How to use M41ST87 tamper detect and RAM clear

AN1879RAM clear dataTable 1.BeforeTamperDuringTamper (1)RAM clear data with different vendorsSRAM Cypress Hyundai Hitachi Cypress Hyundai Samsung STUnitDensity 1 Mb 1 Mb 4 Mb 4 Mb 4 Mb 4 Mb 4 MbV BAT 2.55 2.55 2.55 2.55 2.55 2.55 2.55 VV OUT 2.390 2.390 2.426 2.425 2.400 2.423 2.413 VI IN 0 0 0 0 0 0 0 µATotal I BAT 925.0 582.0 567.0 578.0 752.0 492.0 587.0 nAI SRAM –259.2 –267.2 –272.7 –287.7 –275.9 –382.0 –283.2 µAV IN 2.042 2.030 2.022 2.000 2.018 2.010 2.010 VI IN 316.8 323.0 327.4 339.5 329.8 335.0 336.0 µAV SRAM –0.522 –0.488 –0.465 –0.397 –0.449 –0.425 –0.421 VTotal I BAT 319.2 325.0 329.6 340.8 331.6 336.5 337.4 µAStatusCorrupted1. Typically some SRAMs would take > 10 seconds when V CC is taken to V SS at 25°C to corrupt the memory.Keys:V BAT = Battery voltageV OUT = Voltage outputI IN = Current into charge pump before and during tamper conditionTotal I BAT = Battery backup current of both the M41ST87 and the SRAMI SRAM = Current sink into the external SRAMV IN = Voltage at the charge pump input during tamper conditionV SRAM = Negative voltage produced by the charge pump at SRAM V CC during tamper condition5/9www.BDTIC.com/ST

Tamper timestampAN1879Tamper timestampWhen the device is tampered with, and regardless of which tamper occurs first, a timestamp freezing the update of the clock registers will occur to let the user know when it wastampered 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 todetermine exactly when the tamper event occurred, then read the flag register to see whichtamper condition was triggered. The clock will update to the current time after resetting theTEB bit in the tamper registers. The appropriate TEB bit must always be reset to '0' in orderto read the current time. The tamper detect function operates in V CC as well as in batterybackup.6/9www.BDTIC.com/ST

AN1879ConclusionConclusionWith the increasing frequency of credit card fraud and identity theft, ST is leading the wayprotecting this sensitive data with its new line of secure RTCs. This sensitive data is storedin internal or external memory of most devices like ATM machines or POS terminals. TheM41ST87 solution can provide early detection when these devices have been tamperedwith and clear the RAM before the intruder can access this data.7/9www.BDTIC.com/ST

Revision historyAN1879Revision historyTable 2.Document revision historyDate Revision Changes04-Feb-2004 1 First edition12-Apr-2004 2 Reformatted; update vendor SRAM information (Table 1)03-Jun-2004 3 Correct drawing (Figure 1)16-Jan-2009 4Reformatted document; updated cover page, Clearing theexternal memory with an external charge pump, Figure 1, andRAM clear data.8/9www.BDTIC.com/ST

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