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 1/7
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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