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Operation
Low power operation
Sleep modes generally 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. By default, Digi configures DIO8 (SLEEP_REQUEST) as a
peripheral and during pin sleep it wakes the device and puts it to sleep. This applies to both the UART
and SPI serial interfaces.
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 wakes the device or keeps it awake. Negating SPI_SSEL by driving it high puts the device to
sleep.
Using SPI_SSEL 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 device to sleep whenever the SPI master negates SPI_SSEL
(meaning time is lost waiting for the device to wake), even if that was not the intent.
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 device is one of multiple slaves on the SPI, then the device sleeps while the SPI master talks to
the other slave, but this is acceptable in most cases.
If you do not configure either pin as a peripheral, then the device stays awake, being unable to sleep in
SM1 mode.
Configuration considerations
The configuration considerations are:
How do you select the serial port? For example, should you use the UART or the SPI port?
n
If you use the SPI port, what data format should you use in order to avoid processing invalid
n
characters while transmitting?
What SPI options do you need to configure?
n
Serial port selection
In the default configuration both the UART and SPI ports are configured for serial port operation. In
this case, serial data goes out the UART until the device asserts the SPI_SSEL signal. Thereafter all
serial communications operate only on the SPI interface until a reset occurs.
If you enable only the UART, the devices uses only the UART, and it ignores the SPI_SSEL.
If you enable only the SPI, the device uses only the SPI, and it ignores UART communications.
If neither serial port is enabled, the module will not support serial operations and all communications
must occur over the air. The device discards all data that would normally go to the serial port.
Data format
The SPI only operates in API mode 1. The device does not support Transparent mode or API mode 2
(which escapes control characters). This means that only the AP configuration applies to the UART,
and the device ignores it while using the SPI.
XBee 868LP RF Modules User Guide
Configuration considerations
72
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