ST AN839 Application note

AN839

APPLICATION NOTE

Analog multiple key decoding using the ST6-REALIZER

INTRODUCTION

The ST6 On-chip Analog to Digital Converter (ADC) is a useful peripheral integrated into the silicon of the ST6 family members. One of its practical applications is to decode a number of keys through only one I/O port pin. The technique is to connect the keys by a resistive voltage divider to the converter input. This principle is particularly interesting since it requires only one I/O pin whereas a traditional matrix keyboard requires a high number of I/O pins.

A practical application has been developed with the ST6-REALIZER environment and is illustrated in this note.

Hardware considerations for the keyboard design are reviewed while the software generation by the ST6-REALIZER is described.

June 2008

Rev 2

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Analog multiple key decoding using the ST6-REALIZER

1 PRINCIPLE OF OPERATION

The basic circuit of the decoder consists of a pull-up resistor connected to the ADC input, with the first key directly switching to ground. The following keys are then connected in sequence to the ADC input through serial resistors. The combination of the pull-up resistor, the serial resistors and the pressed key form a resistive voltage divider (Figure 1).

Figure 1. Analog keyboard resistor key matrix

When a key is pressed, the voltage at the ADC input is given by the activated voltage divider, generating a different voltage at the ADC input for each key pressed. If the top key is pressed, the voltage measured is always zero while the default voltage at the ADC input (if no key is pressed) is Vdd.

This analog voltage is converted by the ADC and the digital output value is used to determine which switch is closed. It can be seen that if more than one key is pressed at the same time, the key detected is the key in the chain closest to the ADC input. This allows the keys in the keyboard to be prioritised (Figure 2).

Figure 2. Multiple key press

Depending on the identified key, a direct signal activation can be achieved or a selective jump in the program flow can be performed.

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Analog multiple key decoding using the ST6-REALIZER

2 PRACTICAL REALISATION

2.1 Keyboard hardware description

The serial resistors are selected in order to give an equal distribution of voltage between Vdd (No key pressed) and Vss (Last key pressed) for each switch combination, so as to give the best noise margin between keys. For n keys, the resistor values should be selected such that the voltage for the second key from top is Vdd/n, for the 3rd 2Vdd/n, for the 4th 3Vdd/n and for the nth (n-1)Vdd/n.

Practically the maximum number of keys is limited by the precision of the resistors that gives for each key pressed a voltage value within a window around the theoretical value.

In the case of a 10 keys system, the values (In Ohm) of the Table 1 for the resistors network have been chosen.

Taking into account a +/- 2% resistors, voltage values and conversion results of the Table 2 can be obtained. Vmin is obtained when the serial resistors are at their minimum value and the pull-up resistor Rp is at its maximum value. Vmax is obtained when the serial resistors are at their maximum value while the pull-up resistor Rp is at its minimum value.

Table 1. Used resistors

Rp	R1	R2	R3	R4	R5	R6	R7	R8	R9
10000.00	1100.00	1300.00	1800.00	2400.00	3300.00	5100.00	8200.00	16000.00	51000.00

Table 2. Voltage at the ADC input and 8bit conversion result (5V supply)

Active key	Vmin	Vmax	Conversion result
Key0	0.00	0.00	0-0
Key1	0.48	0.51	24-26
Key2	0.94	1.00	48-51
Key3	1.44	1.52	73-78
Key4	1.94	2.04	99-104
Key5	2.44	2.54	124-129
Key6	2.95	3.05	151-156
Key7	3.45	3.54	179-180
Key8	3.95	4.02	202-205
Key9	4.48	4.52	229-230

The condition no key pressed corresponds to a result of 255.

2.2 Software generation

Thanks to the use of the ST6-REALIZER development tool, the application software is automatically generated from a graphical description of the application.

The functional description of the application includes:

1)The analog input through an ADC to read the value issued by the voltage dividers.

2)Key recognition.

3)Transfer of the result to other functional blocks, or conditional jumps in a state machine.

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