Low power operation

Sleep modes generally work the same on SPI as they do on UART. However, the addition of SPI mode provides an option to configure another pin as a 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.

SPI_SSELcan be configured to both control sleep and to indicate that the SPI master has selected a particular slave device. This configuration provides an advantage where the pin sleep implementation on SPI mode requires one less physical pin. It does have the disadvantage that it puts the device to sleep whenever the SPI master unintentionally negates SPI_SSEL.

To effectively use the pin sharing configuration, the user/design must have control of the SPI_SSELpin to the extent that it can control pin sleep. This makes the SLEEP_REQUEST pin available for a different purpose. Without control of SPI_SSEL while using it for sleep request, the device may go to sleep at inopportune times.

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.