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Figure 14: USRP IO Diagram
SPI, or Serial Peripheral Interface, is a standard used to communicate between electronic
devices. There are five digital lines: MOSI, or master out, slave in; MISO, or master in, slave
out; SCLK, the clock output by the master device; CSEL, or Chip Select; and an optional digital
reset line. The master is the FX2 USB 2.0 controller chip on the USRP [22]. The RFIC is a
slave. All slave chips are controlled by the master. The MOSI, MISO and SCLK lines are
shared between all devices. Master selects which slave device it will output to or take input from
with the CSEL lines. SPI is typically used to read and write data registers on microchips. The
USRP can interface with four devices via SPI: via the TX port on side A; via RX on side A; via
TX on side B; and via RX on side B. The SPI interface is controlled through GNU Radio in
Python. GNU Radio includes functions for the USRP to read and write SPI registers with
optional headers [21]. The RFIC chip on the RFIC daughterboard is connected to the RX SPI
port.
Each SPI register on the RFIC contains one byte, or eight bits, of information. To write a
register on the RFIC, two header bytes are written, then up to 64 bytes of data, which would
therefore set up to 64 registers. The first bit of the header is the write disable bit. It should be set
to zero to write a register. The last bit is an address auto-increment disable bit. If autoincrement is disabled, one may only write or read a single register in a single pass. If autoincrement is enabled, one may write or read up to 64 registers in a single pass. The middle 14
header bits contain the number of the register one wishes to write. The next bytes, up to 64,
contain the data to write to the registers. For example, if, in the header, the write disable bit is
set to zero, the auto-increment bit is set to zero, and the register number is set to 0 and 64 bytes
of data are sent after the header, then registers 0 through 63 on the RFIC would be set with the
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