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STR71x ADC Driving an Analog Keyboard

The associated voltage of the pressed key is given by:

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.

Table 1. Key decision

Key

Resistor

(

Ω)

Voltage

(mV)

Key

Resistor

(

Ω)

Voltage

(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

V

Key

i

V

dd

V

ss

–

()

R

j

j

0

=

i

1

–

∑

R

up

R

j

j

0

=

i

1

–

∑

+

----------------------------------------------------

=