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

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Coded lock using the ST6-REALIZER

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 hardware 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.

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

Figure 1. Block diagram of the coded door lock

From User Interface

	Lock the door
	Change secret code	Sequencing
	Digit Reception Flag	Control
	EEPROM
	Digit value
		Code	to door locking
			system
	Recognition

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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 Figure 2 where four states are existing:

–Door locked

–Code recognition

–Door unlocked

–Secret code entry

Figure 2. State machine description for sequencing control

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 initiates 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 numeric 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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