when two key(0 or 4) pressed same time and hold that what happened

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he all expert please help me i confused what happened when in colom key 0 and key 4 are pressed and hold them ,what display behaviour any effect on segment thanks
 

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Hi,

Obviously the signals of RB4 and RB5 after the resistors are shorted.

Don't know what these lines are used for, because there is no information in your schematic.

Klaus
 

Thats not a clever design. It uses the segment drives as row and column drive/sense lines and has no pull-up/pull-down on the inputs. I would anticipate some strange segment patterns and indeterminate input from the keypad when any two or more keys are pressed simultaneously.

Note the design is 24 years old and uses a long obsoleted PIC. It is only intended as a demo of the ADC inputs, not for practical use.

Brian.
 

of course design is old but one thing i can't understand that when port drive segment my question is that key0 or key4 both pressed then corresponded segment are disturbed(two segment are short)

- - - Updated - - -

KlausST said:
Hi,

Obviously the signals of RB4 and RB5 after the resistors are shorted.

Don't know what these lines are used for, because there is no information in your schematic.

Klaus
Click to expand...

PORTB is using for segment drive
 

Hi,

The idea isn't bad: The display needs to be nultiplexed anyway. Why not with the same wires multiplex the keypad...
A little hardware, and a little piece of additional software and you gain full keypad functionality.

A series diode with each pusbutton, and maybe pullups...and it should work.

Klaus
 


yes i agree with you need some diode or some pullups thanks
 

A series diode with each pusbutton, and maybe pullups...and it should work.
Click to expand...

Perhaps I´m missing something obvious, but with the above circuit, considering that a series resistor is placed at each pin of the port used by keyboard to limit the current sink, it sounds plausible to use it "as is" even with the PIC core. Although I have used the same circuit on 51's familly (which has internal pullup), I do not see why it would not work in another core with a pin change direction feature. By alternating reading and writing of each nibble at a time, several combinations of key pairs pressed together could be detected after a full scan of the two halves of the byte, as can be seen here.
 

It would certainly work but it has a dirty work around to keep it simple. The keypad is multiplexed using only the display segment drives which means keys can only be scanned when all the digits are turned off and the conventional method of using the digit drives can't be used because those pins are also used an analog inputs. Consider what happens when the analog input voltage is raised above Vbe of the digit drive transistors!.
I wonder why they sink 10mA through the potentiometers, strange values :???:

Brian.
 

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