DS5002FP Secure Microprocessor Chip

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observe such relationships. Although it is very unlikely that an application program could be deciphered by

observing vector address relationships, the vector RAM eliminates this possibility. Note that the dummy accesses

mentioned above are conducted while fetching from vector RAM.

The vector RAM is automatically loaded with the user’s reset and interrupt vectors during bootstrap loading.

SECURITY LOCK

Once the application program has been loaded into the DS5002FP’s NV RAM, the security lock can be enabled by

issuing the “Z” command in the bootstrap loader. While the security lock is set, no further access to program/data

information is possible by the on-chip ROM. Access is prevented by both the bootstrap loader firmware and the

DS5002FP encryptor circuits.

Access to the NV RAM can only be regained by clearing the security lock by the “U” command in the bootstrap

loader. This action triggers several events, which defeat tampering. First, the encryption key is instantaneously

erased. Without the encryption key, the DS5002FP is no longer able to decrypt the contents of the RAM. Therefore,

the application software can no longer be correctly executed, nor can it be read back in its true form by the

bootstrap loader. Second, the vector RAM area is also instantaneously erased, so that the reset and vector

information is lost. Third, the bootstrap loader firmware sequentially erases the encrypted RAM area. Lastly, the

loader creates and loads a new random key.

The Security Lock bit itself is constructed using a multiple-bit latch which is interlaced for self-destruct in the event

of tampering. The lock is designed to set-up a “domino-effect” such that erasure of the bit will result in an

unstoppable sequence of events that clears critical data including Encryption Key and Vector RAM. In addition, this

bit is protected from probing by the top-coating feature mentioned below.

SELF-DESTRUCT INPUT

The self-destruct input (SDI) pin is an active-high input that is used to reset the security lock in response to an

external event. The SDI input is intended to be used with external tamper-detection circuitry. It can be activated

and causes the same sequence of events described above for this action. In addition, power is momentarily

TOP LAYER COATING

The DS5002FPM is provided with a special top-layer coating that is designed to prevent a probe attack. This

coating is implemented with second-layer metal added through special processing of the microcontroller die. This

additional layer is not a simple sheet of metal, but rather a complex layout that is interwoven with power and

ground, which are in turn connected to logic for the encryption key and the security lock. As a result, any attempt to

remove the layer or probe through it results in the erasure of the security lock and/or the loss of encryption key bits.

BOOTSTRAP LOADING

Initial loading of application software into the DS5002FP is performed by firmware within the on-chip bootstrap

loader communicating with a PC by the on-chip serial port in a manner that is almost identical to that for the

DS5001FP. The user should consult the DS5001FP data sheet as a basis of operational characteristics of this

firmware. Certain differences in loading procedure exist in order to support the security feature. These differences

are documented below. Table 1 summarizes the commands accepted by the bootstrap loader.

When the bootstrap loader is invoked, portions of the 128-byte scratchpad RAM area are automatically overwritten

with 0’s, and then used for variable storage for the bootstrap firmware. Also, a set of 8 bytes are generated using

the random number generator circuitry and are saved as a potential word for the 64-bit encryption key.

Any read or write operation to the DS5002FP’s external program/data SRAM can only take place if the security lock

bit is in a cleared state. Therefore, the first step in loading a program should be the clearing of the security lock bit

through the “U” command.