Tag Archives: SPI

Remember, chip select pins need to be outputs!

Posted by KarlP on 27 May, 2011 None comments

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)

MRF24J40 with AVR SPI (atmega32u4) part2

Posted by KarlP on 20 February, 2011 3 comments

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

Posted by KarlP on 20 February, 2011 2 comments

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)