60.6.6.3 Automatic Backup Mode

The SECUMOD can switch automatically to Backup mode through a specific protocol involving EXT_IRQ and NRST product pins. This protocol ensures that the SECUMOD does not receive core alarms when VDDCORE is removed. This may occur if VDDCORE is dropping too fast with respect to the time needed by the software to switch to Backup mode.

The only way to avoid this is to anticipate this event soon enough with an external event signaling that the VDDCORE is going to be removed. The EXT_IRQ has been chosen for the external event.

In this protocol, the VDDCORE must be removed only while the NRST pin is low and user reset is enabled in the system controller.

The Automatic Backup mode must first be enabled by writing a '1' to SECUMOD_CR.AUTOBKUP.

If a low-level transition on the EXT_IRQ pin is detected while the Automatic Backup mode is on, the SECUMOD memorizes the current mode setting and switches to Backup mode. It takes up to 5 TD_SLCK cycles to complete.

While the Automatic Backup mode is set, all intrusions generated out of the backup domain are ignored by the SECUMOD.

If a high-level transition on the EXT_IRQ pin is detected while the Automatic Backup mode is on, the SECUMOD mode switches back to the mode it was set to before the low-level transition on EXT_IRQ. It takes up to 5 TD_SLCK cycles to complete.

Warning: The mode cannot be changed to Normal mode by software when being automatically set to Backup mode, between the EXT_IRQ low transition and the EXT_IRQ high transition, if Automatic Backup mode is on. Due to asynchronism between the system clock and TD_SLCK, that cannot be managed safely. To avoid any issue, any attempt to try and change this mode has no effect. To change the mode, first disable the Automatic Backup mode and then set the new mode manually.

If a low level is applied on the NRST pin while the Automatic Backup mode is on and the user reset is enabled in the Reset Controller, the Automatic Backup mode is reset to off in a maximum time of 5 TD_SLCK cycles.

If the Automatic Backup mode is disabled, it does not modify the current mode, but future events on the EXT_IRQ pin will have no effect as long as the Automatic Backup mode is off.

Note that EXT_IRQ is always able to generate an interrupt on the processor, whatever the value of the Automatic Backup mode. See SECUMOD_GPSBR for the selection of the external IRQ signal.

Figure 60-19. Automatic Backup Protocol