XBee‐PRO®900HP/XBee‐PRO®XSCRFModules
©2014DigiInternationalInc. 25
Low Power Operation
In general, sleep modes work the same on SPI as they do on UART. However, due to the addition of SPI
mode, there is the option of another sleep pin, as described below:
By default, DIO8 (SLEEP_REQUEST) is configured as a peripheral and is used for pin sleep to awaken and to
sleep the radio. This applies regardless of the selected serial interface (UART or SPI).
However, if SLEEP_REQUEST is not configured as a peripheral and SPI_SSEL
is configured as a peripheral,
then pin sleep is controlled by SPI_SSEL
rather than by SLEEP_REQUEST. Asserting SPI_SSEL by driving it low
either awakens the radio or keeps it awake. Negating SPI_SSEL
by driving it high puts the radio to sleep.
Using SPI_SSEL
for two purposes (to control sleep and to indicate that the SPI master has selected a
particular slave device) has the advantage of requiring one less physical pin connection to implement pin sleep
on SPI. It has the disadvantage of putting the radio to sleep whenever the SPI master negates SPI_SSEL
(meaning time will be lost waiting for the device to wake), even if that wasn't the intent. Therefore, if the user
has full control of SPI_SSEL
so that it can control pin sleep, whether or not data needs to be transmitted, then
sharing the pin may be a good option in order to make the SLEEP_REQUEST pin available for another purpose.
If the radio is one of multiple slaves on the SPI, then the radio would sleep while the SPI master talks to the
other slave, but this is acceptable in most cases.
If neither pin is configured as a peripheral, then the radio stays awake, being unable to sleep in SM1 mode.
XBee-PRO 900HP Configuration
The three considerations for configuration are:
•How is the serial port selected? (I.e. Should the UART or the SPI port be used?)
•If the SPI port is used, what should be the format of the data in order to avoid processing invalid
characters while transmitting?
•What SPI options need to be configured?
Serial Port Selection
In the default configuration the UART and SPI ports will both be configured for serial port operation.
If both interfaces are configured, serial data will go out the UART until the SPI_SSEL
signal is asserted.
Thereafter, all serial communications will operate on the SPI interface.
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