of non-default options that seemed plausibly useful. Ultimately, I found some magic words: dtparam=spi=on This option makes the kernel on the Pi expose a hardware-assisted SPI peripheral, which seems like an obvious missing option until you realize that fomu-flash actually bit-bangs SPI because the hardware support is insufficient for this application. In any case, I did find that turning that option on makes everything work correctly:  fomu-flash -i
Memory model: AT25SF161 (86)
Memory size: 16 Mbit (01)
Device ID: 14
Serial number: ff ff ff ff
Status 1: 02
Status 2: 00
Status 3: ff

I reported the bug and made a note of this in the documentation so hopefully nobody else has to deal with that problem in the future, even if the root cause is mystifying.

## Success!

With the ability to talk to the configuration flash, it’s then possible to write an actual bitstream. To avoid needing to write one myself, it’s easy to take the LED blinker example from the fomu-tests repository:

fomu-tests/blink$make FOMU_REV=hacker ... Info: constrained 'rgb0' to bel 'X4/Y31/io0' Info: constrained 'rgb1' to bel 'X5/Y31/io0' Info: constrained 'rgb2' to bel 'X6/Y31/io0' Info: constrained 'clki' to bel 'X6/Y0/io1' Warning: unmatched constraint 'spi_mosi' (on line 5) Warning: unmatched constraint 'spi_miso' (on line 6) Warning: unmatched constraint 'spi_clk' (on line 7) Warning: unmatched constraint 'spi_cs' (on line 8) Info: constrained 'user_1' to bel 'X12/Y0/io1' Info: constrained 'user_2' to bel 'X5/Y0/io0' Info: constrained 'user_3' to bel 'X9/Y0/io1' Info: constrained 'user_4' to bel 'X19/Y0/io1' Warning: unmatched constraint 'usb_dn' (on line 13) Warning: unmatched constraint 'usb_dp' (on line 14) ... Info: Device utilisation: Info: ICESTORM_LC: 33/ 5280 0% Info: ICESTORM_RAM: 0/ 30 0% Info: SB_IO: 5/ 96 5% Info: SB_GB: 1/ 8 12% Info: ICESTORM_PLL: 0/ 1 0% Info: SB_WARMBOOT: 0/ 1 0% Info: ICESTORM_DSP: 0/ 8 0% Info: ICESTORM_HFOSC: 0/ 1 0% Info: ICESTORM_LFOSC: 0/ 1 0% Info: SB_I2C: 0/ 2 0% Info: SB_SPI: 0/ 2 0% Info: IO_I3C: 0/ 2 0% Info: SB_LEDDA_IP: 0/ 1 0% Info: SB_RGBA_DRV: 1/ 1 100% Info: ICESTORM_SPRAM: 0/ 4 0% ... Built 'blink' for Fomu hacker$ fomu-flash -w blink.bin
Erasing @ 018000 / 01969a  Done
Programming @ 01959a / 01969a  Done
$fomu-flash -v blink.bin Reading @ 01969a / 01969a Done Programming that to the board yields a blinking LED as expected, so I’ve achieved success in the basic form of this project by getting the FPGA to do something. Further exploration will involve writing gateware with Migen rather than straight Verilog (because I find Verilog to be rather tedious to write) and trying to build a system around a RISC-V CPU (because that sounds interesting). ## Resources If you want to make your own copy of the programming jig or just explore it, you’ve got several options: • View the model at OnShape. This will allow you to view and make changes to the parametric model, which is what you’ll need to make most useful changes to it. • View the STL online. A quick and dirty way to get an interactive view of the model. • Download the STL. If you just want to try to 3D print your own, this is all you need. It may also be useful if you want to make changes using a 3d modeling program (rather than a CAD program). • Download a Solidworks part file. This was just exported from OnShape, but you might prefer this if you want to use SolidWorks to edit the model. All of the official documentation for Fomu is available on Github. For basic information (such as what I referred to when writing up this project), that’s a great starting point. I designed the programming jig in OnShape which is a pretty good and very convenient CAD tool. 1. FPGA vendors don’t publish all the information required to build configuration bitstreams for their hardware, possibly because they wish to support their side business in selling design tool licenses- this despite the fact that (anecdotally, since I can’t recall where I saw it) many FPGA developers say that vendor tooling is one of the biggest annoyances in their work. The open-source tools require a fair amount of painstaking reverse-engineering of chips to create! [return] # Building a terrible 'IoT' temperature logger I had approximately the following exchange with a co-worker a few days ago: Them: “Hey, do you have a spare Raspberry Pi lying around?” Me: [thinks] “..yes, actually.” T: “Do you want to build a temperature logger with Prometheus and a DS18B20+? M: “Uh, okay?” It later turned out that that co-worker had been enlisted by yet another individual to provide a temperature logger for their project of brewing cider, to monitor the temperature during fermentation. Since I had all the hardware at hand (to wit, a Raspberry Pi 2 that I wasn’t using for anything and temperature sensors provided by the above co-worker), I threw something together. It also turned out that the deadline was quite short (brewing began just two days after this initial exchange), but I made it work in time. ## Interfacing the thermometer As noted above, the core of this temperature logger is a DS18B20 temperature sensor. Per the manufacturer: The DS18B20 digital thermometer provides 9-bit to 12-bit Celsius temperature measurements … communicates over a 1-Wire bus that by definition requires only one data line (and ground) for communication with a central microprocessor. … Each DS18B20 has a unique 64-bit serial code, which allows multiple DS18B20s to function on the same 1-Wire bus. Thus, it is simple to use one microprocessor to control many DS18B20s distributed over a large area. Indeed, this is a very easy device to interface with. But even given the svelte hardware needs (power, data and ground signals), writing some code that speaks 1-Wire is not necessarily something I’m interested in. Fortunately, these sensors are very commonly used with the Raspberry Pi, as illustrated by an Adafruit tutorial published in 2013. The Linux kernel provided for the Pi in its default Raspbian (Debian-derived) distribution supports bit-banging 1-Wire over its GPIOs by default, requiring only a device tree overlay to activate it. This is as simple as adding a line to /boot/config.txt to make the machine’s boot loader instruct the kernel to apply a change to the hardware configuration at boot time: dtoverlay=w1-gpio With that configuration, one simply needs to wire the sensor up. The w1-gpio device tree configuration by default uses GPIO 4 on the Pi as the data line, then power and grounds need to be connected and a pull-up resistor added to the data line (since 1-Wire is an open-drain bus). The w1-therm kernel module already understands how to interface with these sensors- meaning I don’t need to write any code to talk to the temperature sensor: Linux can do it all for me! For instance, reading the temperature out in an interactive shell to test, after booting with the 1-Wire overlay enabled: $ modprobe w1-gpio w1-therm
$cd /sys/bus/w1/devices$ ls
28-000004b926f1  w1_bus_master1

## Temperature exporter

Having connected the thermometer to the Pi and set up Prometheus, we now need to glue them together such that Prometheus can read the temperature. The usual way is for Prometheus to make HTTP requests to its known data sources, where the response is formatted such that Prometheus can make sense of the metrics. There is some support for having metrics sources push their values to Prometheus through a bridge (that basically just remembers the values it’s given until they’re scraped), but that seems inelegant given it would require running another program (the bridge) and goes against the how Prometheus is designed to work.

I’ve published the source for the metrics exporter I ended up writing, and will give it a quick description in the remnants of this section.

The easiest solution to providing a service over HTTP is using the http.server module, so that’s what I chose to use. When the program starts up it scans for temperature sensors and stores them. This has a downside of never returning data if a sensor is accidentally disconnected at startup, but detection is fairly slow and only doing it at startup makes it clearer if sensors are accidentally disconnected during operation, since reading them will fail at that point.

#!/usr/bin/env python3

import socketserver
from http.server import HTTPServer, BaseHTTPRequestHandler
from w1thermsensor import W1ThermSensor

SENSORS = W1ThermSensor.get_available_sensors()

The request handler has a method that builds the whole response at once, which is just plain text based on a simple template.

class Exporter(BaseHTTPRequestHandler):
METRIC_HEADER = ('# HELP w1therm_temperature Temperature in Kelvin of the sensor.\n'
'# TYPE w1therm_temperature gauge\n')

def build_exposition(self, sensor_states):
for sensor, temperature in sensor_states.items():
out += 'w1therm_temperature{{id="{}"}} {}\n'.format(sensor, temperature)
return out

do_GET is called by BaseHTTPRequestHandler for all HTTP GET requests to the server. Since this server doesn’t really care what you want (it only exports one thing- metrics), it completely ignores the request and sends back metrics.

    def do_GET(self):
response = self.build_exposition(self.get_sensor_states())
response = response.encode('utf-8')

# We're careful to send a content-length, so keepalive is allowed.
self.protocol_version = 'HTTP/1.1'
self.close_connection = False

self.send_response(200)
self.wfile.write(response)

The http.server API is somewhat cumbersome in that it doesn’t try to handle setting Content-Length on responses to allow clients to keep connections open between requests, but at least in this case it’s very easy to set the Content-Length on the response and correctly implement HTTP 1.1. The Content-Type used here is the one specified by the Prometheus documentation for exposition formats.

The rest of the program is just glue, for the most part. The console_entry_point function is the entry point for the w1therm_prometheus_exporter script specified in setup.py. The network address and port to listen on are taken from the command line, then an HTTP server is started and allowed to run forever.

### As a server

As a Python program with a few non-standard dependencies, installation of this server is not particularly easy. While I could sudo pip install everything and call it sufficient, that’s liable to break unexpectedly if other parts of the system are automatically updated- in particular the Python interpreter itself (though Debian as a matter of policy doesn’t update Python to a different release except as a major update, so it shouldn’t happen without warning). What I’d really like is the ability to build a single standalone program that contains everything in a convenient single-file package, and that’s exactly what PyInstaller can do.

A little bit of wrestling with pyinstaller configuration later (included as the .spec file in the repository), I had successfully built a pretty heavy (5MB) executable containing everything the server needs to run. I placed a copy in /usr/local/bin, for easy accessibility in running it.

I then wrote a simple systemd unit for the temperature server to make it start automatically, installed as /etc/systemd/system/w1therm-prometheus-exporter.service:

[Unit]
Description=Exports 1-wire temperature sensor readings to Prometheus
Documentation=https://bitbucket.org/tari/w1therm-prometheus

[Service]
ExecStart=/usr/local/bin/w1therm-prometheus-exporter localhost 9000
Restart=always

StandardOutput=journal
StandardError=journal

# Standalone binary doesn't need any access beyond its own binary image and
# a tmpfs to unpack itself in.
DynamicUser=true
ProtectSystem=strict
ProtectHome=true
PrivateTmp=true

[Install]
WantedBy=multi-user.target

Enable the service, and it will start automatically when the system boots:

systemctl enable w1therm-prometheus-exporter.service

This unit includes rather more protection than is probably very useful, given the machine is single-purpose, but it seems like good practice to isolate the server from the rest of the system as much as possible.

• DynamicUser will make it run as a system user with ID semi-randomly assigned each time it starts so it doesn’t look like anything else on the system for purposes of resource (file) ownership.
• ProtectSystem makes it impossible to write to most of the filesystem, protecting against accidental or malicious changes to system files.
• ProtectHome makes it impossible to read any user’s home directory, preventing information leak from other users.
• PrivateTmp give the server its own private /tmp directory, so it can’t interfere with temporary files created by other things, nor can its be interfered with- preventing possible races which could be exploited.

## Pi connectivity

Having built the HTTP server, I needed a way to get data from it to Prometheus. As discussed earlier, the Raspberry Pi with the sensor is on a WiFi network that doesn’t permit any incoming connections, so how can Prometheus scrape metrics if it can’t connect to the Pi?

One option is to push metrics to Prometheus, using the push gateway. However, I don’t like that option because the push gateway is intended mostly for jobs that run unpredictably, in particular where they can exit without warning. This isn’t true of my sensor server. PushProx provides a rather better solution, wherein clients connect to a proxy which forwards fetches from Prometheus to the relevant client, though I think my ultimate solution is just as effective and simpler.

What I ended up doing is using autossh to open an SSH tunnel at the Prometheus server which connects to the Raspberry Pi’s metrics server. Autossh is responsible for keeping the connection alive, managed by systemd. Code is going to be much more instructive here than a long-form description, so here’s the unit file:

[Unit]
Description=SSH reverse tunnel from %I for Prometheus
After=network-online.target
Wants=network-online.target

[Service]
User=autossh
ExecStart=/usr/bin/autossh -N -p 22 -l autossh -R 9000:localhost:9000 -i /home/autossh/id_rsa %i
Restart=always
RestartSec=10

[Install]
WantedBy=multi-user.target

Installed as /etc/systemd/system/autossh-tunnel@.service, this unit file tells systemd that we want to start autossh when the network is online and try to ensure it always stays online. I’ve increased RestartSec from the default 100 milliseconds because I found that even with the dependency on network-online.target, ssh could fail when the system was booting up with DNS lookup failures, then systemd would give up. Increasing the restart time means it takes much longer for systemd to give up, and in the meantime the network actually comes up.

The autossh process itself runs as a system user I created just to run the tunnels (useradd --system -m autossh), and opens a reverse tunnel from port 9000 on the remote host to the same port on the Pi. Authentication is with an SSH key I created on the Pi and added to the Prometheus machine in Google Cloud, so it can log in to the server without any human intervention. Teaching systemd that this should run automatically is a simple enable command away1:

systemctl enable autossh-tunnel@pitemp.example.com

Then it’s just a matter of configuring Prometheus to scrape the sensor exporter. The entire Prometheus config looks like this:

global:
scrape_interval:     15s # By default, scrape targets every 15 seconds.
evaluation_interval: 15s

scrape_configs:
- job_name: 'w1therm'
static_configs:
- targets: ['localhost:9000']

That’s pretty self-explanatory; Prometheus will fetch metrics from port 9000 on the same machine (which is actually an SSH tunnel to the Raspberry Pi), and do so every 15 seconds. When the Pi gets the request for metrics, it reads the temperature sensors and returns their values.

## Data retention

I included InfluxDB in the setup to get arbitrary retention of temperature data- Prometheus is designed primarily for real-time monitoring of computer systems, to alert human operators when things appear to be going wrong. Consequently, in the default configuration Prometheus only retains captured data for a few weeks, and doesn’t provide a convenient way to export data for archival or analysis. While the default retention is probably sufficient for this project’s needs, I wanted better control over how long that data was kept and the ability to save it as long as I liked. So while Prometheus doesn’t offer that control itself, it does support reading and writing data to and from various other databases, including InfluxDB (which I chose only because a package for it is available in Debian without any additional work).

Unfortunately, the version of Prometheus available in Debian right now is fairly old- 1.5.2, where the latest release is 2.2. More problematic, while Prometheus now supports a generic remote read/write API, this was added in version 2.0 and is not yet available in the Debian package. Combined with the lack of documentation (as far as I could find) for the old remote write feature, I was a little bit stuck.

Things ended up working out nicely though- I happened to see flags relating to InluxDB in the Prometheus web UI, which mostly have no default values:

• storage.remote.influxdb-url
• storage.remote.influxdb.database = prometheus
• storage.remote.influxdb.retention-policy
• storage.remote.influxdb.username

These can be specified to Prometheus by editing /etc/defaults/prometheus, which is part of the Debian package for providing the command line arguments to the server without requiring users to directly edit the file that tells the system how to run Prometheus. I ended up with these options there:

ARGS="--storage.local.retention=720h \
--storage.remote.influxdb-url=http://localhost:8086/ \
--storage.remote.influxdb.retention-policy=autogen"

The first option just makes Prometheus keep its data longer than the default, whereas the others tell it how to write data to InfluxDB. I determined where InfluxDB listens for connections by looking at its configuration file /etc/influxdb/influxdb.conf and making a few guesses: a comment in the http section there noted that “these (HTTP endpoints) are the primary mechanism for getting data into and out of InfluxDB” and included the settings bind-address=":8086" and auth-enabled=false, so I guessed (correctly) that telling Prometheus to find InfluxDB at http://localhost:8086/ should be sufficient.

Or, it was almost enough: setting the influxdb-url and restarting Prometheus, it was logging warnings periodically about getting errors back from InfluxDB. Given the influxdb.database settings defaults to prometheus, I (correctly) assumed I needed to create a database. A little browsing of the Influx documentation and a few guesses later, I had done that:

$apt-get install influxdb-client$ influx
Visit https://enterprise.influxdata.com to register for updates, InfluxDB server management, and monitoring.
Connected to http://localhost:8086 version 1.0.2
InfluxDB shell version: 1.0.2
> CREATE DATABASE prometheus;

Examining the Prometheus logs again, now it was failing and complaining that the specified retention policy didn’t exist. Noting that the Influx documentation for the CREATE DATABASE command mentioned that the autogen retention policy will be used if no other is specified, setting the retention-policy flag to autogen and restarting Prometheus made data start appearing, which I verified by waiting a little while and making a query (guessing a little bit about how I would query a particular metric):

> USE prometheus;
> SELECT * FROM w1therm_temperature LIMIT 10;
name: w1therm_temperature
-------------------------
time                    id              instance        job     value
1532423583303000000     000004b926f1    localhost:9000  w1therm 297.9
1532423598303000000     000004b926f1    localhost:9000  w1therm 297.9
1532423613303000000     000004b926f1    localhost:9000  w1therm 297.9
1532423628303000000     000004b926f1    localhost:9000  w1therm 297.9
1532423643303000000     000004b926f1    localhost:9000  w1therm 297.9
1532423658303000000     000004b926f1    localhost:9000  w1therm 297.9
1532423673303000000     000004b926f1    localhost:9000  w1therm 297.9
1532423688303000000     000004b926f1    localhost:9000  w1therm 297.9
1532423703303000000     000004b926f1    localhost:9000  w1therm 297.9
1532423718303000000     000004b926f1    localhost:9000  w1therm 297.9

## Results

A sample graph of the temperature over two days:

The fermentation temperature is quite stable, with daily variation of less than one degree in either direction from the baseline.

## Refinements

I later improved the temperature server to handle SIGHUP as a trigger to scan for sensors again, which is a slight improvement over restarting it, but not very important because the server is already so simple (and fast to restart).

On reflection, using Prometheus and scraping temperatures is a very strange way to go about solving the problem of logging the temperature (though it has the advantage of using only tools I was already familiar with so it was easy to do quickly). Pushing temperature measurements from the Pi via MQTT would be a much more sensible solution, since that’s a protocol designed specifically for small sensors to report their states. Indeed, there is no shortage of published projects that do exactly that more efficiently than my Raspberry Pi, most of them using ESP8266 microcontrollers which are much lower-power and can still connect to Wi-Fi networks.