ST PC58 Series, STCU2 Technical Note

TN1239

Technical note

How to change DCF STCU configuration

Introduction

The SPC58 family integrates a dedicated safety module to configure, control and execute
self-test operations. This module is called Self-Test Control Unit (STCU2).

The user can configure the STCU2 in two ways. The first way uses the standard register
access from user application. The second way exploits the Device Configuration Format
(DCF) records.

This document explains the correct procedure to update a STCU configuration which has
been already set by the specific DCF records in UTest memory. It focuses only on the
second way to configure the STCU.

As a prerequisite the reader should be familiar with the functionality of STCU2, DCF, SSCM
and UTest flash. For any details on these topics, refer to the reference manual.

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Contents TN1239

Contents

1 Programming of STCU2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

2 How STCU2 configuration can be updated . . . . . . . . . . . . . . . . . . . . . . . 4

3 Limitation and prerequisites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

4 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

5 Reference documents . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

6 Acronyms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

7 Revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

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TN1239 Programming of STCU2

1 Programming of STCU2

The normal flow to program STCU2 starts by providing the unlock keys and afterwards the
rest of the configuration (left side of Figure 1).

Unlock keys are static keys. The user can find their values in the STCU chapter of the
reference manual.

The STCU loads the configuration only if the user provides the correct unlock keys. If the
unlock keys are not correct, the STCU ignores the following configuration.

The user passes these values to the STCU by writing them into the UTest sector of the
Flash as DCF records as shown on the left side of Figure 1.

A problem occurs in case the user needs to change the STCU configuration. It means that
the user has programmed at least a pair of unlock keys into UTest.

Given that the STCU2 considers only the first valid unlock keys and the following
configuration, after the first acceptance of a configuration, STCU2 does not accept any other
settings even if the provided unlock keys are correct. The SSCM reads the DCF
incrementally from lower to higher addresses.

The right side of Figure 1 shows two different configurations of the STCU. Both
configurations start with the correct unlock keys:

1. First configuration starts at address X

2. Second configuration starts at address Y
Even if the second configuration starts with th e correct unlo ck keys, the STCU ignores it. As

a result, the STCU runs the L/MBIST accordingly with the configuration that starts at
address X.

Next section explains how to instruct the STCU to load the second configuration and discard
the first one.

Figure 1. STCU unlock keys organization

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How STCU2 configuration can be updated TN1239

2 How STCU2 configuration can be updated

As explained in the previous chapter, simply writing a second STCU configuration at
different location does not cause an y change in the configura tion that the STCU loads, such
as the first one.

Since the UTest sector is not erasable, if the user wants to change the STCU con figuration
he must invalidate the first configuration. It means invalidating the first instance of the unlock
keys.

As a consequence, STCU2 reads unlock keys that are not correct and ignores the following
configuration.

With reference to Figure 2, if the user invalidates the unlock keys at address X the STCU
ignores the first configuration and loads the second one.

If needed, the user can repeat the process by adding a third configuration and invalidating
the unlock keys at address Y and so forth.

Figure 2. Make STCU unlock key invalid

Invalidating means overwriting the unlock keys with incorrect keys. The user, however , does
this operation carefully due to the ECC/EDC.

The ECC/EDC protects the UTest content. It means that the UTest contains not only the
data, but also some redundant bits. The hardwar e can correct or detect some errors by
comparing the data and the redundan t bits. This process is transparent from the standpoint
of the user who cannot access the redundant bits.

For this reason, if the user overwrites the unlock keys with random data, there is a risk that
the ECC/EDC logic detects one or more not correctable errors. In case of this event the
sample does not start.

Under those circumstances, the user must adopt specific values that do not cause the
ECC/EDC errors. These values are visible in Table 1.

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