PiFace

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PiFace is a peripheral IO device for the Raspberry Pi. It was announced at the recent Cambridge Raspberry Jam event so I bought one for the princely sum of £30.

It’s designed to fit on-top of the Pi and has an identical footprint with cut-outs to accommodate the Composite video, Ethernet and USB ports. It takes 3.3v and (optionally) 5v from the Pi itself, although there is provision for a separate supply to power the output relays and buffers. While the driver chips will support up to 20V, the relays are designed to only run at 5V, so the external supply needs to be 5v. My board was not fitted with this socket.

Photos

PiFace

View of the PiFace fitted onto a Raspberry Pi

Top View of the PiFace fitted to a Raspberry Pi

Top View of the PiFace fitted to a Raspberry Pi

These photos show the PiFace fitted to a Raspberry Pi. The 9 screw terminals at the bottom/front of the photos are the output pins (Power is the pin on the left), the 2 sets of 3-way screw terminals are the relay outputs and the set of 9 at the top/back are the inputs with 0v being the one on the left, closest to the composite video output.

The PiFace leaves all ports accessible on the Raspberry Pi itself, with the obvious exception of the GPIO port. It uses power and the SPI bus on the GPIO only. It does not make the other GPIO pins available for use.

2 of the outputs are connected to relays which are capable of switching 10amps and 230volts… That’s a 2Kw load however I would not personally like to try to switch anything close to that with those little screw terminals.

Fitting

The PiFace itself is supported only by the large capacitor on the Raspberry Pi board as well as the GPIO connector. If you were to exert a little too much force on the PiFace, it’s possible you may damage the capacitor and also short circuit some of the PiFace’s pins on the HDMI connector. One of those sticky felt pads on the PiFace at the location of the HDMI connector is helpful.

It’s probably not recommended anyway, but do not try to fit the PiFace to your Raspberry Pi when the Pi is turned on – they do it in one of the videos but it caused my Pi to reboot – presumably due to a power spike going through the board.

What You Get

PiFace gives you 8 buffered input pins (with 4 switches on-board connected to 4 of these pins), and 8 buffered output connections. 2 of the output connections are connected to mechanical relays.

You can use the 2 buffered outputs in parallel with the relays, but they will obviously click on/off when you use them. The relay outputs have an LED wired in parallel with the relays and there are 2 other LEDs on the output pins, so with the 4 buttons and 4 LEDs (with the relays clicking away), it’s possible to do some very simple experiments right away.

GPIO

The GPIO is handled by an MCP23S17 SPI/GPIO interface chip. This provides 16 general purpose IO pins with the PiFace dividing them into 8 inputs and 8 outputs.

The Input pins have a 330Ω series resistor and the MCP23S17 is capable of applying internal pull-up resistors making it easy to detect signals that pull to ground such as simple switches without any additional components.

NOTE: Like the GPIO on the Raspberry Pi, the MCP23S17 is a 3.3v chip. Do not attempt to connect it to systems generating more than 3.3v outputs.

The outputs are driven via a standard ULN2803A darlington driver chip.

There is no support for the interrupt capability of the MCP23S17 although this could be achieved by soldering a wire from its interrupt output pins to 2 of the Raspberry Pi’s GPIO pins.

Software

There isn’t any 🙁

At least none that I could find, other than references to a new SPI driver for the kernel, and as I’m using one of Chris Boot’s kernels with his SPI driver installed, (which I think is now back-ported to the official kernel released by the foundation), so I didn’t think there was much point pursuing that line of attack… So armed with the circuit diagram, the MCP23S17 data sheet and some time, I wrote a version of wiringPi to work with the PiFace board. Details here including a software library and demonstration program.

Conclusion

It’s a nice little board. Relatively easy to use, but with no software supplied, could be quite daunting to use! Hopefully someone will produce and publish a Python, scratch, etc. libraries to enable it to be used easily, but until then, there’s wiringPiFace.

I’m somewhat disappointed that there is no way to disable the 2 on-board relays. I wanted to drive 8 LEDs and detect a switch with it to hopefully use it to drive a copy of Mike Cooks Magic Wand the buzzing from the relays is too much to bear and I really don’t want to un-soldering them, so a couple of jumpers would be handy on the next revision…

I feel that the inputs could have been handled better – especially for something intended for classroom use – with e.g. opto-isolators to allow for a range of input voltages from 3 to 12v for example. That would make it much more robust and versatile in a classroom environment… Probably add to the cost, and possibly board size too though, so I can see where a compromise may have been made, however the MCP23S17 is socketed and replacements cost under £1 in the UK.

Also, the circuit diagram shows 2 jumpers that are not fitted to my board – they are attached to the chips address-select lines. The manual for the MCP23S17 says that these pins must be strapped either way, even if they are not being used, so I soldered on some 0.1″ headers and put on a couple of jumpers. I’m hoping this is just an oversight on my early production board… It’s also hard-wired to the Raspberry Pi’s first SPI chip select output – again, a jumper would have been nice, but given that you can’t connect anything else to the Pi when this is connected it’s probably a moot point.

And the inability to connect anything else to the Raspberry Pi’s GPIO connector is somewhat frustrating. I can’t use the PWM output, can’t use the serial port, can’t use the I2C port, nor can I use any of the other GPIO ports that the Pi provides. Maybe one day we’ll see some sort of stacking “shield” arrangement like Arduinos have, however…

The mounting is a little flimsy, I’d like to see a proper spacer pillar in the underside of the board, just to stop it wobbling, although even a sticky-backed felt pad over the HDMI port would be a good start.

I feel it is much less capable than the Gertboard, but at the same time it’s also much simpler and might just be the thing to enable some easy control applications for demos, and even with the 2 relays the ability to drive some low-voltage motors – that combined with its size might just make it a good little board for a floor-crawler robot of some kind. A control board for my PiTrack project? Now there’s a thought…

Cambridge Raspberry Jam: A Box of 200 Rasperry Pi’s…

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I’ll put up another article or 2 if I have time about the Cambridge Raspberry Jam event held on the 14th of July 2012… However…

One of the things that struck me was a comment from Eben that he had a box of 200 Raspberry Pis (for sale) and he noted how easy it was to carry it and made a comment about the potential computing power.

It struck a note with me, as some time back (1988-1995!) I worked for a UK based supercomputer company called Meiko Scientific and one of the projects I worked on was a 256-node supercomputer installed at the Lawrence Livermore National Laboratories in California… (It was one of Meikos CS2 units – Computing Surface mk 2)

This is a picture of that Supercomputer, which at the time was the fastest supercomputer on the planet:

The Meiko CS2 at LLNL

The Meiko CS2 at LLNL

Now, without knowing what you’re looking at, it’s hard to get an idea of how big that is… The cabinets are 2 metres high. The whole is arranged into bays of 3-module widths (4 bays) and 2-module width (the smaller one on a diagonal in the middle). It’s that big “L” shape due to the nature of the cabling between the modules – it goes diagonally under the floor. (The middle sections of each bay is a set of network switches) The stand-alone black boxes are RAID arrays with (from memory) 20 x 4GB drives each.. (they were later moved inside some of the modules, but at that time the mechanicals weren’t in-place)

This was 1993/1994 or thereabouts.

Inside that, amongst other things are 256 compute nodes. 4 to one of the smaller modules that you see. Each smaller module is rated at up to 1 KW in power consumption terms. Those compute modules were about 14 inches square and contained a dual-processor sparc at 66MHz and 128MB of RAM. It ran a variant of Solaris. It booted over the network, but each node also had a local 1GB SCSI drive. (There was an additional control bus and “stuff” associated with that to give us node control like power, monitoring and console which added a little to the board, as well as the high speed data network)

So think about that for a moment… A single Raspberry Pi is probably faster than each of those sparc boards. (and at 700MHz vs 66MHz, it’s a lot faster!) The Pi also has double the RAM. The CS2 boards did have a blisteringly fast (for the time) communications network – 750Mbits/sec with a very low latency between nodes. A lot faster than the Pi’s max. of 100Mbits/sec. into a switched Ethernet network, however I could build an equivalent “supercomputer” based on 256 Raspberry Pis, put it under my desk and power it from a 13A socket. And that’s just using the ARMs – not even thinking of the GPUs…

Still – it’s somewhat interesting to think that Eben can carry a box of 200 Pi’s representing something that 20 years ago was simply incomprehensible to think about at the time.

(Minor update to change network speeds to bits/sec!)

7-Segment LED displays and the Raspberry Pi

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Chatting as you do about “stuff” on the Raspberry Pi, someone on the #raspberrypi IRC channel mentioned LEDs and driving them. Subsequently someone else on the raspberrypi.org forums was asking about then too… And so another little project was born…

So off to my (now usual!) online store, SKPang and a 4-digit, 7-segment display module found its way into my order. (Actually, since it has decimal points, then technically it’s an 8-segment display, but for some reason they’re always referred to as 7-segment displays!)

Raspberry Pi with 7-segment LED display

Raspberry Pi with 7-segment LED display

There are many strategies for driving 7-segment displays – I’ve chosen to use one of the lower-level ones, doing more in software than in hardware to keep the hardware design simple – so as you can see here, it’s almost as simple as it can get. We have 8 GPIO outputs going to the 8 segments of the display (white + yellow wires) and 4 GPIO outputs going to the individual digits common line. (the black and green wires) This is a common anode display, so the anodes of each of the 8 LEDs in a digit are connected together.

To drive the display, we start by setting all GPIO pins to outputs, set the 4 digit GPIO pins to logic zero and the 8 segment GPIO pins to logic 0. Now we need to light up the display one segment at a time…We do this by selecting the digit and driving it’s common connection to logic 1 (this is the common anode), then drive the segments one at a time to logic 0 to illuminate them, pause for a brief period of time (I’m using 500 μS here), then turn it off by setting it to logic 1 and moving on to the next segment. 4 digits, 8 segments per digit, 500 μS per segment means that the total scan time will be 16mS, or just over 60 times a second. That ought to be fast enough to eliminate flicker.

To increase scan frequency, we could light up more than one segment, but as they are all going via the same resistor then the segments will get dimmer the more we turn on… To prevent this, we could use 8 resistors, however then the combined current of 8 GPIO pins being fed from one GPIO pin would be too much for the Pi’s SoC. We could then use a seprate drive transistor to boost the current, but we’re trying to keep things as simple as possible here…

So, we illuminate each segment of each digit in-turn and if we do it fast enough then our persistence of vision will fool us into thinking they’re all turned on all the time.

So now we have a problem… How do we scan through the segments fast enough while at the same time allowing our program to run? And, what happens if,  while scanning the segments, Linux decides to go off and so something else? (Linux is a pre-emptive, multi-tasking, multi-user operating system, after-all!)

This is when we really need a real-time operating system, and while there are some patches to the Linux kernel, they’re not standard in the Pi kernels, and even if they were, they still might not be suitable for use here – to do it “properly” really does require very tight control over the hardware, interrupts, peripherals and so on, and I am not convinced that Linux will give us all that control.

However, there are some simple things we can do to improve things – to the point of making a task like this relatively easy and do-able.

This first thing we can do is tell Linux that we want to have a higher scheduling priority and that we want to be considered for real-time scheduling too. We used the sched_setscheduler() system call to effect this. It’s not perfect – a higher priority process can interrupt your program (and if you want to maintain things like keyboard entry and allowing other programs to run, then you must allow this!) but it’s a good way to give your program a good boost.

The other thing we can do is to use Posix Threads and run what’s effectively a 2nd program concurrently with your main program, and have that 2nd program (although it’s actually a function inside your main program rather than a 2nd program) manage the LED updates.

As long as the LED display routine calls delay() every now and then, then the rest of the system will carry on which the display is kept updating. (It can use other methods to de-schedule itself, but calling a wait function is easiest – I actually call delayMicroseconds() in the display code).

All that remains is to establish a way to communicate between our main program and the display routine and I use a simple global variable here: A string array which I can write to in the main program and read from in the display routine – we don’t need any fancy locking, etc. for something as simple as this.

Even after that, setting the real-time priorities and running the separate display thread, I still see the occasional flicker or glitch on the display. It’s probably no real issue for a display, but imagine if we were driving a stepper motor… One reason I still maintain that Linux is really not the right tool to do that form of “hard” real-time control, but for LED displays? it’s OK.

Any down-sides? Well the overhead of keeping the display updated is between 15 and 20% of the CPU usage on my Pi! So it has to be said that the Raspberry Pi really isn’t the right device to be driving such a display, but it’s good to know that if we had to then we can.

And I’m sure some of the tricks in my program will be useful in other applications. You can find the software here.

Finally another of my most excellent videos:

LAMP on a Raspberry Pi

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So after a conversation on IRC where the general gist was that apache was bloated and slow vs. lighttpd, I thought I’d try a little experiment today – see how long it might take to compile up Apache, PHP and MySQL and get it going on a Raspberry Pi… Then stick a wordpress on it.

(And in-case you found this looking for a comparison, sorry – I don’t have one, lighttpd may well be smaller & faster than Apache, but I’ve not done any tests myself – I was just interested in how well or badly my “usual” apache/mysql/php setup I employ on my server fared on the Raspnerry Pi)

So I decided to compile them from sources – which is what I often do anyway rather then use the Debian supplied versions – mostly to see how long it would take doing it that way, but also to try to only compile in the modules/features, etc. that I needed – hopefully to make it as “lean” as possible while still maintaining enough functionality to run WordPress.

My Pi was running Raspbian with a 3.2.21 kernel, and the memory split at 244MB.

Versions of the “AMP” stack are: Apache: 2.2.22, MySQL: 5.1.61 and PHP: 5.3.10.

MySQL took some 2.5 hours to compile and resulted on about 260KB of swap being used. Apache was a bit quicker at 1.5 hours, but surprisingly it used a little more swap – up to 680KB. The configure and compile for PHP took it up to nearly 900KB. (however I did start up MySQL during this phase to finish the installation of it, but shut it down once I’d finialised the installation)

Watching the system with ‘top’ while it was compiling, it seems the GCC compiler doesn’t really need much more than 60-80MB of RAM, so the few little things that got swapped out are really just making more buffer space – so I imagine the compiler and associated programs stay cached in RAM for the duration while will speed things up somewhat. The bottleneck really seems to be the CPU, so I doubt putting the files on USB drive, NFS, etc. would make any difference to it all – the system runs at >95% CPU running gcc for a good few minutes for each file, so optimising the few seconds of disk IO doesn’t really seem worthwhile.

Typical of top during PHP being compiled:

KiB Mem:    222744 total,   185080 used,    37664 free,    10000 buffers
KiB Swap:   131068 total,      868 used,   130200 free,    71484 cached

  PID USER      PR  NI  VIRT  RES  SHR SWAP S  %CPU %MEM    TIME+  COMMAND                    
14053 root      20   0 83580  73m 6904    0 R  98.1 33.6   3:00.28 cc1

The whole process is very reminiscent of installing the LAMP stack on an older server – a PIII/800MHz many years ago. (And I had about the same amount of RAM then too!)

… Some time later and it’s running.

And so what can I say… It’s slow. I’ve tweaked the apache config somewhat and it’s still slow. What makes it slow… It seems to be a conbination of lots of things – PHP is quite large, so that eats up some RAM. MySQL likes to do fflush() operations which make the underlying filesystem somewhat slow. (however using phpMyAdmin on the server was OK, so maybe wordpress really is a bit big and bloaty afterall!)

Next week (if I have the time and enthusiasm!) I’ll start to try some of the newer servers and configurations and so on… Watch this space, but don’t hold your breath!

Another update to wiringPi

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Thanks to Keith Wright for diligently testing it and pointing out bugs for me – fixed some issues in the gpio command, and while I was at it, I added in 2 new functions to it to allow for easy exporting/unexporting of the /sys/class/gpio interface – so now you can write a little script to do the exports for you, then run your own program that uses that interface (e.g.) a Python, shell, or php script, then un-export them afterwards, all without needing to be root (or use the sudo command)

WiringPi – Another bug, another day…

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Just uploaded a new version of wiringPi  with a few bug fixes. the main one being in digitalWrite when the range check for the pin number was fine when it was wiringPi pins, but not fine for the newly introduced GPIO pin numbers.

I’ve also added in some additions – a shiftIn/Out module and a serial port access module. These are just handy additions, but I use them myself in other projects, so it’s a handy place to keep them.

As usual, fedback welcome!

-Gordon

Raspberry Pi – Game Port IO?

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My little ladder game seems to have generated some interest – or was it my comment about the GPIO port standing for Game Port IO? Who knows, it’s fun anyhow…

But it’s got me thinking – what else can I do with a row of LEDs and a button. The Ladder game isn’t original, it was from a magazine published over 30 years ago, but what else can I think of to use on the same platform…

Some sort of reaction tester? shooter? Will crank up my imagination and see what comes up, but suggestions are welcome!