Motorola APX 5000 Service Manual page 85

Theory of Operation
: Controller
3.2.8.3 Key Retention
The key variables are retained within the MACE IC's memory (SRAM or FLASH). The keys can be
infinite key retention or 30-seconds key retention, depending on how the codeplug is set up. When
set to infinite key retention, the keys are stored in the FLASH memory inside the MACE IC. When set
to 30-second retention, the keys are stored in SRAM, and will be erased when the radio's battery is
removed (after the 30 second delay). The key retention delay circuit controls this time through a
comparator op-amp circuit (U2526). When the battery is removed, VSAVE (nominally 2.5 V) will
eventually drop to 0V after 30 seconds, which will result in a 0V output from U2526. This output is the
input to the CONT_1.875 regulator (U2525), which is nominally 3.3 V. When the regulator's output is
0 V, the keys in MACE's SRAM will be erased.
3.2.8.4 Tamper
The tamper function is intended to erase the encryption keys in a tampering situation. If the radio
chassis is opened during operation, the tamper signal, which is normally connected to ground
through spring contact M2533, will be disconnected from ground. This will be sensed by MACE
through its tamper pin TPR0 (U2510-M2). Once this condition is sensed, the encryption keys will be
erased.
3.2.8.5 Key Zeroize
The encryption keys can also be manually erased if infinite key retention is not turned on in
codeplug, by holding down the Side Top button and emergency buttons during radio power-up.
These two button inputs both connect to a dual transistor Q2537, which will release the
Key_ZEROIZE signal sensed by MACE's TPR0 (U2510-M2). Once this condition is sensed (floating
high), the encryption keys will be erased.
To troubleshoot the encryption circuitry, refer to the flowcharts in
Charts".
Chapter 5 "Troubleshooting
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