Tristate Multiplexing

Tristate multiplexing (or Charlieplexing) is a simple to use technique to expand the number of LEDs that you can light from a simple microcontroller. This post explores the concept, and looks at some interesting calculations we can do when thinking about them.

I’ve been meaning to get around to writing something of a tutorial on the basics of assembly programming using a PIC microcontroller for a while: and this post stared off as the first part of that… But then, in the best traditions of geekery, I got distracted by the interesting mathematics behind the scenes, and instead this post was born. But I’ve not given up on the idea and I will get back to that. Eventually. (But don’t hold your breath). Instead this post is written without any reference to any specific microcontroller architecture, and so you can use the ideas described here on a PIC, an Arduino, and it should even work on a Raspberry Pi (though I’ve not actually tried that yet).

To begin, let’s think about a simple situation where we have a microcontroller which we wish to use to control two LEDs. The simplest implementation is to use one of the microcontroller’s GPIO pins for each LED. This works well for a small number of LEDs; but very quickly gets expensive in terms of the number of pins it uses if you want more than a handful of LEDs; and depending on what else your microcontroller needs to do (and/or how many pins it has available to begin with) it soon becomes unsuitable.

The solution is to multiplex the LEDs – that is to say connect more than one LED to each pin, and use a combination of multiple pins to select the one we want.

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Calling C++ Classes from Python, with ctypes…

I recently found myself needing to call a C++ class from within Python. I didn’t want to call a separate process (as I had previously when using Thrift – see Using Apache Thrift for Python & C++); but rather to call a C++ library directly.

Before I go on I should say that there are a lot of different ways to do this is Python – and I’ve picked one that worked for me. Other techniques are available – and opinion seems quite divided as to which (if any) technique is ‘best’.

To start with we have our C++ class, written in the usual way.

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Simple Python Hello World with Alexa

I’ve recently got myself an Amazon Echo Dot; Amazon’s speech-based, voice-controlled device using Amazon’s Alexa, digital assistant. They’re pretty nice devices; and they’re quite fun to play with (and not too expensive either) – though I’ve not had it long enough to say if it’s actually useful yet…

Obviously the first thing that I did with it was to try to write some code for it. (Actually that’s not quite true – the actual first thing that I did with it was to run through all of the many, funny and often very geeky, easter eggs that have been baked into the system. I think my personal favourite is “Alexa who is the mother of Dragons?” – closely followed by “Alexa: Tea. Earl Grey. Hot.”). But I digress…

Coding for Alexa is actually pretty easy; and appealingly for an old-school back-end programmer like me: there are no visual user-interfaces to worry about. It’s all done with simple JSON; in the form of a pretty simple web application. Perhaps more interestingly there’s no need to worry about the server for that application – since it’s ideally suited for using Amazon’s AWS Lambda Functions.

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Using Apache Thrift for Python & C++

Apache Thrift is a software framework for cross-language: providing what is essentially a remote-procedure call interface to enable a client application to access services from a service — which can be written in the-same, or another language. Thrift supports all of the major languages that you’d expect to use: including Python, C++, Java, JavaScript / Node.js, and Ruby.

Unfortunately, whilst there are quite a few tutorials on how to use Thrift: some of them concentrate on explaining how Thrift is working behind the scenes (which is important, of course): rather than on how to use it. There also aren’t that many that concentrate on using C++. So having spent some time working through some of the tutorials (and the book Learning Apache Thrift), I though I’d have a go at writing something of a practical guide. I’m quite deliberately not going to focus on how Thrift works: if you want that, let me suggest the Apache Thrift – Quick Tutorial. I’m also going to assume that you have Thrift installed on your computer already (again, there are lots of sets of instruction on how to do this — Google will be your friend), and that you’re using a Linux or MacOS computer. You should be able to follow-along with most of this if you’re using Windows: but there will be a few differences, especially when it comes to compiling C++ files.

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RegEx Golf

RegEx Golf
Inspired by the famous XKCD cartoon that started it and by Google’s Peter Norvig over on O’Reilly.com I decided to play some RegEx Golf myself this Easter…

If you’re not familiar with the game – the idea is to write the shortest regular expression possible that will select every element of a list, without selecting any elements from a second list. But what are regular expressions, you ask? Well, they’re a special sequence of characters that specify a particular textual patter – so you can use the appropriate regular expression when programming to automatically extract specific text from a list, file, or other text. There are some great tutorials on the web if you want to learn regular expressions (and you really should – because they’re great) – for example regexone.com

I didn’t want to use lists of Presidents of the United States (as that had been done) – so I decided to have a go at the age-old challenge: animal, mineral, or vegetable

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Arduino I2C with the MCP9808 Temperature Sensor

The Microchip MCP9808 is a high precision temperature sensor with an I2C interface – making it easy to interface with microcontroller or embedded Linux electronics. The chip has a very small form-factor: available as either an 8-pin DFN package – or an 8-pin MSOP package. For hobbiest purposes DFN is almost impossible to use due to the complexity of having to reflow solder under the contacts; and MSOP is quite challenging (though it can be done with care). If you want something easy to use with a breadboard however, the good folks at Adafruit can come to the rescue – as they sell the device pre-soldered to a SIP breakout board. (Also available from Makersify in the UK).

Adafruit MCP9808 Breakout board
Adafruit MCP9808 Breakout board

If all you want to do is get something up and running quickly – then you can’t do better than to use the Adafruit Aurduino library (or if you prefer a Python library for BeagleBone or RPi).

Adafruit have done a great job with these libraries and most users will be delighted that “it just works”; but I want to explore how it all works: how to actually communicate with chip directly – using I2C. This guide written from the point of view of an Arduino; but you should be able to follow along from any other device if you’re reasonably familiar with I2C and low-level coding.

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Arduino & XBee

XBee Module Xbee modules provide a very simple way to add a wireless capability to communications on Arduino. Essentially they let you work in exactly the same way as a direct serial connection between two boards – albeit wirelessly.

The easiest way to connect an Xbee to your Arduino, is to use the Arduino Wireless Shield.

XBee Sheild

It’s possible to connect Xbee’s without a shield – but given the fact that they don’t have the (standard) breadboard 0.1″ pin-pitch, this requires a special breakout board, or some custom hardware. If you’re using an Arduino, I think the shields are definitely the best way to go (and they’re pretty inexpensive).

There are a number of different types of XBee – the most notable distinction is between the original, “series 1” devices (directly using the 802.15.4 wireless protocol) and the “series 2”, ZNet 2.5 (or ZB) modules: which add an additional communications layer over the base-standard. Series 1 and series 2 devices are incompatible with each other (and cannot communicate with each other) – but they are pin compatible with each other: so for point-to-point applications, it’s possible to swap out S1 devices with S2 devices (providing you exchange both ends of the communication).

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GPIO with sysfs on Raspberry Pi (Part 2)

In my last post on using sysfs for GPIO on Raspberry Pi, I showed you how you can use the sysfs file system to manipulate the state of the GPIO pins.

This time, we’re going to explore how to do the same thing programmatically. I’m going to walk you through developing a C library to control the GPIO – but exactly the same principles could be used in any other language.

Why C? Well, mostly because I like C. But proficiency in C is useful if you’re going to be working with embedded systems, and microcontrollers; so it’s a good language to use. I’m also picking C because there are a lot of tutorials & libraries using Python – and few (if any) working with C. None of which is to say that Python isn’t a great language (and if you don’t know it – then I highly recommend learning it too).

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GPIO with sysfs on a Raspberry Pi

GPIO_Closeup

The Raspberry Pi (in case you’ve been living under a rock for the last six-months) is a cheap ($25) ARM Linux computer, primarily designed to be used to help teach kids to learn programming & computer science.

It’s also makes a pretty nice alternative to something like a BeagleBone, if you’re looking to play with some embedded computing technology. Whilst it’s less powerful than BeagleBone (and has less GPIO connectivity), Raspberry Pi does have a few advantages. It’s cheap, and it has an HDMI output – meaning that you can interact with it directly, without the need to ssh in from another computer.

The whole ethos of the Raspberry Pi foundation is to make the device as easy to use as possible: to encourage children (and adults!) to play with it, and learn to program. As with Arduino though, this user-friendly layer doesn’t mean that you can’t get your hands a little dirty and see what’s going on underneath.

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