Datasheet

Operating modes M41ST87Y, M41ST87W
26/54 Doc ID 9497 Rev 10
2.7 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
connect mode bit (TCM
X
). If the selected switch mode is normally open (TCM
X
= '1'), the
tamper pin will be triggered by being connected to V
SS
(if the TPM
X
bit is set to '0') or to V
CC
(if the TPM
X
bit is set to '1'), through the closing of the external switch. When the external
switch is closed, the tamper bit (TB
x
) will be immediately set, allowing the user to determine
if the device has been physically tampered with. If the selected switch mode is normally
closed (TCM
X
= '0'), the tamper pin will be triggered by being pulled to V
SS
or to V
OUT
(depending on the state of the TPM
X
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
TEB
X
= '1.'
If the tamper interrupt enable bit (TIE
X
) 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 register (as seen in Figure 24 on
page 34), a reset of the TIE bit to '0,' or the RST
output is asserted.
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 resetting the TEB
X
bit.
The tamper bits are “read only” bits and are reset only by writing the tamper enable bit
(TEB
X
) to '0.' Thus, when servicing a tamper interrupt, the user should read the flags
register to clear the IRQ
pin, then clear the TEB
x
bit to clear the TB
x
flag, followed by setting
TEB
x
to again enable the tamper circuit.
The tamper detect function operates both under normal power, and in battery backup. Even
if the trigger event occurs during a power-down condition, the tamper flag bit(s) will be set
correctly.
2.8 Sampling
As the switch mode normally closed (TCM
X
= '0') requires a greater amount of current to
maintain constant monitoring, the M41ST87Y/W offers a programmable tamper detect
sampling bit (TDS
X
) to reduce the current drawn on V
CC
or V
BAT
(see Figure 17 on
page 23). When enabled, the sampling frequency is once per second (1Hz), for a duration of
approximately 1 ms.
When TEB
X
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 (TPM
X
= '1') may be bypassed while on V
CC
by
connecting the TPx
IN
pin to V
CC
through an external resistor. This will allow constant
monitoring when V
CC
is “on” and revert to sampling when in battery backup (see Figure 14
on page 20).