We used AR 5212 wireless cards for our experimentation and driver used to run these cards was madwifi-0.9.4

Channel width of the wireless cards was modified by changing the frequency of the reference clock that drives the PLL. The clock value can be changed by writing to a register (AR5K_AR5212_PHY_PLL). We also configured the turbo mode of the card for this purpose (AR5K_AR5212_PHY_TURBO). madwifi-0.9.4 does not come with the capability to write to a register directly, so we also have to modify the driver to add this functionality.

More details:

- address of the turbo config register  AR5K_AR5212_PHY_TURBO  is 0x9804
- address of the PLL register AR5K_AR5212_PHY_PLL is 0x987c
- values of AR5K_AR5212_PHY_TURBO and AR5K_AR5212_PHY_PLL should be as follows:
	5 MHZ :  (0x00 and 0x2ea)
	10 MHZ : (0x00 and 0x1ea)
	20 MHZ : (0x00 and 0xea)
	40 MHZ : (0x03 and 0xea)
- inorder to write to these registers, we added an ioctl "writereg" that can be used with "iwpriv" command.

Actual script :: (works only on AR 5212 cards)
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## usage ./script 2412 5 ath1 (to set ath1 interface to channel 1 with 5 MHz bandwidth)

input=$1;

freq=`echo $input | cut -d " " -f 1`;
width=`echo $input | cut -d " " -f 2`;
iface=`echo $input | cut -d " " -f 3`;
# the fourth arg (epoch number) is for debug, not being used

#echo "freq=$freq width=$width iface=$iface";

iwconfig $iface freq ${freq}M

## set width
if [[ $width -eq 5 ]]
then
    iwpriv $iface writereg 0x9804 0x00
    iwpriv $iface writereg 0x987c 0x2ea
elif [[ $width -eq 10 ]]
then
    iwpriv $iface writereg 0x9804 0x00
    iwpriv $iface writereg 0x987c 0x1ea
elif [[ $width -eq 20 ]]
then
    iwpriv $iface writereg 0x9804 0x00
    iwpriv $iface writereg 0x987c 0xea
elif [[ $width -eq 40 ]]
then
    iwpriv $iface writereg 0x9804 0x03
    iwpriv $iface writereg 0x987c 0xea
else
    echo "Error: Not setting the width"
fi

exit 0;
#####################################


Driver modifications to enable the command "writereg"
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A patch to do this for madwifi-ng is available here: http://nms.csail.mit.edu/~mythili/cmap/madwifi-ng-patch
we made similar modifications to madwifi-0.9.4
####################################