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delay generation

mr.rohit

Delay generation using timer in pic18f46k22

 

I am using timer interrupt for delay generation in pic18f

when i generate  any delay using timer that time this delay is not proper which i want,it showing some difference in time.

i want knw what is the reason of that difference ?

plz help me...

thank u..

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  • Thanks David..

    see in CTMU there is one internal capacitor inbuild

    when we set EDGSTAT1 bit in CTMUCON register that time internal capacitor start charging using internal 5.5uA current source

    after that we have to wait some time for charging thats why we need time in microsecond,then we dissable EDGSTAT1 bit in CTMUCON register ie stop charging of internal capacitor

    so i was asked the question,becoz i want to know what happen if i use time between charging and discharging capacitor is microsecond ?

     

    Can i use the time delay in millisecond using timer interrupt for CTMU ?

     

    according to calculation when i consider time in millisecond that time voltage is increasing

    ex.

    I = 5.5uA

    T = 1ms

    C = 30pf

    V = 183.3v

     

    and  183.3 V on ADC pins for microcontroller  is not possible..

    plz share some tips for me..

    Thank you...image

  • in reply to mr.rohit

    I think you got the CTMU wrong. There is a current source, but the voltage to be measured by ADC can't be higher than Vdd of the PIC. (It is just a regular ADC, with +Vref as Vdd or something else smaller than Vdd.) I hope no manufacturer will design a chip with such self destruction capability image

     

    Your calculation has to count with all the variables. You can set the current value of the current source, you can create variable time delay. The capacity is something you want to measure by measuring the voltage (charge) created by the current source for the time delay.

     

    To your question - with 1 ms delay, you will just measure Vdd, which is meaningless and gives you no answer about the measured capacity.

    Try to stay within Vss and Vdd boundaries otherwise your measurement makes no sense.

  • in reply to koudelad

    ok thank u david..

    i think time delay in microsecond is suitable for this unit..

    i will try to maintain that boudaries...

    thank u so much...image

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