my rants and rambles

While building a MIDI controlled LED suit we accidentally damaged 2 of the Arduino Mega pins. There was no output from Digital 4 and 5 , just floating.

luckily we had some Atmega640 100 pin ICs around. They are from the same family as the Atmega2560 and are pin compatible but has a quarter of the memory.

The only problem is that the Arduino mega 2560 bootloader can't be "just uploaded" to the 640.

I found some source files from http://www.avr-developers.com/bootloaderdocs/index.html (link to file) and then spent a few hours down rabbitholes to compile the bootloader.

There is a pre compiled bootloader for the AtMega640, but it uses UART1 whereas Arduino boards are laid out to use UART0. to save you the effort of recompiling for the 640 the compiled file is on GIThub. I uploaded it to the 640 using an AVRISP MKII, but you can use any flashing tool that works for you. the fuse settings are the same as for the 2560 ( i think we might get away with a smaller bootloader section, but i kept it the same anyway)

mega640.bootloader.low_fuses=0xFF

mega640.bootloader.high_fuses=0xD8

mega640.bootloader.extended_fuses=0xFD

mega640.bootloader.lock_bits=0x0F

add this to your \arduino install folder\hardware\avr\boards.txt

############################################################## mega640.name=Arduino Mega 640 mega640.cpu=640 mega640.upload.tool=avrdude mega640.upload.protocol=wiring mega640.upload.maximum_size=61440 mega640.upload.speed=115200 mega640.bootloader.tool=avrdude mega640.bootloader.low_fuses=0xFF mega640.bootloader.high_fuses=0xD8 mega640.bootloader.extended_fuses=0xFD mega640.bootloader.file=stk500v2/stk500boot_v2_mega2560.hex mega640.bootloader.unlock_bits=0x3F mega640.bootloader.lock_bits=0x0F mega640.build.mcu=atmega640 mega640.build.f_cpu=16000000L mega640.build.core=arduino mega640.build.variant=mega

##############################################################

I recently build a rather large project using Arduino and found myself running out of program space... not SRAM, didnt have any sporadic restarts or "hangs" or glitchy behavior,

just a message saying :

" 33912 bytes (of a 28672 byte maximum) , sketch too big "

I didn't see that coming at all.. mostly because i was doing 2 separate parts of the application and testing them separately, I then combined them and this happened... so here are a few things one can do if you run out of space in Arduino.

don't use floating point math if you don't need to, avoid datatypes like "double" and "float" - in arduino they are the same thing...

The reason is that the Atmega 8 microcontroller does not have a floating point unit... so it is all implemented in software libraries, eating about 2 kbytes of program memory ( FLASH ). So I removed any floating point operations I Had and cleaned up some "if - else" statement and recompiled...

" 31930 bytes (of a 28672 byte maximum) sketch too big "

that's just over 2K of memory, but still too large.. another thing one can do is to tidy up your code, look for places where you do things that are redundant redundant like having multiple "Serial.print("some text"); statements where 1 would be fine. this is a silly example: 

  switch(inputValue)   {   case '1' :     Serial.println("The value was 1");     break;   case '2' :     Serial.println("The value was 2");     break;     case '3' :     Serial.println("The value was 3");     break;     default:       break;   } Can be Replaced by something like :

Serial.print("The value was "); Serial.print(inputValue);

this was a silly example, but its the most simplified way of seeing the error that one can make and how often one has a lot of redundant code... which adds up  eventually. I'll do more posts about memory optimisations like :

using the correct data types ( even in the arduino core files )

avoid too many global variables

removing unused code / things stored in memory that aren't used

removing data that is initialised to a value just be changed again

using C Macros

direct port manipulations

using "inline and "static" functions

smaller buffers for things ( if you know how much data you expect from an SPI or I2C device)

pointers ( this goes with data structures )

The first useful thing I tried was to get airplay working with a Rpi, turns out its

already been done

 really easy

 not sure why my iPad doesnt always pick it up

You will need

a Raspberry pi

an SD card ( 2GB or larger, and a fast one -Class 10 is nice- works great)

form of power for the Rpi ( usb cable works )

a set of speakers that has a 3.5mm Jack, or a male-male 3.5 mm cable if the speakers/amp has a socket, or and amp that has HDMI input ( both video and sound goes thru there), or a TV that takes composite input and a 3.5mm jack to composite ( left and right audio ) cable.. If it can play sound or show video, there are adapters that you can use to go from the Rpi to whatever device you like.

a display device would be nice to help you set up XBMC once its installed

a network connection ( its airplay over WiFi or Ethernet only, no Bluetooth yet)

first, goto raspbmc , and download whichever installer suits you;

there are 4 options ,all fairly easy

installing from a windows machine onto an SD card, The file is small, but it downloads the newest image from a repository when the Rpi boots, so Rpi needs an internet connection

installing from an OS X machine onto an SD card, The file is small, but it downloads the newest image from a repository when the Rpi boots, so Rpi needs an internet connection

a small image to unpack onto the SD card using the details from here

the Full image (~220 MB) to unpack onto the SD card using the details from here

both the last 2 options use this program ( if you are using windows)

first thing to do after you have your image is to format it as FAT32, then upload /unpack your image ( depending on which OS you are using and which image file you have)

once that's done,

insert the SD card into your Rpi,

add network,

add the screen and audio,

give it power.

it should boot up with this image

and then it will load up XBMC.

you will find that its very slow the first time( XBMC will download all the available updates from RaspBMC.

meanwhile : Enable  Airplay under System > Network > “Allow XBMC to receive AirPlay content”