Computing stuff tied to the physical world

Posts Tagged ‘Displays’

Colorful GLCD

In Hardware on May 13, 2011 at 00:01

Whoops! The other day, one of the Graphics Boards here at JeeLabs fell on the floor:

Dsc 2505

Pretty, but not quite what it’s supposed to be :)

Fortunately, I still had a similar black display left over from the initial builds:

Dsc 2507

(ignore the info – node #3 is a radioBlip sketch, even though glcdNode interprets it as a room node)

Sooo… don’t drop the GLCD, it’s brittle! Unless you prefer that first display, of course :)

2×16 LCD’s – yeay!

In News on Jan 21, 2011 at 00:01

At last… the long-awaited order of 2×16 character LCD’s has arrived!

Here’s what one of those boxes looks like:

Dsc 2418

Many of the pending LCD (+ LCD Plug) back-orders have now been sent out. Yippie!

That still leaves 3 more major items which are holding up a couple of packages: Carrier Board PCB’s, Relay Plugs and RBBB’s. The Carrier Boards are currently in Leipzig and will be here either on Friday, or else right after the weekend. The second item which is really hard to get these days, is the relay used on the Relay Plug – they are expected in about a week. The third item is the RBBB, which has been postponed a bit due to a temporary ATmega328 DIP shortage. This has now been resolved, so I expect to have them in by early February.


It’s strange how these supply issues can make such a difference… looks like I can’t seem to focus on anything substantial when there is a large back-log. Pretty silly, really. Oh well.

Sorry for the interruption, I just had to post about this. Tomorrow will be the third Easy Electrons post about transistors, as promised.

GLCD library

In Hardware, Software on Jan 5, 2011 at 00:01

There’s a new GLCD library to drive the 128×64 graphics LCD display on the Graphics Board. The library is called, wait for it… GLCDlib – with a wiki page and a web interface to the source code in subversion. There’s also a ZIP archive snapshot, but it probably won’t get updated with each future subversion change. For some notes about using subversion (“svn”), see this post.

The main class is “GLCD_ST7565″, it has the following members:

Screen Shot 2011 01 04 at 19.52.45

(some longer entries above were truncated, see the website for the full version)

The settings in this library have been hard-coded for use with the Graphics Board, which uses ports 1 and 4 to drive this display. If you want to use this with other I/O connections, you’ll need to change the #define’s at the top of the “GLCD_ST7565.cpp” source file in the library.

Here is the demo used in an earlier post, now included as “glcd_demo.pde” example sketch in the library:

Screen Shot 2011 01 04 at 19.49.49

This produces an output screen similar to this image. Note the use of flash-based string storage with “PSTR” to reduce RAM usage. It not an issue in this example, but more strings tend to rapidly consume RAM, leading to strange and hard-to-find bugs.

The nice thing about GLCDlib, is that you can also use it over wireless. There is a “GLCD_proxy” class, which sends all graphics commands out to another node. Each command is sent as a packet, complete with ACKs, retries, and resends to deal with lost packets.

The “JeePU.pde” example sketch implements the “host”, i.e. a JeeNode with Graphics Board, listening to incoming wireless requests. The “JeePU_demo.pde” sketch shows how to talk to such a remote JeePU node.

Because the transport layer (i.e. wireless or other comms mechanism) is separated out from the main graphics primitives, it is very easy to switch between a locally-connected GLCD and a remote one on a JeePU node. The magic is contained mostly in these lines:

Screen Shot 2011 01 04 at 20.02.40

The only other change needed to use a remote GLCD is to add these lines at the start of setup():

Screen Shot 2011 01 04 at 20.04.57

See the JeePU_demo.pde sketch for an example of how this can be used.

The JeePU node should be running in its own RF12 net group, because clients use broadcasts to send out the graphics commands. They do not need to know the node ID of the JeePU, just its net-group. This also means that multiple GLCD proxy clients can run at the same time, and each could be adjusting a specific part of the same JeePU display … whee, a multi-node status display!

One of the advantages of running the Graphics Board as a JeePU node, is that the other nodes don’t need to load the entire GLCDlib code, in particular they avoid the 1 Kb RAM buffer needed to drive the display.

The graphics code is based on what used to be the ST7565 library by Limor Fried at AdaFruit, which was in turn derived from public domain code attributed to “”.

Several great extensions (and a couple of bug fixes) for the core graphics routines were written by Steve Evans (aka tankslappa on the wiki and forum). Steve also implemented the remote/proxy code and the JeePU “host” and JeePU_demo “client”.

I just hacked around a bit on all that, including renaming things and adding an ACK mechanism to the RF12 wireless layer.

This code is likely to change and extend further, as we come up with more things to do with the current implementation. But for now, enjoy :)

Update – all the code is now at

Touch screen

In Hardware on Dec 21, 2010 at 00:01

Steve Evans (tankslappa on the forum) emailed me recently to present some really exciting developments he’s been working on: a touch screen interface for the Graphics Board:


You’re looking at a little touch screen, placed over the Graphics Board, and as you can see, the size matches up surprisingly well.

Pretty fantastic result! And considerably more precise than I had expected.

He used the touch screen overlay of the Nintendo DS, which can be obtained from eBay. As it turns out, this can be hooked up using 2 digital + 2 analog I/O pins, which is exactly what’s still available on the unused two ports.

All that was needed, were 4 pull-down resistors:


The devil is in the details, though. very small details in fact:


Steve mentioned that it took a considerable amount of patience to get 4 wires soldered to that tiny flexible ribbon, with 0.5 mm between each connection. Clearly, a proper connector would have been a bit more convenient (though just as tiny).

Being resistive and analog, this whole thing needs to be calibrated:

Calibrate 1

The code for this has been added as a sample sketch called touchscreen.pde in the JeeLabs repository.

BTW, this is part of a new GLCDlib library, which I’ll describe in more detail tomorrow. Stay tuned!

Spectrum Analyzer

In AVR, Software on Nov 26, 2010 at 00:01

Intrigued by this post, I wanted to try the same thing with the Graphics Board.

So here goes:

Dsc 2330

A real-time Spectrum Analyzer, with due credit to “deif” on the Arduino forum for making Tom Roberts’ simple 8-bit FFT implementation available (three more people are credited in the source code). On the above display you can see a typical “power spectrum”, with one dominant frequency plus harmonics at fixed intervals.

The sketch can be found here:

Screen Shot 2010 11 23 at 00.14.25

This is almost the same code as the 100 KHz DSO sketch, just extended with FFT.

The LCD is refreshed at maximum speed. As you can see, the lag of the LCD itself is actually quite useful in this context. This did bring a bug to the surface: I’ve noticed that the speed-up I implemented for the ST7565 was slightly too fast for this chip. After running for a while, the display would just turn black and stop refreshing.

So I changed these two lines in ST7565::spiwrite() :

Screen Shot 2010 11 22 at 23.48.10

… to this, slightly slower but equivalent code:

Screen Shot 2010 11 22 at 23.48.43

I’ve also updated my version of the ST7565 ZIP file accordingly.

One more note about this code is that it uses most of the available RAM now: the 1 Kb buffer in the ST7565 library, plus two 256-char arrays to store 256 datapoints for running the FFT on.

With code such as this, it would actually be possible to get rid of the ST7565 buffer, since we’re re-constructing the entire display each time.

But for now, I’ll just leave it at this…

2-channel Logic Analyzer

In AVR, Hardware, Software on Nov 25, 2010 at 00:01

Let’s continue the tiny lab-instrument series ;)

Here’s a logic analyzer which takes 512 samples of 2 bits (DIO2 and AIO2):

Dsc 2318

The “trace” starts when either of the inputs changes. I’ve enabled the internal pull-ups, so in this example you can see that DIO2 has no signal. The AIO2 signal is from a JeeNode running the Arduino IDE’s built-in “ASCIItable” demo, which sends out a bunch of characters at 9600 baud (I took the signal from the USB-BUB connector).

Here is the glcdTracer.pde sketch which implements this:

Screen Shot 2010 11 20 at 17.24.06

Lots of trickery with bits. To keep memory use very low, I’m storing the 512 samples in a 128-byte buffer (512 x 2 bits = 128 x 8 bits). Low values are drawn as a pixel, high values are drawn as a little vertical line.

The code includes a minimal triggering mechanism, i.e. it waits for either of the input signals to change before collecting 512 samples.

Samples are collected at about 10 KHz, but this can easily be changed (up to ≈ 200 KHz with the current code).

Note that it would be possible to double the number of samples to 1024 and display them as 8 groups of 2 signals, but then you’ll have to really squeeze the output to fit it onto the 64×128 pixels on the display.

BTW, there’s some shadowing visible on the display – has to do with how the chip drives the GLCD, no doubt.

Soooo… now we also have a portable Logic Analyzer!

100 KHz DSO

In AVR, Hardware, Software on Nov 23, 2010 at 00:01

You might have been wondering why I created the digital-to-analog converter a few days ago.

Well, because I needed a test signal… to build this thing:

Dsc 2307

You’re looking at a <cough> Digital Storage Scope </cough> with 100 KHz bandwidth :)

First of all: please don’t expect too much. There is no signal conditioning and no triggering whatsoever, and there are no external controls. This is simply a JeeNode plus a Graphics Board. It’s using the built-in ADC, with the conversion clock pushed quite a bit higher than what the Arduino’s analogRead() function will do. This speed comes at the cost of conversion accuracy, which isn’t so important since the Graphics Board display only has 6-bit vertical resolution anyway.

The screenshot shows a 1 KHz sine (from that Bleep! thing, obviously). As you can see, one cycle more-or-less covers the entire x-axis. So that’s about 128 samples per millisecond. This is not the maximum value, the ADC can also work twice as fast – i.e. with a division factor of 4 (ADPS2:0 = 2). This translates to 4 µs per sample.

Using the Nyquist–Shannon sampling theorem again, you can detect a frequency if you sample it at least twice per cycle, so that would have to be a cycle of at least 8 µs, i.e. over 100 KHz. Which is why I decided to call this thing a 100 KHz DSO :)

The code tries to get as many samples as possible into a little 128-byte buffer before doing the rest of the work. The graphics display has a fairly limited response time, so I’m refreshing the display at 5 Hz (it’s still visible up to 50 Hz, but only just…).

I find it pretty amazing what an MPU such as the ATmega can do these days, with just a few lines of C code. Here’s the entire glcdScope.pde sketch:

Screen Shot 2010 11 19 at 02.58.10

The rest of the code is in the same modified ST7565 library as used in the past few days.

There’s lots of room for expansion, this code uses less than 4 Kb.

So there you have it – a very crude, but functional, oscilloscope!

The obligatory clock…

In Software on Nov 22, 2010 at 00:01

Take a Graphics Board, a JeeNode (or JeeNode USB, or JeeSMD), some code, and you get this:

Dsc 2303

I guess it’s sort of the equivalent of “Hello World” for graphical displays by now.

This is my code for it:

Screen Shot 2010 11 21 at 17.38.44

It even includes a blinking colon! :)

There’s a separate header file with the ten bitmaps used for each digit.

Here is a copy of the modified ST7565 code I use.

This clock runs on the ATmega’s 16 MHz clock, which is only accurate to about 0.5% with a ceramic resonator. More importantly, this clock has no notion of real time – it just starts counting when turned on. The value it starts from uses a funky trick available in RTClib: it’s set to the compile time of the sketch … this is actually not a bad choice during development. But for real use, you’ll need to add an RTC Plug or some other means of obtaining the time (such as a DCF77 receiver, if you’re in Europe).

Anyway. At least now you can see that it really is a graphical display.

Indoor temperature, etc.

In Hardware, Software on Nov 20, 2010 at 00:01

Here’s a fun little project…

A Graphics Board with a JeeSMD and two plugs: a Pressure Plug and a partially-populated Room Board.

The result is a geeky little indoor environmental monitoring station:

Dsc 2293

Couple of notes here:

  • I took the lazy way out and merely display the info as text, graphics would be so much nicer!
  • Both sensors can read the temperature, but as you can see, they don’t fully agree…
  • I added the FTDI hookup and used it to power this thing via an AA Power Board.
  • The AA board does double duty, as a little stand to hold this thing slanted upwards.

Adding FTDI was very useful in this case, since it also makes it possible to re-program the JeeSMD, which has no FTDI connector on board. All it needs is a 0.1 µF cap and the 6-pin header:

Dsc 2298

But there’s more to it than that, alas: to be able to upload to the JeeSMD, you need to hook up all the headers on the JeeSMD, i.e. also the PSI and the SPI/ISP headers. Adding those after the fact is very tricky, since you can’t reach the back of the Graphics Board pcb anymore if you soldered the LCD on permanently – as I did.

I had to solder the female headers slightly higher, to just barely reach the pins from the top with a very fine soldering iron tip (I use 0.4 mm). It worked, but as a result, the JeeSMD sits a bit higher on its pins than usual:

Dsc 2297

Here’s the indoor.pde sketch, also added to the Ports library:

Screen Shot 2010 11 17 at 22.47.37


Room Node display

In Software on Nov 17, 2010 at 00:01

Now that there’s a Graphics Board, I thought I’d make a little display with the last few readings from a couple of room nodes around here. Ironically, it’s just a 8×21 character text display for now – no graphics in sight:

Dsc 2281

The information consists of:

  • a packet sequence number (only 4-byte packets are treated as room nodes)
  • the node ID
  • the temperature in °C
  • the relative humidity in %
  • the measured light intensity (0..255)

New readings get added at the bottom, with older readings scrolling upwards.

Unfortunately, the ST7565 library doesn’t have a normal print() & println() API, so the first thing I did was to create a new wrapper class:

Screen Shot 2010 11 16 at 12.42.57

One quirk about this code is that since we’re using a RAM buffer, the ST7565 screen contents needs to be explicitly updated. I solved it by adding a poll() method which you need to call in the main loop. It’ll make sure that the display gets refreshed shortly after anything new has been “printed” (default is within 0.1 s).

Another thing this class does is to scroll the contents of the display one line up when the bottom is reached. It does this in a slightly lazy manner, i.e. the display is not scrolled immediately when a newline is sent to it but when the first character on the next line falls outside the display area – a subtle but important difference, because it lets you use println() calls and the display won’t constantly leave an empty line at the bottom.

Scroll support does require one change to the “ST7565.cpp” source code. This:

    static byte gLCDbuf[1024];

Has to be changed as follows, to make the RAM buffer accessible from other source files:

    byte gLCDbuf[1024];

(should be around line 42 in ST7565.cpp)

With that out of the way, here’s the glcdNode.pde sketch, which has been added to the RF12 library:

Screen Shot 2010 11 16 at 12.53.36

For debugging purposes, the same information shown on the display is also sent to the serial port:

Screen Shot 2010 11 15 at 02.24.28

Note that gldeNode is hard-coded to receive packets from net group 5 @ 868 MHz, as you can see in the call to rf12_initialize().

So now I have a battery-powered wireless gadget which lets me track what our house is trying to tell us!

Update – the ST7565 library needs to be changed a bit more, I now realize. Perhaps the easiest way to do so is to simply insert the following line somewhere near the top of ST7565.c:

    #define buffer gLCDbuf

That way, the buffer will have a more meaningful name when made global by the above-mentioned patch.

Update #2 – no more need for the ST7565 library, use the new GLCDlib instead. The glcdNode demo sketch has been adapted and moved over to it.

Assembling the Graphics Board

In Hardware on Nov 16, 2010 at 00:01

This is going to be a weblog post with lots of pictures…

The Graphics Board is easy to assemble, but there are two things to watch out for:

  • the polarity markings for two of the electrolytic 1µF caps are reversed
  • if you solder the display onto the board, then you can’t easily reach the back anymore

The former is a mini goof-up by yours truly, the latter is something to keep in mind when deciding which headers you are going to use (you can side-step the issue by mounting the display using headers, but that will increase the height of the entire stack).

Here goes. As usual, I’m going to proceed from low to high-profile components, because that lets you put the components in, flip the board over, and press on it to get the components soldered closely to the board.

First the 100 Ω backlight series resistor:

Dsc 2251 2

Note that there’s a solder jumper right above it. The default is to connect the middle pad with the leftmost pad marked “P”. This connects the backlight permanently to the PWR line. If you want to control the backlight through the IRQ signal, you can set the solder jumper to the “I” position instead (see the backlight() docs). This can be changed later if you change your mind.

Then 5 yellow decoupling caps. These are not polarized, but make sure you put them into the right holes:

Dsc 2249

Update: The latest kit version ships with ceramic capacitors replacing the electrolytic capacitors. This gives longer life and simplifies the construction detail since these capacitors are not polarised. See the Wiki for more detail

The next step needs some care, because these two electrolytic caps are marked the wrong way on the silkscreen – they should be placed as follows:

Dsc 2251

The proper way to connect these, is to put long lead on the right and on the bottom, respectively. Or, same thing: put the gray stripes of the plastic case insulation on the left and on the top.

Note: the two caps on the left will only just barely fit when a JeeNode v5 is added on later. If you use a JeeNode v4, it is probably better to place them flat on their side pointing left. When in doubt, make sure you check the mechanical layout before soldering these two leftmost caps!

The other two 1 µF caps go as indicated on the board, they will in fact be oriented exactly opposite to the caps right next to them:

Dsc 2252


Read the rest of this entry »

Meet the Graphics Board

In Hardware on Nov 15, 2010 at 00:01

Sometimes, 2×16 characters (or perhaps a few more) just don’t cut it, when I’d like to present things graphically, or use larger fonts for some of the info. That’s when a “GLCD” would be handy, one which lets you set individual pixels, that is. I was pointed to a nice low-power 128×64 display recently, and decided to create a pcb for it – so here’s the new Graphics Board:

Dsc 2258

Due to the size of the board, it’s not a plug or an add-on to a JeeNode, but the other way around: you can take a JeeNode, JeeNode USB, or JeeSMD, and push it onto this board to power and drive the display.

Ports 1 and 4 are used for the diplay, but ports 2 and 3 are available, and brought out in such a way that plugs can easily be added on. To make a clock, add an RTC Plug. To add a few touch buttons, add a Proximity Plug. To create a fancy Reflow Controller, add a Thermo Plug. Or just use it as is and add a sketch to display messages received over wireless. Endless possibilities…

Here’s the display in action, it’s pretty bright when driven from 5V:

Dsc 2278

It can display up to 8 lines of 21 characters, but it’s also fully graphical of course. BTW, in real life the display looks much more white-on-black.

I particularly like this setup, using the AA Power Board to make this thing completely self-powered:

Dsc 2245

The backlight resistor is chosen such that on 3.3V, this display draws ≈ 6 mA, and on 5V it draws ≈ 19 mA. It will last a few days with an AA Power Board, but when the 100 Ω resistor is replaced by 270 Ω or even 470 Ω, you can get well over a week of battery life on a single AA (assuming the JN itself is sleeping most of the time) – at the cost of dimming the backlight to a fairly low level.

There is a solder jumper on the board which normally connects the backlight to the PWR line. By connecting it to the IRQ line instead, that pin can turn off the backlight under software control (or dim it, using PWM).

The display is based on the ST7565 chip and can be driven by Limor Fried’s great ST7565 LCD library. Note that this library uses a 1 Kb RAM buffer.

Here’s the glcd_demo.pde sketch which generates the above display:

Screen Shot 2010 11 14 at 17.20.36

I included logic to put the ATmega and radio into power down mode to let me measure the display’s current consumption. While active, they draw another 5..30 mA.

The Graphics Board is now in the café and the shop.

UpdateHere is a copy of the ST7565 code I use.

New LCD Plug

In Hardware on Oct 16, 2009 at 00:01

There’s a new LCD Plug on its way – hopefully this one has the correct pinout.

Here’s the schematic:

Picture 7.png

This board is based on the same MCP23008 chip as the Expander Plug. Six outputs are used to drive the LCD panel (in 4-bit mode), one output controls power to the backlight, and one spare I/O line is available for a button or a LED – this is brought out to a 2-pin connector.

A breadboard version of a similar setup was described in an earlier post. The software to drive this plug has also been presented before.

The layout of the LCD Plug is as follows:

Picture 3.png

It departs slightly from the normal plug layout, because this board is intended to mount on the back of the LCD panel. There are two solder jumpers to select between PWR and +3V for both logic and backlight. R3 is a current limiting resistor for the backlight, it can be bypassed with a third solder jumper if not needed.

Now I get to play the waiting game again, to see if it comes out ok…

No LCD Plug yet

In Hardware on Oct 1, 2009 at 00:01

As mentioned before, the LCD Plug will need to be redone. The MCP23008 chip pinout was completely wrong…

But hey, it does look nice, doesn’t it?


Here’s how it’s supposed to go on the back of any LCD with a standard 16-pin header:


The board can be mounted very close to the LCD because it uses SMD parts:


Alternately, it could be flipped over and mounted alongside the LCD.

Note how the solder pad configuration is on the other side, so it can be adjusted once mounted. Not included in this first prototype is a small trim pot to adjust the contrast level.

So the real LCD Plug will take a bit longer – hopefully the next board will be without these crazy mistakes…

Another display option

In Hardware on Sep 29, 2009 at 00:01

Here’s another display option, using a 16×32 LED panel by Futurlec:


The pattern was hand-coded, to help me figure out which pin is which. The board has 3 16-bit shift registers and corresponding LED drivers. This display can be much brighter than in this picture, I’m using PWM to dim it way down and reduce average current consumption (I think it was 40 mA i.s.o. over 250 mA).

The board is powered from 5V and requires 6 output pins – I’m just using the DIO + AIO pins from three different ports for this test.

Note the 470 µF cap hooked up to the power lines of port 4 to help avoid glitches – switching 32 LEDs on and off at 1 KHz can generate a lot of noise!

Sample code for this display can be found here.

It’d be fun to have this display in a nice enclosure and drive it from packets received over wireless from another JeeNode!

Mounting options

In Hardware on Sep 27, 2009 at 00:01

I’ve been getting some questions about mounting options for the JeeNode lately. And once some plugs are out, that same issue is going to come up for these too, no doubt.

There are no mounting holes in the JeeNode. The reason for this is that it would increase the size by quite a bit (one hole isn’t enough, even two is debatable), which in turn could actually reduce the options for using a JeeNode since it would be larger than the current design.

The basic solution for this for me, is to treat the JeeNode as a module that plugs onto something else – be it a generic board such as the Jeeboard idea I described a while back, or a custom board created for a specific application.

Here is an example of a JeeNode with an LCD display:

Picture 2.png

Note that the JeeNode is mounted upside down, with the components facing downwards.

But where do you go from there? One approach would be to place a transparent acrylic plastic panel over this entire thing, with some stand-off’s to create a sandwich consisting of base panel, JeeNode and other components, and the acrylic panel on top. That’s the geeky way to do it: make the electronics visible!

I’m not sure I’d always want that. I also like things which are “closed”, with only control/display functions visible, and the whole thing having a nice and calm front plate.

This would be an intermediate solution – a clean front panel hiding all the techy stuff underneath it:

Picture 3.png

Still open on the side, where the connectors, power jacks, and even simple controls switches could be located.

The reason why all this awkward compromising is needed of course, is that with only single units (or say a few dozen kits), there is simply no way to get custom enclosures made that are affordable and also really look sweet – a JeeNode is not an iPhone!

Here are a few other options I’ve been pondering about:

  • Cardboard – design a fold-up box and print it out as template for a DIY cardboard enclosure. Would be very low-cost, but not very sturdy. And it looks a bit cheap…
  • Wood – this is the classical enclosure: a lot of work, but you can make it look any way you like, if you’re willing to go through all that. Takes a lot of craftsmanship.
  • Sandwiched foam – of the kind used as poster board for photographs. Again, a DIY design which you’d have to cut out and assemble yourself. Probably not easy to get real clean edges, cutouts, and holes. Not truly sturdy.
  • Acrylic front – as described above: a base pcb with everything mounted on it and the acrylic as cover on top. The you-can-see-what’s-inside option. Not easy to do yourself without a laser cutter or CNC router.
  • Aluminum front – this is a variation of the acrylic front. End result is no longer transparent, but creating a clean panel with labels is not trivial.
  • Prefab box – this is what many hobbyists do: get a plastic box, and drill holes and cut-outs in it, possibly also with an aluminum faceplate. Again, hard to get a nice clean result without a CNC router. But to be honest, I don’t think these (mostly black) boxes look that spiffy…

I really would love to find a nice generic solution for this. Right now, since pcbs are already being made completely to specification, and because they end up looking quite nice, I’m inclined to look for solutions with either the front, the back, or perhaps even both made of pcb material. With SMT components, most if not all through-holes can be avoided, leaving the back side available for all sorts of visible cutouts and labeling.

So what’s the solution? Just let everyone figure this out for themselves? Start saving for a laser cutter or a CNC router to be able to provide simple but accurately machined parts? To be honest, I wouldn’t mind doing the CNC thing – but until it becomes clear what a desirable result should look like, there’s little point picking any tool!

And then there’s plugs and daisy-chained plugs…

If you’ve got further suggestions or ideas on how to come up with kits that not only work well but also make the end result look really nice, please let me know!

Generalized LiquidCrystal library

In AVR, Software on Sep 26, 2009 at 00:01

Let’s dive into some C++ code for a change…

The Arduino version 0017 IDE has a nice library for driving standard character LCD’s, called… “LiquidCrystal”. The nice thing is that it mixes in the print() / println() functionality which is also used in the Serial library.

That makes it very easy to change a sketch to use either the serial port or the LCD screen to display results. Here’s how – let’s say you used this code in your sketch to print a value to the serial port:

Picture 2.png

The first thing to do is to generalize this slightly:

Picture 3.png

Functionally, nothing has changed. That’s the whole point, because you can develop and extend the sketch while testing everything via the serial port, as usual. Just use “Output” everywhere.

To change the output to the LCD, replace that “#define” line by the following lines of code:

Picture 4.png

This adds a bit of logic to easily switch between serial and LCD, without having to change a thing in the rest of the sketch. The “(2, 3, 4, 5, 6, 7)” parameters have to be set to the pin numbers actually used for your specific setup, of course.

So far, so good, although none of this applies to the I2C-connected LCD I’m working on. But by extending and re-organizing it a bit, it is possible to re-use most of the code of the original LiquidCrystal library.

The first thing I had to do was to rip apart the general and the pin-specific logic. That’s where C++’s abstract base classes and virtual member functions come in. I created a new “LiquidCrystalBase” class which has all the original code except for the pin-specific parts:


The “virtual … =0″ is C++’s funny way of saying it doesn’t need definitions for these three functions yet. The result is an incomplete class – you can’t create objects of type LiquidCrystalBase, you can only derive other subclasses from it. Tricky stuff, but bear with me…

Note that all the pin-specific member variables have been removed as well.

The next step is to re-define the original LiquidCrystal class in terms of LiquidCrystalBase:

Picture 5.png

If you compare this to the original code, you’ll see that most functions member definitions are missing. That’s because they have already been defined in what is now LiquidCrystalBase, and the new LiquidCrystal inherits everything from it.

What LiquidCrystal does add, is a definition and implementation for each of the virtual config(), send(), and write4bits() members. It is sort of “filling in” the missing functionality. So the result is… a LiquidCrystal class which does exactly the same as the original.

That seems pretty useless, but it isn’t – what we now have is a generic part and a specialized part. Which means it is now very easy to implement a variant which works exactly like the original, but going through the I2C port interface instead:

Picture 6.png

Nice. Only a little bit of new code was needed to create a completely different hardware interface which is fully compatible with the original LiquidCrystal class.

Here’s the original example, using this new LCD-via-I2C library:

Picture 8.png

Note that the I2C port is available as “myI2C” and can be shared with other devices by defining them here as well. You could even daisy-chain multiple LCD displays on the same port if you wanted to.

That’s all there is to it. The sketch code itself remains the same. Welcome to the world of abstraction, à la C++ !

Update – the final version and source code are described in this followup post.

LCD Plug prototype

In Hardware on Sep 24, 2009 at 00:01

For one of the numerous projects here at Jee Labs, I’m going to need a little 2×16 LCD panel hooked up to a JeeNode. Time to get the breadboards and jumpers out:


On the left is a breadboard-friendly 5V power supply.

The chip is an MCP23017, which is a 16-bit GPIO expander for the I2C bus. For the LCD, I only need 8 pins but this particular project needs a few more, hence the MCP23017 i.s.o. of the MCP23008. What’s the project? Well, for now let me just say that it’s about Volts and Amps (no, not because it’s an electrical circuit).

Here’s the MCP23008, as originally planned for use with LCD panels:

Picture 2.png

The chip is running at 3.3V, matching the I2C bus, but the LCD is powered at 5V. This should not be a problem since all connections on the LCD are inputs, and a 3.3V “high” level ought to be sufficient as 5V “high” level as well. The PNP transistor for backlight control will need a second resistor between B and E, though.

Next step is to come up with code for this I2C setup!

Bi-color LED

In Hardware on Sep 23, 2009 at 00:01

In trying to save precious I/O lines, I wanted to use one line for three states:

Picture 2.png

Red (logic 1), green (logic 0), and off (tri-state).

The way this SMD bi-color LED is connected is:

Picture 3.png

The idea being that together, the forward voltages of these LEDs is more than 3.3V, so they can’t both light up. Both are rated at 2.0 V typical (2.4 V max) forward voltage.

And in fact it almost works as expected. The only problem is that when the I/O pin is left floating, the red LED still lights up dimly – it’s hard to see in the photo, but it really does.

These LEDs are rated at 80 mA peak and 30 mA continuous, btw – so I figured I wouldn’t need a resistor, at least for the test. For a more permanent setup, a 100 Ω resistor between this circuit and the I/O pin would probably be better.

I wonder if there is a simple way to turn the red LED completely off – a 1N4148 diode between 3.3V and the red LED perhaps? That might also dim it a bit, since it’s brighter than the green one.

Geek news: this is post # 2^8 :)

Update – the extra diode helps. Here’s a better test, showing red, orange, green and off:

Picture 2.png

The “orange” isn’t truly convincing with the clear LED I’m using, but its reflection on a piece of white paper looks ok.

Four states on a single I/O pin!