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DRIVING AN ANALOG KEYBOARD WITH THE ST7 ADC

6.3 EXTENSION FOR WAKE UP

With the ST7, it is possible to generate a wake-up function. This can be achieved by a mod-ifi-

cation of the circuit. The pull-up resistor must be connected to an additional port pin. During

sistor.

Figure 9. Keyboard with wake-up circuitry.

During the wait for a key to be pressed, the I/O pin used for the pull-up is switched into a high

impedance state (e.g. open drain output mode). The pin used as the ADC input while polling

is switched to interrupt input with pull-up mode.

So if any key is pressed an interrupt will be generated if the voltage at this pin is below the

Schmitt trigger low level threshold. The serial resistors in the keyboard chain must not be too

high in this case, therefore the maximum number of keys is reduced in comparison to normal

mode.

ADC

I/0

analog

input

AINx

ST7

key 0

key 15