M41ST87YMX6 datasheet(22/42 Pages) STMICROELECTRONICS

22 / 42 page

M41ST87Y, M41ST87W

22/42

Tamper Detection Operation

The Tamper Pins are triggered based on the state

of an external switch. Two switch mode options

are available, “Normally Open” or “Normally

Closed,” based on the setting of the Tamper Con-

nect Mode Bit (TCMX). If the selected switch mode

is Normally Open (TCMX = '1'), the Tamper Pin will

be triggered by being connected to VSS (if the

TPMX Bit is set to '0') or to VCC (if the TPMX Bit is

set to '1'), through the closing of the external

switch. When the external switch is closed, the

Tamper Bit will be immediately set, allowing the

user to determine if the device has been physically

tampered with. If the selected switch mode is Nor-

mally Closed (TCMX = '0'), the Tamper Pin will be

triggered by being pulled to VSS or to VOUT (de-

pending on the state of the TPMX Bit), through an

internal pull-up/pull-down resistor as a result of

opening the external switch.

When a tamper event occurs, the Tamper Bits

(TB1 and/or TB2) will be immediately set if TEBX =

'1.'

If the Tamper Interrupt Enable Bit (TIEX) is set to a

'1,' the IRQ/OUT pin will also be activated. The

IRQ/OUT output is cleared by a READ of the Flags

of the TIE Bit to '0,' or the RST output is enabled.

Note: In order to avoid an inadvertent activation of

the IRQ/OUT pin due to a prior tamper event, the

Flag Register (0Fh) should be read prior to reset-

ting the TEBX Bit.

The Tamper Bits are “Read only” bits and are reset

only by writing the Tamper Enable Bit (TEBX) to '0.'

The Tamper Detect function operates both under

normal power, and in battery back-up. Even if the

trigger event occurs during a power-down condi-

tion, the bit will be set correctly.

Sampling

As the Switch Mode Normally Closed (TCMX ='0')

requires a greater amount of current to maintain

constant monitoring, the M41ST87Y/W offers a

programmable

Tamper

Detect

Sampling

Bit

(TDSX) to reduce the current drawn on VCC or

VBAT (see Figure 17., page 19). When enabled,

the sampling frequency is once per second (1Hz),

for approximately 1ms.

When TEBX is disabled, no current will be drawn

by the Tamper Detection Circuit. After a tamper

event has been detected, no additional current will

be drawn.

Note: The oscillator must be running for Tamper

Detection to operate in the sampling mode. If the

oscillator is stopped, the Tamper Detection Circuit

will revert to constant monitoring.

Note: Sampling in the Tamper High Mode

(TPMX = '1') may be bypassed while on VCC by

connecting the TPXIN pin to VCC through an exter-

nal resistor. This will allow constant monitoring

when VCC is “On” and revert to sampling when in

battery back-up (see Figure 14., page 16).

Internal Tamper Pull-up/down Current

Depending on the capacitive and resistive loading

of the Tamper Pin Input (TPXIN), the user may re-

quire more or less current from the internal pull-up/

down used when monitoring the Normally Closed

switch mode. The state of the Tamper Current Hi/

Tamper Current Low Bit (TCHI/TCLOX) deter-

mines the sizing of the internal pull-up/-down.

TCHI/TCLOX = '1' uses a 1MΩ pull-up/-down re-

sistor, while TCHI/TCLOX = '0' uses a 10MΩ pull-

up/-down resistor (see Figure 18., page 19).

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