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AN1879
Application note
How to use the M41ST87W tamper detect and RAM clear
Introduction
The M41ST87W is a supervisory family circuit that provides the industry with the latest in onchip
security solutions. The tamper detection and RAM clear circuit can be used in any system to protect
sensitive data from tampering. This chip can be used to secure a wide range of applications from credit
card machines and point-of-sale (POS) terminals to electric data meters. The M41ST87W features the
ability to detect and timestamp any tampering of the system, and corrupt the device memory when the
event occurs. This prevents the intruder from accessing data stored in memory by clearing the device
memory and/or external RAM when the tampering event occurs.
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Contents
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Contents
1
Description....................................................................................... 3
1.1
How it works ...................................................................................... 3
1.2
Clearing the external memory with the tamper registers ................... 3
1.3
Clearing the external memory with an external charge pump ........... 3
1.4
RAM clear data ................................................................................. 5
1.5
Tamper timestamp ............................................................................ 5
2
Conclusion ....................................................................................... 6
3
Revision history .............................................................................. 7
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Description
1
Description
1.1
How it works
The M41ST87W device provides two independent tamper input pins, TP1IN and TP2IN that
can be used to monitor two separate signals. These two tamper input pins can be set to
indicate that a tamper event has occurred by either 1) closing a switch (normally open) to
ground or VOUT or 2) opening a switch that was previously closed (normally closed) to
ground or VOUT. The closing and opening of the switch is configurable using bits that are set
in the tamper registers.
The M41ST87W device includes 128 bytes of internal RAM that the user has the option of
clearing by setting the TEB and CLR bits in the tamper registers.
1.2
Clearing the external memory with the tamper registers
The M41ST87W can also clear the external, battery-backed up SRAM of the device by
setting the TEB and CLREXT bits in the tamper registers. To clear/corrupt the external
memory, VCC of the SRAM can be taken to ground. However, certain SRAMs require a
significant amount of time for the memory to be corrupted if VCC is simply grounded. To
corrupt the memory in a reasonable amount of time, one can take VCC of the SRAM to a
negative voltage. By taking VCC to a negative voltage, the input protection diode turns on
and goes into conduction mode so that it corrupts the memory.
1.3
Clearing the external memory with an external charge pump
An external charge pump device should be used with the M41ST87W to drive VCC of the
SRAM to a negative voltage during the tamper condition. Figure 1: "Circuit connections"
shows how to connect this circuit. When using the M41ST87W with the charge pump
device, the user must also provide two additional MOSFETs to isolate VOUT of the
M41ST87W from the output (OUT) of the charge pump during normal operation, and from
VOUT of the M41ST87W device during the tamper condition. During normal operation the
TPCLR signal will be forced low, disabling the charge pump. When disabled, the output of
most charge pumps will be forced to ground. In order to allow proper operation of the
SRAM, MOSFET(1) must be "off" to isolate VCC of the SRAM from the charge pump output.
At this same time, the P-channel MOSFET(2) will be "on" to provide the supply voltage for
the SRAM.
During a tamper condition, the TPCLR signal will be forced high, controlling the inhibit pin of
the DC regulator. This will put the regulator in standby mode for tCLR. The tCLR is the tamper
clear timing where the regulator will be switched off for 1, 4, 8, or 16 seconds, depending
on the setting of the CLRPW1 and CLRPW0 bits in the register. The TPCLR signal also
enables the charge pump. When the charge pump is enabled, OUT generates a negative
voltage on the VCC pin of the SRAM (for a programmable period of time), causing data
corruption. The M41ST87W must be isolated from the VCC of the SRAM to avoid data
corruption of the M41ST87W due to forward biasing of the parasitic diode of the
M41ST87W VOUT output. This is accomplished by using the TPCLR signal to turn the Nchannel MOSFET(1) "on," while turning the P-channel MOSFET(2) "off."
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Description
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Figure 1: Circuit connections
Inverting
Charge
Pump
IN OUT
C2
SHDN
Inhibit
VCC
5V
Regulator
VIN
Negative Output
(–1 x VIN)
(1)
M41ST87W
VCC
TPCLR
CAP+ CAP–
C1
TP1IN
(2)
TP2IN
VOUT
VCC
SCL
ECON
E
WDI
SDA
RSTIN1
RST
RSTIN2
SQW/FT
EX
Pushbutton
Reset
PFO1
PFI1
Low-Power
SRAM
To RST
To LED Display
To NMI
PFO2
PFI2
VSS
VBAT
1.
2.
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IRQ/OUT
F32k
N-channel MOSFET
P-channel MOSFET
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To INT
To 32kHz
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1.4
Description
RAM clear data
Depending on the process technology used to manufacture the external SRAM, clearing
the memory may require varying durations of negative potential on the VCC pin. The
M41ST87W device allows the user to program the time needed for their particular
application. The control bits CLRPW0 and CLRPW1, located in the day register, determine
the duration of the tCLR pulse width during a tamper event (see Figure 2: "Tamper output
timing" ). Thus, users can control the voltage and duration of the negative pulse enabling
them to configure the circuit for many different LPSRAMs.
Figure 2: Tamper output timing
TPCLR
t CLRD
t CLR
t rec
RST
High-Z
VOUT
IRQ/OUT
High-Z
ECON
Tamper
Event
(TB Bit set)
Note: see M41ST87W datasheet for timing details.
1.5
Tamper timestamp
When the device is tampered with, and regardless of which tamper occurs first, a time
stamp freezing the update of the clock registers will occur to let the user know when it was
tampered with. The tamper bits (TB1 or TB2 in the flag register) will be set immediately.
Therefore, when tampering occurs, the user may elect to first read the time registers to
determine exactly when the tamper event occurred, then read the flag register to see which
tamper condition was triggered. The clock will update to the current time after resetting the
TEB bit in the tamper registers. The appropriate TEB bit must always be reset to '0' in order
to read the current time. The tamper detect function operates in VCC as well as in battery
backup.
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Conclusion
2
AN1879
Conclusion
With the increasing frequency of credit card fraud and identity theft, ST is leading the way
protecting this sensitive data with its new line of secure RTCs. This sensitive data is stored
in internal or external memory of most devices like ATM machines or POS terminals. The
M41ST87W solution can provide early detection when these devices have been tampered
with and clear the RAM before the intruder can access this data.
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Revision history
Revision history
Table 1: Document revision history
Date
Revision
Changes
04-Feb-2004
1
First edition
12-Apr-2004
2
Reformatted; updated vendor SRAM information (Table 1)
03-Jun-2004
3
Corrected drawing (Figure 1: "Circuit connections" )
16-Jan-2009
4
Reformatted document; updated cover page, Section 1.3: "Clearing the external memory
with an external charge pump" , Figure 1: "Circuit connections" , and RAM clear data
16-Oct-2013
5
Removed Table 1 (RAM clear data with different vendors) and updated Section 1.4: "RAM
clear data"
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