Actions
Forum Thread Details
  • Replies 17 replies
  • Subscribers 475 subscribers
  • Views 1609 views
  • Users 0 members are here
Related

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..

Parents

  • Hello,

    just a few thoughts:

    • The internal 16 MHz oscillator is supposed to by factory-calibrated to +/- 1 %. By using 4x PLL you multiply the error up to 4 times. What delay are you trying to achieve and what is the delay you actually measure?
    • I haven't programmed a PIC for a while, but I would suggest to do the configuration to T0CON at once (and as a final step, enable it).
    • Timer0 consists of TMR0L and TMR0H registers. What is the line "TMR0 = 254" supposed to do? BTW it is not recommended to set a value of an enabled counter.
    • The ISR is also important and comes as a factor into the equation. What do you do and how in your ISR?
  • in reply to koudelad

    hello

     

    i am trying to achieve 10us delay and i am getting time period on DSO is 17us

    Timer0 is 8-bit timer so i set maximum count ie 254

     

     

    check this page

    PIC Timer Calculator

     

     

    thank u..

  • in reply to mr.rohit

    Well... A calculator is useful only after you fully understand, what it calculates. There is no information about age or function...
    First, trust the datasheet of PIC18F46K22. Microchip tries to have backwards compatibility to earlier released PICs, however, an actual datasheet is the only trustworty document about the device. Chapter 11 is all about Timer0.

     

    Now comes the math homework image

     

    Your internal oscilator deliveres 16 MHz * 4 (for PLL) = 64 MHz
    If you want to use the internal oscilator as a clock source for Timer0, it uses Fosc / 4 = 16 MHz.
    Then, optionally, you can use a prescaler of 1:2 - 1:256 and the timer can be used as 8 / 16 bits.

    The period for 16 MHz is 1 / 16 000 000 = 62.5 ns
    To achieve 10 us delay, you need to delay for 10 * 10^-6 / 62.5 * 10^-9 = 160 cycles. This perfectly fits into an 8 bit timer, without using a prescaler.

     

    If you multiply the period of 62.5 ns by your 254 value, you get 16 us. (I guess the additional 1 us goes to Timer0 setting after enabling and the ISR.)

  • in reply to koudelad

    hello david...

    I am new in Embedded world and this is realy helpfull for me to understand timer interrupt

    thank u so much david..imageimage

  • in reply to mr.rohit

    I am glad I could help. And welcome to element14 community image

    To be honest, when I started with 16F PICs (13 years ago), I only had some book that didn't provide much explanation. Then I opened the datasheet and realized that it is the only source of reliable information (and erratas, that are related to specific products).

  • in reply to koudelad

    hmm...

    Really u have lots of experience in Embedded..

    nice to meet u..image

Reply Children
No Data