AvaLAN Wireless AW900mSPI-10 - Manual (Page 5)

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AW900mSPI User’s Manual
Technical Summary
The AW900mSPI modules from AvaLAN Wireless have an on-board Baseband proces-
sor, the XC1220 from AvaLAN Wireless. The XC1220 handles all RF timing, data buff-
ering, AES encryption, RF data transfers and statistics gathering. The XC1220 must
be connected to a host microcontroller via a Serial Peripheral Interface (SPI) or
a Universal Asynchronous Receiver/Transmitter (UART). AvaLAN’s packaged radios,
such as the AW900XTR, include a host microcontroller that provides an Ethernet
interface. The purpose of this RF-only module is to allow you to create other ap-
plications and interfaces with your own electronics.
The host microcontroller is responsible for configuring the keys that the XC1220
uses for RF communication/encryption, as well as transferring data to and from the
XC1220. The XC1220 features an 8kB transmit FIFO and a 5kB receive FIFO. The RF
communication topology that the modules use is a point to multipoint star topology.
There is one RF master Access Point (AP) and up to 63 RF slave Subscriber Units (SU).
Data from the AP can be sent to one specific SU or broadcast to all SUs. Broadcast
data has no retransmissions and is not guaranteed to reach all SUs. Data from an SU
is always sent to the AP with retransmissions.
Data is divided up into blocks for RF transmission. This division of the data allows
for better interference immunity and re-transmission performance.
The digital interface to the AW900mSPI may be SPI or UART, depending upon which
firmware is running in the XC1220.
Serial Peripheral Interface (SPI) is a full duplex synchronous serial interface. SPI is
a master-slave interface, with the master providing the synchronous clock.
Universal Asynchronous Receiver/Transmitter (UART) is an asynchronous serial
interface that allows data to be transmitted without a clock signal, but the sender
and receiver of the data must agree in advance on the timing parameters and spe-
cial bits are added to each data byte to synchronize the sending and receiving units.
Selecting SPI or UART:
The choice of interface is up to the user and governed by the user's application and
the nature of the host microcontroller. UART is more common, being closely related
to RS-232. It is also places fewer demands on the host microcontroller. The UART
interface is limited to 115,200 bits per second, while SPI may be run as high as 7
megabits per second. Whether the AW900mSPI uses SPI or UART is a choice that is
controlled at boot up. By connecting a 10K resistor from Pin 8 (Error Flag) to Vcc
(pull up), the AW900mSPI will boot up in UART mode. If the resistor is connected
instead to ground (pull down), it will boot up in SPI mode.