37.3.9 Sleep Mode Operation

The UPDI PHY layer runs independently of all sleep modes, and the UPDI is always accessible for a connected debugger independent of the device’s sleep state. If the system enters a sleep mode that turns the system clock off, the UPDI cannot access the system bus and read memories and peripherals. When enabled, the UPDI will request the system clock so that the UPDI always has contact with the rest of the device. Thus, the UPDI PHY layer clock is unaffected by the sleep mode’s settings. By reading the System Domain in Sleep (INSLEEP) bit in the ASI System Status (UPDI.ASI_SYS_STATUS) register, it is possible to monitor if the system domain is in a sleep mode.

It is possible to prevent the system clock from stopping when going into a sleep mode by writing to the Request System Clock (CLKREQ) bit in the ASI System Control A (UPDI.ASI_SYS_CTRLA) register. If this bit is set, the system’s sleep mode state is emulated, and the UPDI can access the system bus and read the peripheral registers even in the deepest sleep modes.

The CLKREQ bit is by default ‘1’ when the UPDI is enabled, which means that the default operation is keeping the system clock in ON state during the sleep modes.