Category Archives: avr

version control of tools – part 2

Oh look! again! Remember kids, it’s not just compilers that you should keep around, but libc/binutils, and all libraries you’re ever going to use.

Today it’s avr-libc, and this gem:

In file included from ../../common/FreqCounter.h:17:0,
                 from ../../common/FreqCounter.c:35:
/usr/avr/include/util/delay.h: In function '__vector_21':
/usr/avr/include/util/delay.h:246:2: error: __builtin_avr_delay_cycles expects a compile time integer constant

Apparently it was always actually a requirement in the notes, but everyone did it anyway. Now, you need to write a wrapper function like this sort of shit:

void do_what_I_say_delay_us(int us) {
     while (us-- > 0) {

Hooray, isn’t that better for everyone! See for more information, if you can read carefully enough :)

version control of tools

Ahh, it’s always the right thing to do, but normally it’s so heavy to check in things like GCC/Binutils versions into every project. It’s the sort of thing you do if you’re a big serious company, working on big serious things, with big serious support. Besides, I’m not relying on anything weird or esoteric right?


../../common/pjrc/usb_debug_only.c:96:45: error: variable 'device_descriptor' must be const in order to be put into read-only section by means of '__attribute__((progmem))'
static uint8_t PROGMEM device_descriptor[] = {

Thanks for nothing avr-gcc updates. I understand what you’re getting at, but still, this used to compile, and put things in PROGMEM. Now it fails the compile. GCC bug in question is, or at least, that’s where it came from. A nice case of, “everybody’s using this code, but it was never actually meant to work, so let’s make it fail for them all, like it was meant to anyway.”

Compilers are awesome.

Code Size changes with “int” on 8bit and 32bit platforms

I was looking for a few bytes extra flash today, and realized that some old AVR code I had, which used uint8_t extensively for loop counters and indexes (dealing with small arrays) might not be all that efficient on the STM32 Cortex-M3.

So, I went over the code and replaced all places where the size of the counter wasn’t really actually important, and made some comparisons. I was compiling the exact same c file in both cases, with only a type def changing between runs.

Compiler versions and flags

platform gcc version cflags
AVR avr-gcc (GCC) 4.3.5 -DNDEBUG -Wall -Os -g -ffunction-sections -fdata-sections -Wstrict-prototypes -mmcu=atmega168 -funsigned-char -funsigned-bitfields -fpack-struct -fshort-enums
STM32 arm-none-eabi-gcc (GNU Tools for ARM Embedded Processors) 4.6.2 20120316 (release) [ARM/embedded-4_6-branch revision 185452] -DNDEBUG -Wall -Os -g -ffunction-sections -fdata-sections -Wstrict-prototypes -fno-common -mcpu=cortex-m3 -mthumb

And… here’s the results

counter type avr-size arm-none-eabi-size
unsigned int 1318 844
uint8_t (original) 1160 856
uint_least8_t 1160 856
uint_fast8_t 1160 844
int 1330 820
int8_t 1212 872
int_least8_t 1212 872
int_fast8_t 1212 820

I would personally say that it looks like ARM still has some work to go on optimizations. If _least8 and _fast8 take up more space than int it’s not really as polished as the avr-gcc code yet. For me personally, as this code no longer has to run on both AVR and STM32, I’ll just use int.

So, after extending this a bit, my original conclusion about the fast_ types not being fully optimized with arm-gcc were wrong. It’s more that, on AVR, your “don’t care” counters should be unsigned for smaller size, while on STM32, they should be signed (Though I still think it’s dodgy that int_least8_t resulted in bigger code than int_fast8_t) Also, even if signed is better in the best case, the wrong signed is also the worst case. Awesome.

Static analysis for AVR code with Splint

Over on #avr, static analysis came up, particularly, getting Splint to run. _abc_ put this guide together, which I’ve reproduced verbatim below.

Running splint(1) on avr-gcc source code

Version 0.0 - tested 15.05.2012

by _abc_

software versions:
splint 3.1.2 compiled from source
avr-gcc-4.5.1 as supplied by Atmel (binary build for Linux)

Splint is a static C source code analyzer which can help
keep C source files and headers tidy and consistent. It can
also prevent a lot of silly errors, which is more important
in embedded C code than elsewhere. Many embedded code error
signals involve smoke, sparks, and lost devices.

Splint was never designed to run on embedded C, with its
extensions. In addition to that, splint has not been maintained
that much. The Wikipedia article on splint mentions that is is
'not relevant' as software anymore and that its Wikipedia page
should be removed. I disagree with that statement.

The following shows how splint (3.1.2) can be run successfully
on abr-gcc source code in 2012.


1. Install splint (it compiles cleanly from source on any gnu
compiler equipped system, using ./configure && make && sudo
make install). Alternately, get the distribution specific
package of splint if it exists.

2. Make a copy of your embedded source files for backup in case
something goes wrong.

3. Get used to the fact that the splint manual page is not so
useful. Running splint -help [something] 2>&1|less is much more
so. The pdf manual which comes in the package of splint 3.1.2 is
dated 2003 (proudly, on the first page). That's okay, since the
C standard most used today is dated 1999 (C99)...

4. Copy the following commands into a shell command file in
your project directory, and edit the relevant parts (path to
avr installed toolchain include, source file name etc):

# 644
# V0.0 tested 15.05.2012 by _abc_ @
# run the splint static source analyzer on a avr-gcc C source code file,
# in a avr-gcc compatible way.
# initial revision, tested with avr-gcc, throws lots of warnings on avr-gcc
# includes but runs AS LONG AS:
# i. __fuses and __lockbits sections are hidden from splint using 
#    #ifndef S_SPLINT_S
#      ...
#    #endif
# ii. any binary constants (0b000f) in the C source are either rewritten as
#   decimal or hex numbers, OR their sections are guarded as above, and
#   binary or hex values are provided instead.
# Comment: the splint parser should be patched to accept binary constants.
# The usual makefile used for project building should have a new section
# (target) called lint or splint which should lint the source. It can be
# easily written by adding the code below:

## adjust to suit your installation
AVR_TOOLCHAIN_INSTALLED_LOCATION="/some/where/" ;# edit me

SRC="mysource.c" ;# edit me, ONE source file per shell file for now

DEFS="-D__AVR_ATmega8__" ;# etc, edit this

## actually run
splint \
	-preproc \
	-unrecogcomments \

5. while editing the required areas of the command file above, also 
read its header. It explains how the source can be splint proofed. It is
really easy. The -unrecogcomments comment is needed because there are
some semantic comments in the header files which throw errors. The other
ways to splint proof your code are explained by executing the commands:

  splint -help parseerrors 2>&1|less

in a shell.

6. splint should really be updated with embedded C extensions, especially
the 0b0001 style binary constant support.

7. avr-libc maintainers should run splint as above on sources which include
them (hello_world.c with just #include  and #include  are
enough for a start), as splint shows more than 90 warnings on running through
the avr-libc headers included by the simple main alluded to above at the
time of this writing.

-- the end --

Note, I haven’t actually used splint myself, on AVR code or regular desktop code. I do however endorse static analysis, having used PMD and friends on java code quite a lot. If you don’t like splint, or are looking for alternatives, perhaps something still being maintained, abcminiuser suggests Cppcheck, which covers both C and C++. There’s also the commercial Coverity and gimpel. I’ve not used any of these, but it seemed like such a worthwhile record that I was loathe to lose the discussion the depths of IRC logs.

Updated MRF24J40 arduino driver

Thanks to pkarck my changes to improve the C library for Microchip’s MRF24J40 802.15.4 modules has been ported to the Arduino library. I’d written an Arduino library some time ago, but hadn’t been maintaining it as I don’t really deal with Arduino much these days. However, it’s gotten some love :) It’s now easier to use, and has support for the power amplifier on the MRF24J40MB modules, if you’re using those. It also makes some quirks I was dealing with in a mixed xbee/mrf24j40 module environment optional. In general, this collects up some of the various reported issues over the last year or so, and I didn’t have to do anything at all!

This is, “teh awesome”.


Fridge Controller – temperature and motor run time, reporting over 802.15.4

One of my most finished projects, mostly because it’s simply much more important than having a battery powered thermometer sitting on a shelf. I brew beer, and serve beer in a converted fridge in my loungeroom. One day, the beer got slow, and what came out was suspicously cold. Turns out the thermostat was broken, so as long as the fridge had power, it was cooling, which resulted in deeply frozen kegs of beer.

It’s a pretty old fridge, that I got free, so I thought about simply getting a new (second hand) fridge and doing the conversion again. A newer fridge would probably be quieter, use less energy, all good things. But, not all fridges are the same shape, and finding one that fit three kegs, and doing all the work of the conversion again just felt like a lot of work. And goddamnit, I’ve been building sensor nodes, I always planned on having nodes that could control things too, so maybe I should just get down to business.

In the end there wasn’t much too it. I’m not doing any fancy PID control, just a set point, and a minimum motor run time, and minimum motor rest time. Initially, it didn’t even listen to any controls from my network, it reported temperature and motor status, but that was it. I had no beer! This was too important to have offline for weeks while I played around 24-480

I got a (massivly overspecced) 25A SSR on ebay, and after a bit of thought about how to keep the cost down, came up with the following schematic:

Full schematic for the fridge controller

The clever bit, if I can call it that, was to not even think about making my own power supply for the control logic. Big companies can make 10 gazillion CE marked, compact, safe switch mode power supplies, and consumers can throw them out by the gazillion. I was just going to have a normal euro 2 prong socket, and a surplus phone charger plug pack with the jack cut off and soldered to the board. Presto, cheap _and_ safe. Far far cheaper, faster and easier than I could have done myself. Of course, the extra socket and jacks take up space, but you can only have so much cake.

There’s really not an awful lot more to it than that. I tested this with a teensy board first, because I could use the nice friendly USB port for debugging, then switched to the ATtiny84, and after a bit of fiddling to get USI working for SPI, it all “just worked” I was suitably impressed when the fridge turned on and off at the right times :)

If you want to make this cheaper, you can just drop the 802.15.4 radio altogether. It worked well enough to keep my beer cold but not frozen before I finished the code to listen for new parameters over the air. But, being able to tweak it’s settings is a nice thing.

The TMP36, or similar, is wired up on a chopped up length of headphone cable. This is the bit that’s easiest to get wrong. Take care with the pinouts of whatever headphone socket you use, and the way you wire which lead to which part of the 3.5mm stereo plug. (If you get it wrong, you’ll read temperatures like 50, then 80, then 90 degrees Celsius, which will actually be _correct_ if you touch the sensor!)

Things I would have liked to have done:

  • Make the headphone socket mount flush on the wall of the box. Just takes more money and time to get the mounting perfect.
  • Use a panel mount socket for both input and output. It would be much tidier, but it takes yet more space, and yet again, more money

Other notes

The SSR I got really needs 3V+ to control. I was mistakenly feeding it with about 2V, from the wrong side of a resistor divider, early on in testing, and the red LED on the device would light up, so I expected it to properly be switching the live side. However, it seems 2V was enough for the LED, but not enough to actually switch. As soon as I gave it 3V, it behaved perfectly.

There’s no LCD display. Which might have been nice, but really, how often do you look at the temperature of your fridge? Besides, because it’s reporting every 10 seconds to “karlnet” it becomes just another node that the rest of my system stores in databases, graphs, or uploads to pachube

The software has a fairly nice way of working with saved state in EEPROM I learnt recently. I’m quite happy with it :) However, in general, the code is a little bit harder to read, because it contains all the debug for a teensy board, with #defines separating the live code from the test code. This is however a fully fledged real demo of my updated MRF24J40 library code


Parts list below.





25A SSR, 3-25V control, 24-480VAC output


7.99 US


grounded euro socket


195 ISK


grounded euro plug


195 ISK


Ungrounded euro socket


181 ISK


3 strand power cable


363 ISK


lunch box


499 ISK


Green LED









1 (optional)



3.5mm stereo socket, board mount




MCP1702, 3.3V regulator






1.58 US


1uF capacitors




5V DC plugpack/wallwart


500 ISK

Second hand store

2×3 pin header for AVR programming


sockets and header pins to comfort




The mouser parts should be available as Shared project 3411228a84 You can substitute something else for the TMP36, that’s just what I had wired up.

Because remember, Digikey are evil, and still refuse to recognise that the 802.15.4 encryption was removed from export restrictions years ago. Digikey, in their infinite wisdom REFUSE TO SHIP 802.15.4 modules to Iceland. We’re terrorists or something.

PetitFatFs SD card bootloader for ATmega2560 / Arduino Mega 2560

ChaN’s PetitFatFs, is a pretty neat plain C library dealing with Fat file systems (like on SD cards) with as little code as possible. It even includes sample bootloader code for AVR, which tries to open a file on an SD card, and if it exists, write it to flash, before booting the user application.

It’s a pretty neat base, though you may/probably will want to add a little bit functionality. The directions that come with the code are pretty good for getting this working. [1] However, for boards based around the ATmega2560, or ATmega2561, which contain >128k program space, a couple of things just don’t work…

First, the code for jumping into the user code:


This is fairly commonly seen pattern, but least with GCC 4.3.3 (WinAVR 20100110) this generates incorrect code. It gets turned into an EICALL, but without setting EIND. This results in the bootloader continually resetting into itself. (And, if the file on the sd card exists, repeatedly overwriting flash as fast as possible, not really very good for your flash lifetime!)

The alternative, which correctly generates a jump to the real 0, (These big ATmega’s have a 22 bit Program Counter, not a 16 bit one) is simply:

    asm volatile("jmp 0000");

The other required piece of the puzzle, is actually an AVR-libc FAQ item. You need to add -mrelax to your linker options.

As best I can tell, both of these additions should actually “do no harm” for regular sized AVRs as well, but I don’t have any boards handy to try that out with.

By the way, if you’re using the SD card on an arduino ethernetshield, below is the the only change you need in asmfunc.S. Just to save you from working out what the real name for “digital pin 4” is C land.

#define	DDR_CS	_SFR_IO_ADDR(DDRG), 5	// MMC CS pin (DDR, PORT)

Now, if only I could convince avrdude to erase all the flash, but only program the 4-8kb of bootloader, instead of insisting that it needs to program the entire 256k of flash each time I make a tweak to the bootloader. (139 seconds to program using a FabISP)

[1] The only thing I found unclear at first was setting up the “DI” and “DO” pins. Given that I was choosing pins that were on my mega2560, I assumed these were Data Out and Data In, when seen from the point of view of the mega2560. They are not, they are from the point of view of the SD card. (This is actually shown on the schematic image that comes with the PetitFatFs sample download)

Updated MRF24J40 library code, now easier to use

I’ve just substantially updated my library code for dealing with Microchip’s MRF24J40 modules on AVR microcontrollers, making it much much easier to use. It’s now based on callbacks for handling rx/tx packets, so you don’t need to know anywhere near as much about the module’s internals anymore. The tradeoff is a bit more ram usage.

mrf_reset and mrf_init now take parameters to ports and pins, so you no longer need to mess around with defines in the library headers any more.

#include "lib_mrf24j.h"
    mrf_reset(&PORTB, PINB5);   // reset pin
    mrf_init(&PORTB, PINB0);  // chip select pin

You no longer need to manually work with the interrupts, you just need to set up the ISR for the pin you’ve chosen to connect to the MRF24J40’s INT pin, for example, for my case, using INT0. (Remember to add a EIMSK |= _BV(INT0); somewhere too!)

ISR(INT0_vect) {

The interrupt handler takes care of reading in received rf data, and keeping it in a local buffer. It always contains the most recent packet. To do something with the received data, call mrf_check_flags() pretty often. It takes two function pointers as parameters, one which will be called for rx, and one for tx. Here’s a snippet from my main().

    // set the pan id to use
    // set our address
    while (1) {
        // Call this pretty often, at least as often as you expect to be receiving packets
        mrf_check_flags(&handle_rx, &handle_tx);
        if (time_to_send()) {
            mrf_send16(0x4202, 4, "abcd");

And here’s some example rx/tx callback handlers.

void handle_rx(mrf_rx_info_t *rxinfo, uint8_t *rx_buffer) {
    printf_P(PSTR("Received a packet: %d bytes long\n"), rxinfo->frame_length);
    printf_P(PSTR("Packet data:\n"));
    for (int i = 0; i <= rxinfo->frame_length; i++) {
        printf("%x", rx_buffer[i]);
    printf_P(PSTR("\nLQI/RSSI=%d/%d\n"), rxinfo->lqi, rxinfo->rssi);
void handle_tx(mrf_tx_info_t *txinfo) {
    if (txinfo->tx_ok) {
        printf_P(PSTR("TX went ok, got ack\n"));
    } else {
        printf_P(PSTR("TX failed after %d retries\n"), txinfo->retries);

The library code is available here, and also on github (lib_mrf24j)

Complete demo code for a module connected to a PJRC Teensy 2.0 board is here and also on github (You’ll probably have to edit paths in the Makefile)

FatFs with Arduino Mega 2560 and EthernetShield micro SD card

This is based on FatFs 0.8b, the latest release at the time of writing. We wanted to use the SD card slot on the standard arduino ethernet shield, and there’s a standard arduino provided SD library for working the sd card slot. However, despite the claims in the arduino FAQ, where it says, “… the C/C++ microcontroller libraries are under the LGPL” and “Using the Arduino core and libraries for the firmware of a commercial product does not require you to release the source code for the firmware. The LGPL does, however, require you to make available object files that allow for the relinking of the firmware against updated versions of the Arduino core and libraries. Any modifications to the core and libraries must be released under the LGPL.” this is actually untrue. Even on the SD card library reference page or Notes on using SD cards no mention is made of the fact that the SD library is actually GPLv3. Because the library is based on the GPLv3 sdfatlib

This is a pretty glaring omission, and a rather rude piece of code to just slip in.

Fortunately, there is also FatFs, and it’s tiny sister, PetiteFs, which are both under a BSD style license. They’re also plain C, and a lot more portable, with the sample code working on not just AVR, but ARM, i386, h8, and on and on.

So, I thought it should be possible to get that working with our mutant arduino here. Two websites were very useful in getting me started in the right direction, (in addition to the FatFs documentation)

What did I actually have to do?

  1. Download FatFs sample, and grab diskio.h, ff.c, ff.h, ffconf.h, integer.h and mmc.c and put them in my project
  2. Replace allll instances of BYTE with BYTEF (or anything else you want) in those files. BYTE is already defined (and differently) by the Arduino codebase
  3. In diskio.h, I had to add the following c++ wrapper
    #ifdef	__cplusplus
    extern "C" {

    at the start, and close it at the end.

  4. Edited the ffconf.h to suit, (This is up to you…)
  5. in mmc.c, I added #include "WProgram.h"
  6. in mmc.c, I replaced the #defines for CS_LOW and CS_HIGH with digitalWrite(4, LOW/HIGH). For the Arduino Ethernet shield, chip select is on pin 4.
  7. in mmc.c, the three power_* functions were stubbed out. The ethernet shield doesn’t support this.
  8. in mmc.c, the disk_timerproc function had the write detect and card inserted logic removed, again, the ethernet shield doesn’t support this.
  9. in mmc.c, because I wanted write support, I had to provide a function, DWORD get_fattime(), which simply returns 1. This is the file create time, and if you don’t really care what timestamp you get, a fixed value is fine. You would normally hook this up to an RTC or something.

    That’s about it. Then, just be careful to make sure that SPI is set up properly (taken care of if you’re using the ethernet on the ethernet shield) and, very importantly, make sure to include pinMode(4, OUTPUT) to make sure that the SD cards chip select pin is actually an output. If you forget this, it will sometimes work, and sometimes fail, and be very very very unpredictable.

    Oh, And you need to set up a timer to call disk_timerproc() every 10ms. The circleofcurrent page shows how to work around this with fixed loops if you’d prefer, but I just set up Timer2 to do this. This list of arduino resources used seems to imply that Timer2 is safe for us, but your mileage may vary. Here’s how we set up Timer2

            TCCR2A = _BV(WGM21);  // CTC MODE
    	TCCR2B = _BV(CS22) | _BV(CS21) | _BV(CS20); // clkdiv 1024
    	TIMSK2 = _BV(OCIE2A);			/* Output compare match interrupt */
    	OCR2A  = 156;         // 156 ticks at 16Mhz/1024 = ~10ms
            // Needs to be turned on before we start trying to wake up the sd card!
            // fatfs disk init code below...

    This was all done on an Arduino Mega 2560 board.

Remember, chip select pins need to be outputs!

I was struggling with (very) unreliable file reads and writes while trying to get FatFs running on an Arduino Mega 2560 board, with the standard Arduino Ethernet Shield. The ethernet connection was continually dropping and reconnecting as well.

What was missing? I hadn’t set the pin used for chip select on the SD card to be an output. As soon as I added that, my file operations became 100% reliable, and my ethernet connection stopped bouncing. Excellent!

(I’ll try and package up what we needed to get FatFs working, but it might not be today)