how to interface a resistive touch screen to lpc2148 (arm7) microcontroller

[ANJUSHA]

hai,

    i want to know how to interface a touch screen to arm7, in which inbuilt ADC is present.

[AJISH ALFRED]

Hi Abiram,

Your touch screen should have two analog output pins, X-cordinate & Y-cordinate. Read the value of both using two seperate ADCs, and the result is the location (X,Y) of a touch.

[ANJUSHA]

 thank you sir…….

sir can you give an idea of the logic involved in writing code for interfacing a touch screen to lpc2148.

 

example.like for a motor ,logic is presented as on and off…two cases.similarly for touch screen 

can you tell me sir.

[AJISH ALFRED]

Hi Abiram,

Good question. You asked for the logic only, while most people ask for a working code staraight away. I’m impressed. If you have the perfect logic, you can make your controller to do any task. Code is nothing but a representation of your logic, right?

Regarding your question,

The aim is to sense a touch and find its co-rdinates (X,Y) on the touch screen.

You have to connect the X and Y probs to seperate ADCs.

You must initialize the ADC in such a way that it should perform continuous digital conversion.

Now your code should do the following continuously

 – read the ADC1

 – read the ADC2

 

If there is no touch the values will be the maximum. If they are 10-bit ADC, 1023 will be the maximum value. This value will drop suddenly whenever you touch the screen. That’s how you sense a touch.

Now the location of the touch is the reading from both the ADC at that moment.

 

I can represent the logic as follows

 

step1: initialize the adc

step2: read adc1

step3: if adc1 value less than a threshold value, say 550 then go to step6, otherwise go to next step

step4: read adc2

step5: if adc2 value less than a threshold value, then go to next step, ortherwise go to step2

step6: read and store the value of adc1 as X cordinate and ADC2 as Y cordinate of the touch

 

Any further doubts, please ask.

[ANJUSHA]

 

hello sir,

 

 Thank you sir.your explaination is perfect sir.i am trying according to the steps given sir.now i want to define 6 functions to control 6 operations in a system.so sir could you tell me depending on factor of touch screen ,we should divide (x,y) values .so that 6 functions are to be controlled through touch screen……….likewise ex: if we take keypad we had 0 to 9 digits.so 9 functions can be controlled on selecting the digits.the same logic has to be implemented using touch screen.how sir?can you tell me.

[AJISH ALFRED]

K. Assume your touchscreen is divided into 6 areas, like 6 switches. You select them and find in which range the X and Y varies in that particular area.

Use these values to check in which area the touch has been sensed.

[karthik]

Hi,
I have written a code such that the o/p values of touch screen are converted to digital values and transmitted through UART so that I can easily note down the touch screen digital values of x-y coordinates . When monitoring the digital values of ADC keeps varying a lot!!! Please help.. Any suggestions?
I have connected the pins of touchscreen directly to my controller. Is there any necessity of any external circuit?
Thanks in advance.

[SHAH DISHANT H.]

Hi,

 

What is the variation you are getting ?

 

Tell the range. actually the step size for LPC2148 is very small so you can expect a variation.

[Author]

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