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Help me interface a keypad to an expanded mode hc11

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ZeleC

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hello all
im trying to interface a keypad to my expanded mode hc11 .(i dont want to connect dirrectly to the uC.)
i made an interface circuit using the 74hc245 and 74hc573 chips but im having some problems .
Can anyone give me some help or links about some circuits to interface a keypad to a uC using the databus.
thx
 

Re: keypad

I think it is very easy,how ever you must have resistor bank to pull high and you should use 74HC244 instead of 74HC245 as a Input port
Good luck !
 

Re: keypad

ZeleC said:
hello all
im trying to interface a keypad to my expanded mode hc11 .(i dont want to connect dirrectly to the uC.)
i made an interface circuit using the 74hc245 and 74hc573 chips but im having some problems .
Can anyone give me some help or links about some circuits to interface a keypad to a uC using the databus.
thx
Click to expand...
put diodes between the output and the keyboard rows. Use pullup resistors on each column.
 

Hey Zelec,

Elaborate on the problems yr having...might be easy to solve
 

Re: keypad

Hi,

See this app. note from IAR if you do not want to use 74c922 :

h**p://www.cs.ccu.edu.tw/~pahsiung/courses/ese/resources/project1/state_machine.pdf

* = t
 

Re: keypad

hello friends,
her is my problem im using my microcontroller in expanded mode and the data is transfered from the keypad to the uC via a flatcable
the main problem that im having is that the readings sometimes is not right : for example if i have a keypad with the following
1 2 3
4 5 6
7 8 9
and i pressed the key 1 i would have 2 r 3isplayed on my LCD
the prblem hapens in the same row (pressed 5 i would get 6 or 4 )
this problem is minimized if the flat cable is short.
So here is a part of my problem
thank you guys for your help
 

Re: keypad

Hi,

Try to alternate one data signal with one ground in your flat cable. This will avoid diaphony between two consecutive data signals.

Make some software filtering before assuming a key recognition. That means you should read 2 or 3 time the same pressed key in 10 or 20ms interval.
 

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Re: keypad

Maybe you are not giving the active line enough time to settle.
Increase your delay between line scan,and read.
 

Re: keypad

Hi,

Another idea. I dont see any latch in your first post. Do you :

1) use buffered address lines as rows (for exemple) and do you read the columns through the data bus, or

2) do you use the data bus to set the rows and then read the columns ?

If you have choosen the second option, you first need to latch the rows by writing to a lacth and in a second cycle read the columns. The latch should be open collector to avoid shorts between outpouts of opposit level when pressing two or more keys on the same column
 

Re: keypad

here is my schematics
one thing i came up with accident is that i replaced th nand 74hc00 with 74ls00 the problem was solved
i dont know why could someone give me some explanation is it the low voltage present on the output of the LS series that solved the problem or is ther some thing else in the design that is wroong...?
thx you friends
 

Re: keypad

Hi

I had a look at your schematic. If I well understand it, all your signals are carried by the flat cable. I think the power supply of all the chips do also. I suggest to add a pull resistor between each signal (R/W, ADxx, W etc.) and the 5V.

If you do not have any, add a 100nF ceramic capacitor between +5V and Gnd of each chip and a big 10uF or 47uF between +5V and Gnd at the place where the flat cable is connected to your PCB.
 

Re: keypad

hmmm, it be nice for me, well, any have the c code of the 4 wire keypad? ASM its CHINESE for me! LOL
 

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