AvaLAN Wireless AW900mSPI-10 - Manual (Page 20)

AW900mSPI User’s Manual
PAGE 20 Technical support (650) 384-0000 www.avalanwireless.com
channel information back from the XC1220. The host microcontroller must delay
while the XC1220 completes the scan before reading any data. In SPI mode, the
XC1220 will use the Data Ready line (pin 9) to indicate when the scan is complete
and the data is available. In UART mode, the host microcontroller needs to issue the
command and wait for data to be returned.
getRSSIReadings SPI Mode: 0x09 UART Mode: 0x08
b7 b6 b5 b4 b3 b2 b1 b0
Byte 1 SS3 SS2 SS1 SS0 EXP3 EXP2 EXP1 EXP0
Note that this command is an exception to the general rule that “get” commands
receive data and “set” commands send it. This command must be followed by send-
ing one byte of configuration information and then reading back a variable number
of data bytes.
SS0 to SS3 is the frequency step size (valid numbers are 1, 2, 4, 8).
EXP0 to EXP3 is the base-2 exponent of the number of samples to collect and aver-
age together at each frequency step.
EXP3...0 Number of Samples
0 1
1 2
2 4
3 8
4 16
5 32
6 64
7 128
8 256
The number of samples and frequency step size affects the amount of time it takes
to scan the band. A step size of 1 and exponent of 8 takes approximately 2 seconds
to scan the band. A step size of 8 and exponent of 32 takes approximately 300ms
to scan.
Although a higher step size and lower exponent scan much faster, a complete pic-
ture of the band may not be formed. Devices that only transmit for a very short
period of time may be missed with a fast scan.