delay generation

Hello,

there are too many variables. The more details you add, the more we would be able to help you.

Basically, I would focus on the following:

1) oscillator frequency and stability - this is crucial - are you using an integrated RC or some kind of external oscillator? (What are the conditions your design works in? e.g. variable temperature..)

2) is your timer (and prescaler / postscaler) set correctly?

3) do you work with the timer properly? (for example if you generate periodically an interrupt, but have poorly written ISR, it looses precision; the same if multiple interrupts occur at once or even worse, the same source of interrupt occurs multiple times, before you finish the ISR)

4) What makes you think that you have a difference in time delay? Do you have any measurement instrument? What delays are you trying to achieve?

i am  using DSO for time period measurement

i want time delay in microsecond for CTMU unit in pic18f46k22

i am using internal oscillator with PLL - 64MHz (16MHz  + 4xPLL),

my timer register detail is

T0CONbits.T0PS = 1;                 //timer0 prescalar enable    

    T0CONbits.PSA = 1;

    T0CONbits.T0PS0 = 0;

    T0CONbits.T0PS1 = 0;

    T0CONbits.T0PS2 = 0;

    T0CONbits.T08BIT = 1; //8 BIT MODE

    INTCONbits.TMR0IE = 1; //Enable TIMER0 Interrupt

    T0CONbits.TMR0ON = 1; //Now start the timer!

    T0CONbits.T0CS = 0;

    TMR0 = 254; //load count in TMR0 register

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?

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..

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

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.)

hello david...

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

thank u so much david..

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

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).

hmm...

Really u have lots of experience in Embedded..

nice to meet u..

How precise and how accurate?

Accurate timing comes down to a time base and synchronization.

You can accomplish these tasks depending upon your requirements.

Please share them if you want a good answer.

DAB

hello...

I am working with Capacitive sensor using CTMU in pic18f46k22..I am refering AN1375 App note for CTMU..

this microcontroller having CTMU unit for capacitive sensor.in CTMU registers , i am using 5.5uA internal current source,and 14us delay which is generated by timer interrupt for CTMU.

calculations....

c = 30 pf

I = 5.5uA

T = 14us

v = ?

according to mathematical calculation

v = (I*T)/C

v = (5.5uA * 14us)/30pf

v = 2.56v

my question is ,

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

plz help me, i am not able to understand the accurate calculations..

thank u..