Saturday, March 5, 2011

AR9280 TX errors - why it's important to care about how your hardware works..

One of the issues that I've been having with my AR9280 NIC is that MCS rates aren't being reliable.

Having fiddled with the TX power control with the AR9160 (and I'll write about that soon!) I remembered that badly-configured TX power control would result in the transmit side spitting out a (sometimes very) unclean spectral mask, with noise all over the place.

I did a little digging into how the AR9280 TX-side calibration works. It turns out that there's two methods of TX power control - closed-loop (with a power-detector ADC which compares the signal to a calibrated level) and open-loop (which I'm not yet sure about.) The AR9280 NIC that I have has the "open loop TX power control" bit set in the EEPROM, which indicates it needs open-loop TX power rather than closed-loop. A little digging shows that FreeBSD's HAL doesn't include closed-loop TX power control or any of the newer AR9280 calibration code (in particular the open-loop temperature compensation code.)

Long-story short, I've added the open-loop TX calibration and temperature compensation code from ath9k to the FreeBSD HAL (but I haven't yet committed it and won't until I figure out how to make it tidy) and suddenly all MCS rates TX perfectly fine.

So if you're using an AR9280 NIC, please keep a look out for when I commit this stuff and let me know if it improves things.

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