I set up my rasp pi b+ and U3 mining rig using the instructions in this forum post, with screen and cgminer.
The command relevant to over clocking the raspi that I used is the following:
sudo ./cgminer --bmsc-options 115200:0.57 -o POOL -u USERNAME -p PASSWORD --bmsc-voltage 0800 --bmsc-freq 1286
Those last two options (bmsc-voltage and bmsc-freq) are what over clocked the rasp pi. But I think I had to compile cgminer and configure it to include bmsc. Mine typically runs with a hash rate in the high 50's or low 60's, but maybe not quite 62-63 GH/s.
Also, I did try to push the U3 voltage and frequency higher by playing with the settings as detailed in the U3 manual. However, I could never get the U3 to run stably at settings any higher than what I gave above. I'm not sure if it makes a difference, but I'm actually running two U3's on the one rasp pi and cgminer instance.
Update: In the interest of providing a complete answer in one place, here's the full text from the linked forum post:
Run a fresh install of Raspbian using NOOBS, after the install type ifconfig
and make a note of the IP address, so that you can remotely
SSH into your Pi. Login via PuTTY with the default username/password,
then raspi-config if you would like to change the hostname, default
password and so on.
Now update the Pi:
sudo apt-get update && sudo apt-get upgrade
Install dependencies:
sudo apt-get install libusb-1.0-0-dev libusb-1.0-0 libcurl4-openssl-dev libncurses5-dev libudev-dev screen libtool automake pkg-config libjansson-dev screen
Clone cgminer from here:
git clone https://github.com/bitmaintech/cgminer
Install, configure and also enable bmsc options:
cd cgminer
sudo ./autogen.sh
export LIBCURL_CFLAGS=’-I/usr/include/curl’
sudo ./configure --enable-bmsc
sudo make
Test cgminer is working correctly by running the following (input your
own pool, username and password):
sudo ./cgminer --bmsc-options 115200:0.57 -o POOL -u USERNAME -p PASSWORD --bmsc-voltage 0800 --bmsc-freq 1286
Type Q to exit cgminer...
Now, add cgminer to start automatically when the Pi is powered on and
create a screen session:
sudo nano /etc/rc.local
Remember, Ctrl-X to exit, then type Y and press Enter to save changes. Add this just above
exit 0, again enter your own pool, username and password:
cd /home/pi/cgminer
screen -dmS cgminer ./cgminer --bmsc-options 115200:0.57 -o POOL -u USERNAME -p PASSWORD --bmsc-voltage 0800 --bmsc-freq 1286