Need help to understand circuit!!

Hi,

Greetings of the day!!!

I have got a display board which has 3 common anode 7 segment displays, 7 individual LED's and 4 switches.

Segments are driven by using port 0 of P89V51RD2 micro controller (through 74HCT574 latch IC). The common terminals (COM1 to COM4 as indicated in schematic) are also driven by using port 0 of P89V51RD2 micro controller (through another 74HCT574 latch IC and the output of latch IC if given to ULN2003). COM1, COM2, COM3 and COM4 is nothing but the output pins of ULN2003.

Pin no 14 on connector CN1 (common point where anode's of all diodes are connected) is connected to port 1.4 (pin no 5 of micro controller).

With this schematic I can't figure out how the micro controller is reading the state of switch and how it is differentiate that which switch has been pressed??

Please advice.

Regards,
Paddy

With this schematic I can't figure out how the micro controller is reading the state of switch and how it is differentiate that which switch has been pressed?

Click to expand...

You'll hardly understand if you assume a common anode display. LTS367 is common cathode. In this case, the switches are scanned along with segment multiplexing. Consider a pull-up resistor at the common switch input.