AN1809 Datasheet, PDF (4/8 Pages) STMicroelectronics – STR71x ADC DRIVING AN ANALOG KEYBOARD

STR71x ADC Driving an Analog Keyboard
R_up: The pull-up resistor by which the v_key is maintained to Vdd when no key is pressed.
The associated voltage of the pressed key is given by:
i–1
∑ ^{(Vdd – Vss) Rj}
VKeyi = ---------------------------_i---_–----^j_{1---=------0--------}
∑ ^{Rup +}
Rj
j=0
If more than one key is pressed at the same time, the key detected will be the next key in the
chain closest to the ADC input. This means that the key recognition is managed by priority.
1.3.1 Key recognition
The number of keys which can be detected depends on the tolerance of the resistors used.
This mean that we must take a voltage range corresponding to each key when we convert the
result.
The resistor value and the equivalent voltage of each key are given by the following table.
R_up =1200 Ω
Table 1. Key decision
Key
Resistor
Voltage
Key
Resistor
Voltage
(Ω)
(mV)
(Ω)
(mV)
0
0
0
8
220
1121
1
56
111
9
270
1273
2
68
234
A
390
1442
3
82
366
B
560
1616
4
100
507
C
820
1788
5
120
654
D
1200
1946
6
150
810
E
1800
2084
7
180
966
F
2800
2200
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