Oct
04
Filed Under (Random) by mark on 04-10-2005

I bought one of these a while ago, and discovered that the remote control didn’t work. After lots of looking at the board, and the help of an accomplished friend with a multimeter we were able to discover roughly what it does. In essence, it’s really dumb hardware :) There’s an IR demodulator on the end of a wire in a nice box that you put somewhere. That goes back eventually to a flip-flop. Curiously, the output from the demodulator is connected to the clock signal of the flip-flop. This causes the output to be passed through back to GPIO pin 5 of the bt878a chip, which then triggers an interrupt. I’m not clear on how it triggers an interrupt from the datasheet (it’s not GPINT, and in fact it doesn’t seem to set any status bits in INT_MASK). You then prod GPIO pin 4 which is connected to the reset line of the flip-flop. That in fact clears the interrupt, and off you go again. So basically the interrupts give you the edges of the IR pulse train.

With that information, you then have a driver time the interrupts. The supplied remote is RC5, and the edge transition bit pattern can be converted directly back to an RC5 code. Actually works quite nicely, and with a bit of tuning I have it feeling nice and smooth. Obviously the driver isn’t limited to just that remote, all you need to do is cook your own keymap. Since, weirdly, the remote seems to have the RC5 address set to TV, it interferes with my TV. So I cooked myself a keymap for another remote that I have (a Hauppauge grey remote, which is RC5 as well), and I use that instead. This nasty interrupt method seems to feel much smoother than polling the on-board IR thing on the PVR cards, although I will be working on fixing that.

I posted a linux driver to the video4linux list (one that uses the input layer, like ir-kbd-gpio or ir-kbd-i2c). You can probably find the patch in list archives (or if you are interested and can’t find it — ask and I’ll send it to you).

Anyway, it was a bit of fun and that’s my second remote control driver. I think I quite like playing with hardware.


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