Honey, I Shrunk The Arduino

As you might be able to tell from recent posts, I've been doing quite a bit of work with an Arduino. I've now got at least one project that I'd like to make a little more permanent, rather than it just being a bunch of components on a breadboard.

The easiest option would probably be to solder the components onto a prototyping shield (for example, these budget models from oomlout). There are two problems with this approach though. Firstly not every component is ideally suited for the form factor of an Arduino shield (for example a 16x2 character LCD takes up most of the prototyping space), and secondly to be any use a shield still needs to be connected to an Arduino. While Arduino's aren't extortionately expensive at just over £20 (depending where you buy them from), that's quite a cost to add to a project.

There are cheaper versions of the Arduino available, such as the Arduino Mini, which would bring the cost down to around £10 and which might be ideal for some projects. If you want to move your project onto a custom PCB though, you probably don't want to have to solder on a second PCB. The solution is to build the Arduino components directly into your circuit rather than relying on a pre-built solution. This should be cheaper than buying an Arduino and allows you to lay out the components in the most efficient way on a custom PCB.

The problem of course is figuring out how to construct an Arduino from the component parts. I spent quite a while trawling the internet trying to find the answers, but couldn't find a single page that collected everything you needed to know in one place, so I'm sure you can guess what the rest of this post is going to cover :)

The first decision you have to make is which Arduino compatible microcontroller you are going to base your circuit around. This will depend on the number of input/output pins you need as well as the size of the compiled sketch you intend to use. Given that I'm prototyping my ideas using an Arduino UNO, I decided to stick with the ATmega328P-PU chip the UNO is based around. This is by far the most expensive component you will need to buy, but you can pick them up for around £3. You will find that a number of places sell them with the Arduino bootloader programmed in, for which they charge a premium. Unless you are simply wanting to replace the chip in an existing Arduino UNO you don't actually need to use the bootloader, so save yourself a few pounds and buy the plain un-programmed chips.

Most of the tutorials on using a ATmega328P-PU expect you to run the chip at 16MHz (the same speed as the UNO) but this requires you to provide an external clock signal using a 16MHz crystal. As components mean costs, I decided I'd be a cheapskate and run the chip just using the internal 8MHz clock. We don't need any other components in order to programme the chip, but we do need to tell the Arduino software how to work with the chip.

Extra boards/chips are made available through the Arduino software by adding them to the boards.txt file in the hardware/arduino/ folder of the Ardunio installation (for full details of the format of this file try this Arduino wiki page). After a fair amount of messing around (starting from this example) I eventually figured out the following definition:

atmega328p8mhz.name=ATmega328P (8 MHz internal clock)

atmega328p8mhz.upload.protocol=stk500v1
atmega328p8mhz.upload.maximum_size=30720
atmega328p8mhz.upload.speed=9600

atmega328p8mhz.bootloader.low_fuses=0xE2
atmega328p8mhz.bootloader.high_fuses=0xDF
atmega328p8mhz.bootloader.extended_fuses=0x07
atmega328p8mhz.bootloader.unlock_bits=0x3F
atmega328p8mhz.bootloader.lock_bits=0x0F

atmega328p8mhz.build.mcu=atmega328p
atmega328p8mhz.build.f_cpu=8000000L
atmega328p8mhz.build.core=arduino:arduino
atmega328p8mhz.build.variant=standard

I'm sure that to most people (me included) that isn't self explanatory, so let's step through it to see what it means.

Firstly, line 1 simply assigns a human readable name to the board definition (this is what will apeartr on the Boards menu). Lines 3-5 state that we will be uploading to the chip using the right protocol to use the Arduino as the ISP (In System Programmer), the maximum sketch size, and the speed at which we will upload to the chip. Unfortunately, at least in version 1.0 of the Arduino software the upload speed seems to be ignored which will come back to bite us later.

Lines 7 to 11 specify the fuse settings. These are essentially used to configure the chip. I used Engbedded's fuse calculator to figure out the correct settings. Note that I'm not using the default fuse settings that the calculator suggests as I've turned off the Divide clock by 8 internally option. I found that otherwise the chip seemed to be running at 1MHz rather than 8MHz (i.e. the blink example sketch while setup to blink at 1 second intervals appeared to be actually blinking at 8 second intervals). I also specified the smallest bootloader space option, given that we aren't going to use one. Lines 13 and 14 specify the chip type and speed and finally lines 15 and 16 specify which internal libraries to compile the sketch against (in this example the standard variant which is the UNO).

So now that the Arduino software knows how to programme the board, we need to turn our existing Arduino UNO into an ISP which we can use to programme the chip. Now there is an example sketch called ArduinoISP bundled with the Arduino software, but try as I might I couldn't get it to work. Instead I'm using a slightly improved version released by Adafruit. Simply download the new version into your sketchbook folder and then upload it to your Arduino UNO the same way you would any normal sketch. Now wire the ATmega382P-PU you want to programme to the Arduino as shown to the left (this is also documented in the comments at the top of the ArduinoISP sketch, which also talks about adding LEDs for feedback; feel free to add these if you wish). Now in the IDE, from the Tools menu set the Programmer to ArduinoISP and the board to ATmega328P (8 MHz internal clock).

Now before we can actually programme the chip we need to set the fuses to the correct values as discussed above. To do this simply choose Burn Bootloader from the Tools menu (note that this is slightly misleading in that we aren't burning a bootloader as we didn't specify one in boards.txt, but this menu item sets the fuses as well as burning the bootloader if specified). Hopefully this will "just work", but for some reason my version of the IDE uses the wrong upload speed and as such this fails. Fortunately it's fairly easy to do manually. The easiest way is to get the IDE to generate the command line for you. To do this open the Preferences dialog from the File menu, and turn on the verbose output during upload. Now try to burn the bootloader again. It will still fail, but the first message will be the command line it is trying to use. On my machine this is:

/usr/share/arduino/hardware/tools/avrdude -C/usr/share/arduino/hardware/tools/avrdude.conf -v -v -v -v -patmega328p -cstk500v1 -P/dev/ttyACM0 -b19200 -e -Ulock:w:0x3F:m -Uefuse:w:0xFF:m -Uhfuse:w:0xD9:m -Ulfuse:w:0x62:m

This command is setting the baud rate to 19200 which is an awful lot faster than both the upload speed we specified in boards.txt and the baud rate specified in the ArduinoISP sketch, both of which use 9600. So simply change the -b19200 to -b9600 and execute the command manually (from a terminal or command prompt etc.). Hopefully that should work correctly. Once you have the fuses set, assuming you don't want to change the way the chip is configured, you won't need to do this step in the future.

Now you are ready to actually upload a sketch to the new chip. I usually test that everything is working using the simple Blink example that comes with the Arduino IDE. As you have already connected the Aruino to pin 13 of the new chip (this doubles as the SCK pin) I usually change the sketch to use pin 8 which is the bottom right pin if you have yours in the same orientation as the above diagram, so that you can see the sketch working without having to change any of the wiring used for programming. Now to upload to the new chip you need to use the "Upload Using Programmer" option from the File menu (making sure that the programmer and board are still set correctly) and not the Upload button on the toolbar. If setting the fuses failed the first time and you had to manually change the baud rate then you will probably run into the same problem here as well, but the solution is the same (run the command from the IDE then copy it out, change the baud rate, and then run it manually). Once it has uploaded successfully you should find your LED is blinking on and off at one second intervals.

Now, as the main aim was to remove the need to dedicate a full Arduino UNO to each project we need to power the chip in some other way than by being connected to the UNO. Fortunately this is also fairly simple as you can see from the setup to the left. This uses just four components; a voltage regulator, two capacitors, and a 9V battery connector.

I've actually found that the supply from a fresh 9V battery is stable enough that you can do without the two capacitors, but as they are stipulated by the datasheet for the voltage regulator I've included them.

Minimal Arduino Part List
Part	Unit Cost
ATMEL ATmega328P-PU	£3.02
5V Voltage Regulator	£0.40
100µF Capacitor	£0.151
9V Battery Connector	£0.60

These extra components are, as you can see from this table, much cheaper than the main ATmega328P-PU, and adding the two capacitors adds just 30p to the cost. In total the components add up to just £4.32 + VAT for a grand total of £5.19 -- or around half the cost of the Arduino Mini, or a quarter of the cost of an Arduino UNO. The circuit could be made even cheaper if you were willing to buy the components in bulk; for example if you buy 10 ATmega328P-PU then they cost just £1.63 each, so you could make 10 of these circuits for just £3.52 each.

The one thing I haven't tried yet is using any analog inputs with this setup. The datasheet for the ATmega328P-PU says that the AREF pin needs to be connected to power via a low-pass filter to remove noise if the analog to digital converter is being used. I've seen a number of websites that at least suggest this isn't necessary, especially if powering via a battery, rather than an alternating current based power source. I'll figure this out in due course but even if I do need to add extra components they will be priced in the pence so won't add much to the overall cost.