How to make better keypad schematic for noisy environments?

[mystery]

Keypad and noise

Hi all

I have designed a system with 8x8 keypad with the attached shematic. My system works well in the lab. But the working environment is vey noisy. The row and column pins are connected to the MSP430F149 port 5 and port 2 respectively. There are glithes on the keypad lines which causes unwanted interrupts. And sometimes the noise comming from the keypad lines causes the microcontroller to be reset. How can we make better the keypad schematic for the very noisy environments?

Thanks

 

[neuralc]

Keypad and noise

Try to put small caps (1nF) on each input.

HH

NeuralC

 

[gidimiz]

Re: Keypad and noise

Hi,

You can doone of 2 things or even both:
1. Add 100R resistor on each input ( nsted of the Ferrite bead and before the diode ). That will also protect your MCU from static electricaty. As you know our fingures are full of that stuff the the MSP430 is very sansative...
2. I didnt see your complete layout, but try to change the configuration so the lines will be on high ( pull up resistors ) and when you press the keys, then you will intruput on falling edge ( ground ).
Good luck.

 

[checkmate]

Re: Keypad and noise

Have you implemented debouncing on your keypad code?

 

[mystery]

Re: Keypad and noise

Have you implemented debouncing on your keypad code?

Yes I have implemented debouncing.

 

[gidimiz]

Re: Keypad and noise

Add 100R resistor on each input ( nsted of the Ferrite bead and before the diode ). That will also protect your MCU from static electricaty

I made a small mistake, the resistors need to be just before the pines of the MCU and after the pull up not before!

 

[GrandAlf]

Keypad and noise

As gidimiz suggested, it is good practice to to use active low inputs, as most MCUs have ports that are tied high, it is common sense. Lower value resistors will also help 1-4k7 is ok, will increase current though. I have had trouble in the past with keypads layed out like yours, always now use low ips if possible. Also make sure that the debounce is at least 20ms.

Added after 8 minutes:

Just another thought,

You probably wont need the diodes. Pullup resistors on the input port only. Output low, look for low inputs. Much better for noise. Probably you could lose the ferrites as well, unless it is a long cable.

 

[the tran]

Re: Keypad and noise

Check this link out. Hope it helps
**broken link removed**

 

[mystery]

Keypad and noise

Thanks for all your responses
I connect a 10nf on each column input of the microcontroller , and it seems ok