ANALOG KEYBOARD

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EXTENSION FOR WAKE UP

ST6 family members with the Analog input

capacity can also generate a wake-up opera-

tion (from WAIT or STOP modes) on the

pressing of a key. This can be achieved by a

modification of the circuit shown in figure 1.

but to an additional I/O port bit. During key

polling, this additional port bit is set to output

mode active high, thus effectively switching

the pull-up resistor must be high enough to

give no significant voltage drop, or the result-

ing error must be calculated and taken into

account. The other I/O bit is used as the Ana-

log input to the ADC as in the original circuit.

During the wait for the key press, the first I/O

pin, used to pull the pull-up resistor high to

pedance state (e.g. open drain output mode).

The second I/O pin, used as the ADC input

while polling, is switched to the interrupt input

with pull-up mode. The internal pull-up is in

the range of 100k, in comparison to the 1k -

10k of the external resistor used during poll-

ing. If any key is now pressed an interrupt will

be generated if the voltage at the second I/O

pin is below the Schmitt trigger low level

threshold. The serial resistors in the keyboard

chain must not be too high in this case, there-

fore the maximum number of keys is reduced

in comparison to the normal mode.

Figure 3. Keyboard wake-up circuit

Figure 4. Keyboard reading

Figure 5. Interrupt configuration