ANALOG KEYBOARD

4/15

PRACTICAL LIMITATIONS

Theoretically, for an ideal power supply, ADC and resistors, 255 keys could be detected. Practi-

cally however, it is necessary to take into account potential errors coming from:

- the power supply - the key resistivity - the resistor tolerance - the ADC error

The power supply tolerance can normally be neglected providing noise is not present at a fre-

quency within or above the frequency range of the RC delay of the resistive divider, as the ADC

reference is normally provided by the power supply of the ST6. For ST6 family members with ex-

The sensitivity of the key can normally be neglected, as the resistance of the divider is high in

comparison to it. If the key resistivity is significant, it should be added to the “serial” pull-down re-

sistance of the different dividers. The key resistivity variation must also be added to the toler-

ance of the serial pull-down resistor (see resistor tolerance following).

The resistor tolerance affects the tolerance of the dividers. Two situations must be taken into ac-

count:

a) minimum value of pull-up combined with maximum values of pull-down = maximum voltage of

the divider at the ADC input.

b) maximum value of the pull-up combined with the minimum values of pull-down = minimum

voltage at the ADC input. These two cases give the maximum voltage variation of each divider

(see Table 3). The voltage variation ranges of two dividers must not overlap otherwise the key

cannot be decoded, even with an ideal converter.

Active

Key

R -2% ( )

R +2% ( )

S0

00

S1

1078

1122

S2

2352

2448

S3

4116

4284

S4

6468

6732

S5

9702

10098

S6

14700

15300

S7

22736

23664

S8

38416

39984

S9

88396

92004

Table 3. Effective Divider Resistors RX

Realistic converters require a margin be-

tween the range of variation. In the case of a

significant variation in the key resistivity, the

maximum resistivity of the key has to be

added to the value of the pull-down resistor in

case a). For case b) no error needs to be

added as the resistivity cannot be less than 0

Ω.