PNI CompassPoint V2Xe - Manual (Page 8)

V2Xe User Manual – May 2012 Page 6
4.2 Mechanical Installation
4.2.1 Mounting Orientation
The V2Xe should be mounted horizontally and as level as possible. The arrow on the top
of the board should point in the direction of travel or line of sight.
4.2.2 Locate away from changing magnetic fields
It is not possible to calibrate for changing magnetic anomalies. Thus, for greatest
accuracy, keep the V2Xe away from sources of local magnetic distortion that will change
with time; such as electrical equipment that will be turned on and off, or ferrous bodies
that will move. Make sure the V2Xe is not mounted close to cargo or payload areas that
may be loaded with large sources of local magnetic fields.
4.2.3 Location-verification testing
Location-verification testing should be performed at an early stage of development to
understand and accommodate the magnetic distortion contributors in a host system.
Place the compass in a fixed position, then move or energize suspect components while
observing the output to determine when they are an influence.
4.3 SPI Hardware Protocol
The V2Xe is a slave SPI device. The host must supply the required eight clock pulses to
transfer a byte to and from the V2Xe. In addition a “Slave Select” (SSN) line must be held
low prior to any transmission.
The V2Xe assumes the clock polarity to be zero (CPOL=0) which means the clock idles low.
The clocking phase is shown in the diagram below, where CHPA = 0.
The V2Xe samples the “Master-Out-Slave-In” (MOSI) line during the rising edge of the
clock and on the falling edge of the clock, the “Master-In-Slave-Out” (MISO) will have the
next valid bit value.
The V2Xe communicates in half duplex mode. When the V2Xe is in receive mode (waiting
for valid bytes), it will transmit a zero-byte (0x00) while receiving bytes. During
transmission (response frames), the V2Xe will ignore the MOSI line but it is recommended
that the host transmit zero-bytes while receiving.