Introduction
Recently, I got my hands on a Raspberry Pi and one of the first things I wanted to do with it was to turn it into my complete AVR development environment. As part of that I wanted to make avrdude be able to program an AVR directly from the Raspberry Pi with no programmer. I know there is this linuxgpio programmer type that was recently added, but it is so recent that it isnât yet included in the repos and it also requires a compile-time option to enable it. I noticed that the Raspberry Pi happens to expose its SPI interface on its expansion header and so I thought to myself, âWhy not use this thing instead of bitbanging GPIOs? Wouldnât that be more efficient?â Thus, I began to decipher the avrdude code and write my addition. My hope is that things like this will allow the Raspberry Pi to be used to explore further embedded development for those who want to get into microcontrollers, but blew all their money on the Raspberry Pi. Also, in keeping with the purpose that the Raspberry Pi was originally designed for, using it like this makes it fairly simple for people in educational surroundings to expand into different aspects of small computer and embedded device programming.
As my addition to avrdude, I created a new programmer type called âlinuxspiâ which uses the userspace SPI drivers available since around Linux ~2.6 or so to talk to a programmer. It also requires an additional GPIO to operate as the reset. My initial thought was to use the chip select as the reset output, but sadly, the documentation for the SPI functions mentioned that the chip enable line is only held low so long as the transaction is going. While I guess I could compress all the transactions avrdude makes into one giant burst of data, this would be very error prone and isnât compatible with avrdudeâs program structure. So, the GPIO route was chosen. It just uses the sysfs endpoints found in /sys/class/gpio to manipulate a GPIO chosen in avrdude.conf into either being in a hi-z input state or an output low state. This way, the reset can be connected via a resistor to Vcc and then the Raspberry Pi just holds reset down when it needs to program the device. Another consequence which I will mention here of choosing to use the Linux SPI drivers is that this should actually be compatible with any Linux-based device that exposes its SPI or has an AVR connected to the SPI; not just the Raspberry Pi.
Usage
So, down to the nitty gritty: How can I use it? Well, at the moment it is in a github repository at https://github.com/kcuzner/avrdude. As with any project that uses the expansion header on the Raspberry Pi, there is a risk that a mistake could cause your Raspberry Pi to die (or let out the magic smoke, so to speak). I assume no responsibility for any damage that may occur as a result of following these directions or using my addition to avrdude. Just be careful when doing anything involving hooking stuff up to the expansion port and use common sense. Remember to measure twice and cut once. So, with that out of the way, I will proceed to outline here the basic steps for installation and usage.
Installation
The best option here until I bother creating packages for it is to  do a git clone directly into a directory on the Raspberry Pi and build it from there on the Raspberry Pi itself. I remember having to install the following packages to get it to compile (If I missed any, let me know):
- bison
- autoconf
- make
- gcc
- flex
Also, if your system doesnât have a header at âlinux/spi/spidev.hâ in your path, you probably need to install that driver. I was using Arch Linux and it already had the driver there, so for all I know its always installed. You also should take a look to make sure that â/dev/spidev0.0â and â/dev/spidev0.1â or something like that exist. Those are the sort of endpoints that are to be used with this. If they do not exist, try executing a âsudo modprobe spi_bcm2708â. If the endpoints still arenât there after that, then SPI support probably isnât installed or enabled for your kernel.
After cloning the repo and installing those packages, run the â./boostrapâ script which is found in the avrdude directory. This will run all the autoconf things and create the build scripts. The next step is to run â./configureâ and wait for it to complete. After the configure script, it should say whether or not âlinuxspiâ is enabled or disabled. If it is disabled, it was not able to find the header I mentioned before. Then run âmakeâ and wait for it to complete. Remember that the Raspberry Pi is a single core ARM processor and so building may take a while. Afterwards, simply do âsudo make installâ and you will magically have avrdude installed on your computer in /usr/local. It would probably be worthwhile to note here that you probably want to uninstall any avrdude you may have had installed previously either manually or through a package manager. The one here is built on top of the latest version (as of May 26th, 2013), so it should work quite well and be all up to date and stuff for just using it like a normal avrdude. I made no changes to any of the programmer types other than the one I added.
To check to see if the avrdude you have is the right one, you should see an output similar to the following if you run this command (tiny-tim is the name of my Raspberry Pi until I think of something better):
Valid programmer types are:
arduino         = Arduino programmer
avr910Â Â Â Â Â Â Â Â Â Â = Serial programmers using protocol described in application note AVR910
avrftdi         = Interface to the MPSSE Engine of FTDI Chips using libftdi.
buspirate       = Using the Bus Pirate's SPI interface for programming
buspirate_bb    = Using the Bus Pirate's bitbang interface for programming
butterfly       = Atmel Butterfly evaluation board; Atmel AppNotes AVR109, AVR911
butterfly_mk    = Mikrokopter.de Butterfly
dragon_dw       = Atmel AVR Dragon in debugWire mode
dragon_hvsp     = Atmel AVR Dragon in HVSP mode
dragon_isp      = Atmel AVR Dragon in ISP mode
dragon_jtag     = Atmel AVR Dragon in JTAG mode
dragon_pdi      = Atmel AVR Dragon in PDI mode
dragon_pp       = Atmel AVR Dragon in PP mode
ftdi_syncbb     = FT245R/FT232R Synchronous BitBangMode Programmer
jtagmki         = Atmel JTAG ICE mkI
jtagmkii        = Atmel JTAG ICE mkII
jtagmkii_avr32Â Â = Atmel JTAG ICE mkII in AVR32 mode
jtagmkii_dw     = Atmel JTAG ICE mkII in debugWire mode
jtagmkii_isp    = Atmel JTAG ICE mkII in ISP mode
jtagmkii_pdi    = Atmel JTAG ICE mkII in PDI mode
jtagice3Â Â Â Â Â Â Â Â = Atmel JTAGICE3
jtagice3_pdi    = Atmel JTAGICE3 in PDI mode
jtagice3_dw     = Atmel JTAGICE3 in debugWire mode
jtagice3_isp    = Atmel JTAGICE3 in ISP mode
linuxgpio       = GPIO bitbanging using the Linux sysfs interface (not available)
linuxspi        = SPI using Linux spidev driver
par             = Parallel port bitbanging
pickit2Â Â Â Â Â Â Â Â Â = Microchip's PICkit2 Programmer
serbb           = Serial port bitbanging
stk500Â Â Â Â Â Â Â Â Â Â = Atmel STK500 Version 1.x firmware
stk500generic   = Atmel STK500, autodetect firmware version
stk500v2Â Â Â Â Â Â Â Â = Atmel STK500 Version 2.x firmware
stk500hvsp      = Atmel STK500 V2 in high-voltage serial programming mode
stk500pp        = Atmel STK500 V2 in parallel programming mode
stk600Â Â Â Â Â Â Â Â Â Â = Atmel STK600
stk600hvsp      = Atmel STK600 in high-voltage serial programming mode
stk600pp        = Atmel STK600 in parallel programming mode
usbasp          = USBasp programmer, see http://www.fischl.de/usbasp/
usbtiny         = Driver for "usbtiny"-type programmers
wiring          = http://wiring.org.co/, Basically STK500v2 protocol, with some glue to trigger the bootloader.
Note that right under âlinuxgpioâ there is now a âlinuxspiâ driver. If it says â(not available)â after the âlinuxspiâ description, â./configureâ was not able to find the âlinux/spi/spidev.hâ file and did not compile the linuxspi programmer into avrdude.
Configuration
There is a little bit of configuration that happens here on the Raspberry Pi side before proceeding to wiring it up. You must now decide which GPIO to sacrifice to be the reset pin. I chose 25 because it is next to the normal chip enable pins, but it doesnât matter which you choose. To change which pin is to be used, you need to edit â/usr/local/etc/avrdude.confâ (it will be just â/etc/avrdude.confâ if it wasnât built and installed manually like above). Find the section of the file that looks like so:
programmer
id = "linuxspi";
desc = "Use Linux SPI device in /dev/spidev*";
type = "linuxspi";
reset = 25;
The âreset = â line needs to be changed to have the number of the GPIO that you have decided to turn into the reset pin for the programmer. The default is 25, but thatâs just because of my selfishness in not wanting to set it to something more generic and having to then edit the file every time I re-installed avrdude. Perhaps a better default would be â0â since that will cause the programmer to say that it hasnât been set up yet.
Wiring
After setting up avrdude.conf to your desired configuration, you can now connect the appropriate wires from your Raspberry Piâs header to your microchip. A word of extreme caution: The Raspberry Piâs GPIOs are NOT 5V tolerant, and that includes the SPI pins. You must do either one of two things: a) Run the AVR and everything around it at 3.3V so that you never see 5V on ANY of the Raspberry Pi pins at any time (including after programming is completed and the device is running) or b) Use a level translator between the AVR and the SPI. I happen to have a level translator lying around (its a fun little TSSOP I soldered to a breakout board a few years back), but I decided to go the 3.3V route since I was trying to get this thing to work. If you have not ever had to hook up in-circuit serial programming to your AVR before, perhaps this would be a great time to learn. You need to consult the datasheet for your AVR and find the pins named RESET (bar above it), MOSI, MISO, and SCK. These 4 pins are connected so that RESET goes to your GPIO with a pullup resistor to the Vcc on your AVR, MOSI goes to the similarly named MOSI on the Raspberry Pi header, MISO goes to the like-named pin on the header, and SCK goes to the SPI clock pin (named SCLK on the diagram on elinux.org). After doing this and double checking to make sure 5V will never be present to the Raspberry Pi, you can power on your AVR and it should be able to be programmed through avrdude. Here is a demonstration of me loading a simple test program I made that flashes the PORTD LEDs:
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kcuzner@tinyâtim:~/avrdude/avrdude$ sudo avrdude âc linuxspi âp m48 âP /dev/spidev0.0 âU flash:w:../blink.hex
[sudo] password for kcuzner:
avrdude: AVR device initialized and ready to accept instructions
Reading | ################################################## | 100% 0.00s
avrdude: Device signature = 0x1e9205
avrdude: NOTE: âflashâ memory has been specified, an erase cycle will be performed
        To disable this feature, specify the âD option.
avrdude: erasing chip
avrdude: reading input file â../blink.hexâ
avrdude: input file ../blink.hex auto detected as Intel Hex
avrdude: writing flash (2282 bytes):
Writing | ################################################## | 100% 0.75s
avrdude: 2282 bytes of flash written
avrdude: verifying flash memory against ../blink.hex:
avrdude: load data flash data from input file ../blink.hex:
avrdude: input file ../blink.hex auto detected as Intel Hex
avrdude: input file ../blink.hex contains 2282 bytes
avrdude: reading onâchip flash data:
Reading | ################################################## | 100% 0.56s
avrdude: verifying ...
avrdude: 2282 bytes of flash verified
avrdude: safemode: Fuses OK
avrdude done.  Thank you.
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There are two major things to note here:
- I set the programmer type (-c option) to be âlinuxspiâ. This tells avrdude to use my addition as the programming interface
- I set the port (-P option) to be â/dev/spidev0.0â. On my Raspberry Pi, this maps to the SPI bus using CE0 as the chip select. Although we donât actually use CE0 to connect to the AVR, it still gets used by the spidev interface and will toggle several times during normal avrdude operation. Your exact configuration may end up being different, but this is more or less how the SPI should be set. If the thing you point to isnât an SPI device, avrdude should fail with a bunch of messages saying that it couldnât send an SPI message.
Other than that, usage is pretty straightforward and should be the same as if you were using any other programmer type.
Future
As issues crop up, I hope to add improvements like changing the clock frequency and maybe someday adding TPI support (not sure if necessary since this is using the dedicated SPI and as far as I know, TPI doesnât use SPI).
I hope that those using this can find it helpful in their fun and games with the Raspberry Pi. If there are any issues compiling and stuff, either open an issue on github or mention it in the comments here.
Read More:Â Raspberry Pi as an AVR Programmer
