The MPU can assert the PWRHLD pin via the SHDN command (which is a VDDBU-powered I/O) to shut down all regulators and enter Backup mode. All regulators are also shut down by the action of the SHDN signal. NRST is asserted low.
Depending on the presence of a backup supply (supercap populated) and by action on the wakeup signal connected at nSTRT (SW1 push button), the MCP16502 initiates a turn-on sequence.