Using a MCP4921 or MCP4922 as a SPI DAC for Audio on Raspberry Pi


I’ve been playing recently with using a MCP4921 as an audio DAC on a Raspberry Pi Zero W, although a MCP4922 would be equivalent (the ’22 is a two channel DAC, the ’21 is a single channel DAC). This post is my notes on where I got to before I decided that thing wasn’t going to work out for me.

My basic requirement was to be able to play sounds on a raspberry pi which already has two SPI buses in use. Thus, adding a SPI DAC seemed like a logical choice. The basic circuit looked like this:

MCP4921 SPI DAC circuit

Driving this circuit looked like this (noting that this code was a prototype and isn’t the best ever). The bit that took a while there was realising that the CS line needs to be toggled between 16 bit writes. Once that had been done (which meant moving to a different spidev call), things were on the up and up.

This was the point I realised that I was at a dead end. I can’t find a way to send the data to the DAC in a way which respects the timing of the audio file. Before I had to do small writes to get the CS line to toggle I could do things fast enough, but not afterwards. Perhaps there’s a DMA option instead, but I haven’t found one yet.

Instead, I think I’m going to go and try PWM based audio. If that doesn’t work, it will be a MAX219 i2c DAC for me!


Introducing GangScan


As some of you might know, I am a Scout Leader. One of the things I do for Scouts is I assist in a minor role with the running of Canberra Gang Show, a theatre production for young people.

One of the things Gang Show cares about is that they need to be able to do rapid roll calls and reporting on who is present at any given time — this is used for working out who is absent before a performance (and therefore needs an understudy), as well as ensuring we know where everyone is in an environment that sometimes has its fire suppression systems isolated.

Before I came along, Canberra Gang Show was doing this with a Windows based attendance tracking application, and 125kHz RFID tags. This system worked just fine, except that the software was clunky and there was only one badge reader — we struggled explaining to youth that they need to press the “out” button when logging out, and we wanted to be able to have attendance trackers at other locations in the theatre instead of forcing everyone to flow through a single door.

So, I got thinking. How hard can it be to build something a bit better?

Let’s start with some requirements: simple to deploy and manage; free software (both cost and freedom); more badge readers than what we have now; and low cost.

My basic proposal for such a thing is a Raspberry Pi Zero W, with a small LCD screen and a RFID reader. The device scans badges, and displays a confirmation of scan to the user. If the device can talk to a central server it streams events to it; otherwise it queues them until the server is available and then streams them.

Sourcing a simple SPI LCD screen and SPI RFID reader from ebay wasn’t too hard, and we were off! The only real wart was that I wanted to use 13.56mHz RFID cards, because then I could store some interesting (up to 1kb) data on the card itself. The first version was simply a ribbon cable:

v0.0, a ribbon cable

Which then led to me having my first PCB ever made. Let’s ignore that its the wrong size shall we?

v0.1, an incorrectly sized PCB

I’m now at the point where the software for the scanner is reasonable, and there is a bare bones server that does enough roll call that it should be functional. I am sure there’s more to be done, but it works enough to demo. One thing I learned while showing off the device at coffee the other day is that it really needs to make a noise when you scan a badge. I’ve ordered a SPI DAC to play with, which might be the solution there. Other next steps include a newer version of the PCB, and some sort of case solution. I’ll do another post when things progress further.

Oh yes, and I’ll eventually release the software too once its in a more workable state.


HomeAssistant configuration


I’ve recently been playing with HomeAssistant, which is quite cool. Its not perfect — for example it broke recently for me without any debug logs indicating problems because it didn’t want to terminate SSL any more, but its better than anything else I’ve seen so far.

Along the way its been super handy to be able to refer to other people’s HomeAssistant configurations to see how they got things working. So in that spirit, here’s my current configuration with all of the secrets pulled out. Its not the most complicated config, but it does do some things which took me a while to get working. Some examples:

  • The Roomba runs when no one is home, and let’s me know when its bin is full.
  • A custom component to track when events last occurred so that I can rate limit things like how often the Roomba runs when no one is home.
  • I detect when my wired doorbell goes off, and play a “ding dong” MP3 in the office yurt out the back so I know when someone is visiting.
  • …and probably other things.

I intend to write up interesting things as I think of them, but we’ll see how we go with that.


Learning from the mistakes that even big projects make


The following is a blog post version of a talk presented at pyconau 2018. Slides for the presentation can be found here (as Microsoft powerpoint, or as PDF), and a video of the talk (thanks NextDayVideo!) is below:


OpenStack is an orchestration system for setting up virtual machines and associated other virtual resources such as networks and storage on clusters of computers. At a high level, OpenStack is just configuring existing facilities of the host operating system — there isn’t really a lot of difference between OpenStack and a room full of system admins frantically resolving tickets requesting virtual machines be setup. The only real difference is scale and predictability.

To do its job, OpenStack needs to be able to manipulate parts of the operating system which are normally reserved for administrative users. This talk is the story of how OpenStack has done that thing over time, what we learnt along the way, and what I’d do differently if I had my time again. Lots of systems need to do these things, so even if you never use OpenStack hopefully there are things to be learnt here.

Continue reading Learning from the mistakes that even big projects make


Rejected talk proposal: Design at scale: OpenStack versus Kubernetes


This proposal was submitted for pyconau 2018. It wasn’t accepted, but given I’d put the effort into writing up the proposal I’ll post it here in case its useful some other time. The oblique references to OpensStack are because pycon had an “anonymous” review system in 2018, and I was avoiding saying things which directly identified me as the author.

OpenStack and Kubernetes solve very similar problems. Yet they approach those problems in very different ways. What can we learn from the different approaches taken? The differences aren’t just technical though, there are some interesting social differences too. Continue reading Rejected talk proposal: Design at scale: OpenStack versus Kubernetes


Accepted talk proposal: Learning from the mistakes that even big projects make


This proposal was submitted for pyconau 2018. It was accepted, but hasn’t been presented yet. The oblique references to OpensStack are because pycon had an “anonymous” review system in 2018, and I was avoiding saying things which directly identified me as the author.

Since 2011, I’ve worked on a large Open Source project in python. It kind of got out of hand – 1000s of developers and millions of lines of code. Yet despite being well resourced, we made the same mistakes that those tiny scripts you whip up to solve a small problem make. Come learn from our fail.

Continue reading Accepted talk proposal: Learning from the mistakes that even big projects make


pyconau 2018 call for proposals now open


The pyconau call for proposals is now open, and runs until 28 May. I took my teenagers to pyconau last year and they greatly enjoyed it. I hadn’t been to a pyconau in ages, and ended up really enjoying thinking about things from topic areas I don’t normally need to think about. I think expanding one’s horizons is generally a good idea.

Should I propose something for this year? I am unsure. Some random ideas that immediately spring to mind:

  • something about privsep: I think a generalised way to make privileged calls in unprivileged code is quite interesting, especially in a language which is often used for systems management and integration tasks. That said, perhaps its too OpenStacky given how disinterested in OpenStack talks most python people seem to be.
  • nova-warts: for a long time my hobby has been cleaning up historical mistakes made in OpenStack Nova that wont ever rate as a major feature change. What lessons can other projects learn from a well funded and heavily staffed project that still thought that exec() was a great way to do important work? There’s definitely an overlap with the privsep talk above, but this would be more general.
  • a talk about how I had to manage some code which only worked in python2, and some other code that only worked in python3 and in the end gave up on venvs and decided that Docker containers are like the ultimate venvs. That said, I suspect this is old hat and was obvious to everyone except me.
  • something else I haven’t though of.

Anyways, I’m undecided. Comments welcome.

Also, here’s an image for this post. Its the stone henge we found at Guerilla Bay last weekend. I assume its in frequent use for tiny tiny druids.


A pythonic example of recording metrics about ephemeral scripts with prometheus


In my previous post we talked about how to record information from short lived scripts (I call them ephemeral scripts by the way) with prometheus. The example there was a script which checked the SMART status of each of the disks in a machine and reported that via pushgateway. I now want to work through a slightly more complicated example.

I think you hit the limits of reporting simple values in shell scripts via curl requests fairly quickly. For example with the SMART monitoring script, SMART is capable of returning a whole heap of metrics about the performance of a disk, but we boiled that down to a single “health” value. This is largely because writing a parser for all the other values that smartctl returns would be inefficient and fragile in shell. So for this post, we’re going to work through an example of how to report a variety of values from a python script. Those values could be the parsed output of smartctl, but to mix things up a bit, I’m going to use a different script I wrote recently.

This new script uses the Weather Underground API to lookup weather stations near my house, and then generate graphics of the weather forecast. These graphics are displayed on the various Cisco SIP phones I already had around the house. The forecasts look like this:

The script to generate these weather forecasts is relatively simple python, and you can see the source code on github.

My cunning plan here is to use prometheus’ time series database and alert capabilities to drive home automation around my house. The first step for that is to start gathering some simple facts about the home environment so that we can do trending and decision making on them. The code to do this isn’t all that complicated. First off, we need to add the python prometheus client to our python environment, which is hopefully a venv:

pip install prometheus_client
pip install six

That second dependency isn’t a strict requirement for prometheus, but the script I’m working on needs it (because it needs to work out what’s a text value, and python 3 is bonkers).

Next we import the prometheus client in our code and setup the counter registry. At the same time I record when the script was run:

from prometheus_client import CollectorRegistry, Gauge, push_to_gateway

registry = CollectorRegistry()
Gauge('job_last_success_unixtime', 'Last time the weather job ran',

And then we just add gauges for any values we want to add to the pushgateway

Gauge('_'.join(field), '', registry=registry).set(value)

Finally, the values don’t exist in the pushgateway until we actually push them there, which we do like this:

push_to_gateway('localhost:9091', job='weather', registry=registry)

You can see the entire patch I wrote to add prometheus support on github if you’re interested in an example with more context.

Now we can have pretty graphs of temperature and stuff!