AN3133 datasheet(11/43 Pages) STMICROELECTRONICS

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AN3133

Real-time clock overview

Doc ID 16956 Rev 5

11/43

1.5

RTC tamper detection

The RTC includes 3 tamper detection inputs. The active level can be configured

independently for each tamper input. Each tamper input has an individual flag (bit

RTC_ISR2_TAMPxF). A tamper detection event generates an interrupt when the

RTC_TAMPCR1.TAMPIE bit is set.

This interrupt can wake up the device from Active-halt mode.

Figure 5.

Example of tamper detection circuit

C1 C2 and C3 are optional (filtering can be performed by software).

The tamper inputs are sampled at a programmable rate from 1 Hz to 128 Hz (with RTCCLK

at 32.768 kHz). This reduces power consumption as the pull-up is applied only during the

precharge time, once every sampling period. Consequently, a trade-off must be made

between the sampling frequency, which impacts the tamper detection latency, and the

consumption due to the pull-up resistor.

Biasing can be performed using the MCU I/Os pull-up resistors (RTC_TCR2.TAMPPUDIS =

). When the precharge is enabled, the length of the pulse during which the internal pull-up

is applied is programmable from 1 to 8 RTCCLK cycles, in order to support different

capacitance values. The RTC_TAMPx pin level is sampled at the end of this pre-charging

pulse (see Figure 6). When the internal pull-up is not applied, the I/Os Schmitt triggers are

disabled in order to avoid extra consumption if the tamper switch is open.

Figure 6.

Tamper sampling with pre-charge pulse

Note:

In Figure 6, Point B indicates where input voltage sampling is performed.

TamperDet_01

STM8L

Device

RTC_TAMP1

RTC_TAMP2

RTC_TAMP3

Tamper

Switch

Tamper

Switch 2

Tamper

Switch 3

C1 (optional)

C2 (optional)

C3 (optional)

RTCCLK

TamperDet_02

Floating

Input

Switch

opened

Switch

closed

1 cycle pre-charge

2 cycles pre-charge

4 cycles pre-charge

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