23.4.9.2 Low-power Tamper Detection and Anti-Tampering

Low-power debouncer inputs (WKUP0, WKUP1) can be used for tamper detection. If the tamper sensor is biased through a resistor and constantly driven by the power supply, this leads to power consumption as long as the tamper detection switch is in its active state. To prevent power consumption when the switch is in active state, the tamper sensor circuitry must be intermittently powered, and thus a specific waveform must be applied to the sensor circuitry.

The waveform is generated using RTCOUTx in all modes including Backup mode. Refer to the section “Real-Time Clock (RTC)” for waveform generation.

Separate debouncers are embedded, one for WKUP0 input, one for WKUP1 input.

The WKUP0 and/or WKUP1 inputs perform a system wakeup upon tamper detection. This is enabled by setting SUPC_WUMR.LPDBCEN0/1.

WKUP0 and/or WKUP1 inputs can also be used when VDDCORE is powered to detect a tamper.

When SUPC_WUMR.LPDBCENx is written to ‘1’, WKUPx pins must not be configured to act as a debouncing source for the WKUPDBC counter (WKUPENx must be cleared in SUPC_WUIR).

Low-power tamper detection or debounce requires RTC output (RTCOUTx) to be configured to generate a duty cycle programmable pulse (i.e., OUT0 = 0x7 in RTC_MR) in order to create the sampling points of both debouncers. The sampling point is the falling edge of the RTCOUTx waveform.

The following figure shows an example of an application where two tamper switches are used. RTCOUTx powers the external pull-up used by the tamper sensor circuitry.

The debouncing period duration is configurable. The period is set for all debouncers (i.e., the duration cannot be adjusted for each debouncer). The number of successive identical samples to wake up the system can be configured from 2 up to 8 in SUPC_WUMR.LPDBC. The period of time between two samples can be configured by programming RTC_MR.TPERIOD. Power parameters can be adjusted by modifying the period of time in RTC_MR.THIGH.

The wakeup polarity of the inputs can be independently configured by writing SUPC_WUMR.WKUPT0 and/ or SUPC_WUMR.WKUPT1.

In order to determine which wakeup/tamper pin triggers the system wakeup, a status flag is associated for each low-power debouncer. These flags are read in SUPC_SR.

A debounce event (tamper detection) can perform an immediate clear (0 delay) on the first half the general-purpose backup registers (GPBR). SUPC_WUMR.LPDBCCLR bit must be set.

Note that it is not mandatory to use the RTCOUTx pin when using the WKUP0/WKUP1 pins as tampering inputs in any mode. Using the RTCOUTx pin provides a “sampling mode” to further reduce the power consumption of the tamper detection circuitry. If RTCOUTx is not used, the RTC must be configured to create an internal sampling point for the debouncer logic. The period of time between two samples can be configured by programming RTC_MR.TPERIOD.

The following figure illustrates the use of WKUPx without the RTCOUTx pin.