I use it to save pins on my microcontroller. The 74HCT238 is a 3 to 8 decoder/converter and decodes a 3 pin binary input to a decimal output. PC0 is used for the buzzer and PC1 to PC3 go to the 74HCT238. The communication with the clock and EEPROM goes via an I 2C protocol and uses PC4 and PC5. Port B is used for ISP programming (PB3 to PB5 + PC6 for reset) and to sent the pulses for the steppermotors to the drivers (PB0 to PB2). When we look at the microcontroller, you can see that Port D is entirely used for the lcd and the 2 buttons (INT0 and INT1). The 2 diodes should prevent the 9V battery to 'charge' when the 12V is on and vice versa. The only thing I added are a 'power-on led' and a connection for a 9V battery. The powersupply should be pretty forward: it is a standard 5V supply used on many projects.
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