Interview technical Question about keypad

[tst]

today I attend an interview, the interviewer ask me an question and i duno how to answer it.
Can you connect the 4x4 keypad to the PIC microcontroller if there is only 1 I/O port for you to use? The interviewer ask me go home and find it out myself. Anyone have the idea?

[Brian Griffin]

today I attend an interview, the interviewer ask me an question and i duno how to answer it.
Can you connect the 4x4 keypad to the PIC microcontroller if there is only 1 I/O port for you to use? The interviewer ask me go home and find it out myself. Anyone have the idea?

It could.

The keypad is connected to a resistor network first, and then to a astable 555 timer and each key can generate a specific frequency, just like a musical keyboard.

The square wave is fed into the I/O pin.

The MCU intepretes the frequency.

And get the key information from the lookup table.

[tst]

Thanks Brian. Do you mind to teach me how to connect it? Let say that I am using cytron 4x4 keypad, there are total 8 pin. so I need 8 astable 555timer?

Astable 555 timer: http://www.kpsec.freeuk.com/555timer.htm#astable
4x4keypad: http://www.cytron.com.my/viewProduct.ph ... wUYOr1yFg=

[ABSF]

today I attend an interview, the interviewer ask me an question and i duno how to answer it.
Can you connect the 4x4 keypad to the PIC microcontroller if there is only 1 I/O port for you to use? The interviewer ask me go home and find it out myself. Anyone have the idea?

Wow, is this a trick question? Because 1 I/O port can be a port with 8 bits.....

But it can be done. Just read this application Note at Trick #7

http://ww1.microchip.com/downloads/en/d ... 40040b.pdf

Allen

[ABSF]

There is a disadvantage using the above method. If you press more than one key, the ADC would be confused and would give you the wrong keypress.

If you use Brian's method. The 4x4 keypad may have to be broken into 16 push button switch connected to 16 unique resistors. A keypress will connect one of the 16 resistors into the 555 osc replacing your R2. This will give a unique frequency (preferably in audio range). The square wave will be counted by PIC to determine which key is pressed.

Allen

[tst]

The 4x4 keypad may have to be broken into 16 push button switch connected to 16 unique resistors.

Thanks Allen. How to connect it to 16 resistor since there is only 8 pin for the 4x4 keypad. The unique resistor you mean are bulit-in resistor in keypad?

A keypress will connect one of the 16 resistors into the 555 osc replacing your R2. This will give a unique frequency (preferably in audio range). The square wave will be counted by PIC to determine which key is pressed.

what is R2? How is the connection looks like? Do you mind to sketch it? All the keypad pin (8pins) are connected together to 1 555timer? Sorry that I have too many question. Thanks a lot.

[robosang]

There is a disadvantage using the above method. If you press more than one key, the ADC would be confused and would give you the wrong keypress.

If you use Brian's method. The 4x4 keypad may have to be broken into 16 push button switch connected to 16 unique resistors. A keypress will connect one of the 16 resistors into the 555 osc replacing your R2. This will give a unique frequency (preferably in audio range). The square wave will be counted by PIC to determine which key is pressed.

Good method! Learn something new today

[tst]

Anybody can answer my question?

[Brian Griffin]

Anybody can answer my question?

ABSF has given you very clear hints about the keypad. Have you read about the operation of the 555 timer?