ST AN840 Application note
AN840
APPLICATION NOTE
Coded lock using the ST6-REALIZER
INTRODUCTION
The trend is to provide more and more applications with security features. This is requested to insure data
confidentiality, access control, or simply user identification. Non Volatile Memory is usually required to
store the identification or secret code. With the embedded EEPROM provided on the ST62 MCU, onechip solutions can be achieved with the associated cost and density advantages.
A coded lock system has been developed with the ST6-REALIZER environment and is described in this
note. This development tool is used to get an automatic generation of the application software.
June 2008 Rev 2 1/7
Coded lock using the ST6-REALIZER
Lock the door
Change secret code
Digit Reception Flag
Digit value
to door locking
system
Sequencing
Control
EEPROM
Code
Recognition
VR02063
From User Interface
1 APPLICATION OVERVIEW
The application described here consists of a coded lock for a door. A secret code is loaded into the lock
system to allow the door to be unlocked only if the same access code is entered. When loaded, the secret
code is stored in the non-volatile memory (EEPROM) embedded in the ST62 MCU. This ensures that the
data is retained even after a voltage cut-off and insures the security is maintained along the time.
The application provides the following features:
– Secret code recording (3 digits).
– Recognition of entered access code.
– Door lock control.
All the featured functions of the system are managed in the final application under software control by the
MCU. However, using the ST6-REALIZER allows this application software to be generated by a functional
description of the system. Thus the software development can be achieved by a system oriented or hard
ware like description.
Only the core of the application, the secret code storage in EEPROM and the code recognition, are described as a generic base for various applications. Any kind of user interface could be used (keyboard, IR
or RF solution for instance), while the output signal can activate any kind of circuit. In the present case, it
is assumed that the user interface provides the following 4 inputs:
– Digit reception flag
– Digit value (0..9)
– Change secret code
– Lock the door
The digits used for the code are serially received, announced by the activation of a flag.
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2 PRACTICAL REALISATION
2.1 Functional description
By restricting the application description to its core (the operations sequencing, the secret code storage in
the EEPROM and the code recognition), the resulting application block diagram is represented in
Figure 1. Block diagram of the coded door lock
Figure 1
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Coded lock using the ST6-REALIZER
2.1.1 Sequencing control
The most important feature is the sequencing control of all the operations. Two main items must be considered: the transition between the different working modes (Door locked, Access code entry, Door unlocked and Secret code entry) and the management of the serial flow of the numeric values received
through the user interface.
The transition between the working modes follows conditional rules described in the ST6-REALIZER with
the help of a state-machine as described in
– Door locked
– Code recognition
– Door unlocked
– Secret code entry
Figure 2. State machine description for sequencing control
Figure 2 where four states are existing:
The conditional occurrences are generated either through the user interface (Lock, Change secret code),
or through some functional sub-blocks of the application (One digit received, Right code, Wrong code,
Last digit). For instance, the occurrence of the condition One digit received in the Door locked state initi
ates the process Code recognition.
The management of the serial flow of the digits entered (0..9) is achieved with a counter of received digits
Figure 3). Depending whether it is the 1st, the 2nd or the 3rd digit (In the case of a 3 digit code), the nu-
(
meric value issued by the interface is stored in the associated memory location. When the 3rd digit is received, the counter is reset. In addition, the reception flags of the 1st digit and 3rd digit are used in the
state-machine to initiate respectively the code recognition and conclude the Secret code entry.
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