Motorola APX 5000 Service Manual page 54

Sec 1: 3-12 Basic Theory of Operation: Controller Section
The Bluetooth feature is accompanied by a Low-Frequency (LF) detection circuit that is also located
on the expansion board. The LF circuit provides the ability of a secure pairing connection with a
Bluetooth accessory. Once a radio has the Bluetooth feature enabled, a user can tap their LF
enabled Bluetooth audio accessory with the radio at the pairing spot to establish a secure Bluetooth
connection. The LF circuit uses a 125 kHz radiated signal to communicate the secure pairing
information between the Bluetooth accessory and low-frequency receiver. The low-frequency
receiver is programmed by the AVR32 processor through a dedicated SPI bus and transfers the
pairing data through a dedicated UART.
There is a digital accelerometer on the expansion board that detects the 3-axis force of gravity which
can be used to determine the radio's orientation. The accelerometer's position is communicated to
the AVR32 processor through a SPI bus.