AN979 Datasheet, PDF (12/14 Pages) STMicroelectronics – DRIVING AN ANALOG KEYBOARD WITH THE ST7 ADC

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
key polling, this pin is in output mode and active high, thus switching V_DD to the pull-up re-
sistor.
Figure 9. Keyboard with wake-up circuitry.
ST7
V_SS
I/0
ADC ^analog
R_up
input
AINx
R₀
key 0
R₁₄
key 15
V_SS
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.
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