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DRIVING AN ANALOG KEYBOARD WITH THE ST7 ADC

Then we have a resistive divider

So the corresponding voltage of each key is given by the values of the resistors. An equal dis-

pressed.

4.2 PRACTICAL LIMITATIONS

Theoretically, with an 10-bit ADC, 1023 keys can be decoded. But potential errors must be

taken into account. They can come from the power supply, the key resistivity, the resistor tol-

erance, the ADC conversion errors.

The resistor tolerance is the main limitation as usually 5% tolerance resistors are used. It is

advised to use a 1% tolerance resistor for the pull-up. Changing this resistor greatly improves

the keyboard as the pull-up has an influence on every key.

The ADC total unadjusted error (TUE) for the device is specified as 4 LSB. So, it has to be

taken into account to avoid any key decision error.

These parameters will reduce the number of keys that can be efficiently decoded.

i - 1

Σ

j = 0

i - 1

Σ

j = 0