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System overview UM0886
14/30 Doc ID 16916 Rev 1
4.1.6 S-Touch-based keypad
The demonstration board incorporates an S-Touch keypad based on the STMPE1208SQTR
to navigate through the menus. This keypad is a 12-key daughterboard.
Figure 3 shows the
placement of the keys.
4.1.7 Power supply unit
The power block is the main module of any hand-held system. This application requires
several voltages (3.3 V, 5 V, 6.5 V) to drive the various board components. The board can be
used in standalone/hand-held mode with two batteries. The circuit is implemented so that
the batteries are charged individually and in parallel by dedicated battery chargers, and
while discharging (application running on the batteries) they are connected in series. This
switching is done through a relay.
If the DC adaptor is plugged in, the voltage from the voltage regulator output excites the
relay, which changes the series connection of the batteries to a parallel connection and all
the power is taken from the adaptor.
Figure 12. Power block of STM32-based parking ticket vending machine
Figure 13. Voltage regulators used in power block