Category Archives: wireless

MRF24J40 with arduino (teensyduino)

Earlier, I’d been working in pure C land, but I know not everyone uses pure C, and sometimes, the arduino environment is a nice easy way to get something prototyped.

So, I turned my mrf24j40 library into an arduino library! It supports all the basic stuff for sending and receiving 802.15.4 frames in a non-beacon network. It’s been tested so far with teensyduino, but it doesn’t use any teensy specific features, so it should work just fine on any arduino style board that has SPI and three spare pins for the reset, interrupt and chip select pins.

You can get the library from github, and like any arduino library, just extract it into your arduino/libraries directory. There are examples for tx only, rx only, and two way data.

Get the MRF24J40 arduino library

The examples are about the extent of the documentation so far, but just email me with questions!

MRF24J40 with AVR SPI (atmega32u4) part3

Update 2011 Oct 6 See Christopher’s comments below! Specifically, line 172 might need to be removed, if you’re using this with a pure mrf24j40 network, or a network that doesn’t contain xbee series 1 devices!
Update 2011 May 29 The sample code and library have been substantially upgraded. See my newer post for the details

And now I have TX working too. The microchip datasheet is _reallly_ sparse on details, and you have to go and get a copy of the actual IEEE 802.15.4 spec (either 2003, or 2006, we’re only concerned with the specification of the MAC header)

I had some problems trying to reconcile bit ordering between the IEEE spec (MSB rightmost, most of the time) and the Microhip docs (MSB leftmost, most of the time) but from there it pretty much just worked. I have yet to try out acknowledgements, I was trying to keep it pretty simple, but working code is working code.

Except… The microchip docs say that you write out one byte with the header length, one byte with the frame length, (header plus packet body) then the header, then the packet body. No gaps anywhere. Try as I might, I had to leave a two byte gap after the header, and before the packet data, when writing to the MRF24J40’s tx normal fifo. (And allow for this in the “length” fields)

I thought I had the addressing modes wrong, and my first two bytes of packet data were being used as some sort of header field I didn’t understand, but I tried out 4 different addressing modes, and in all cases, the data on the received side passed all checksums, and was only consistent with there being a _gap_ in the fifo.

Strange, but workable, as long as you know it’s there. It still makes me feel uncomfortable though.

For reference, I’m using 16 bit (short) addressing for both source and destination, PAN ID header compression (only destination PAN ID is sent) with no security.

The library code is over at github and allows sending packets to existing series one xbee listeners just like this…

mrf_reset();
mrf_init();
// set our PAN ID
mrf_pan_write(0xcafe);
// set our local source address
mrf_address16_write(0x6001);
// send something to a given node
mrf_send16(0x4202, 4, "abcd");

MRF24J40 with AVR SPI (atmega32u4) part2

Update 2011 May 29 The sample code and library have been substantially upgraded. See my newer post for the details.

Good progress last night and today. I get an interrupt for each received packet, working in both promiscuous mode and normal mode.

A fair bit of code to get here, compared to using xbees, but SPI is much much faster, and you don’t need to dick around with baud rates and getting clock timing perfect. UARTs are terrible. From the datasheet, and the little bit of information you can find on the web, it seems that these modules have some rather odd silicon errata. It seems the original datasheet was completely wrong. This is the only thing that I can think of to explain why you need to set up so many registers just to get it to work. The defaults in the silicon are all worthless. Oh well :) It works :)

Note to self: be careful to use mrf_read_long() when you want to use a long address, the address spaces overlap, so using mrf_read_short works, just doesn’t return anything useful.

Working code to get this far, with an AVR host is over at github

MRF24J40 with AVR SPI (atmega32u4) part1

Ouch, this took a lot longer to get started than I thought. SPI is meant to be easy right? All my SPI reads from the MRF24J were returning 0xff.

Turns out that, even with the /SS pin disconnected, if you don’t explicitly set it as an output pin in the DDR register, the AVR falls out of SPI mode if it ever goes low.

So, even though the pin is not connected, (I’m using a general IO pin to do chip select on the radio module) nothing worked until I explicitly made it an output.

And now, presto, I can read data from the MRF24J properly now! Now we can finally move on to the rest of this bring up.

(The MRF24J40 is an 802.15.4 module, with a SPI interface. It’s about 6€, vs about 20€ for xbees, and is on a standard 2.54mm pin spacing, instead of xbee’s 2mm spacing)