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Timing calculations on PIC18f

mr.rohit

Hi guys,
Was exploring PIC18F controllers timing specification. Unable to understand a couple of things in time generation sequence for based on the same.

 

 

Breadboard arrangement:

MUC:PIC18f46k22

Fosc = (XTAL) = 16MHz (with 22pf decoupling)

Fcpu = Fosc/4 = 4Mhz,

Tcpu = 1/4000000 = 0.25us = 250ns

 

Based on my observations I have noticed the following (pls. refer to attached images):

Case 1:

while(1){

LATDbits.LATD2 = 0;  //turn off led exec#0

LATDbits.LATD2 = 1;  //turn on led exec#1

LATDbits.LATD2 = 0;  //turn off led exec#2

LATDbits.LATD2 = 1;  //turn on led exec#3

}

 

Observations:

Noted for exec#0 and exec#2 Time period = 500ns

After which the pin stays high for exec#3 Time period 750ns and low for exec#0 time period 240ns.

In my opinion every 8-bit instruction is getting executed every 250ns in this case, the rest of the period where the signal is high the MCU is still awaiting instructions to fullfill 4MIPS.

https://drive.google.com/open?id=0B1ujJMA_1UnySUdKMi1JeVhMUWM

 

Case2: (Using ternary operator)

while(1){

LATDbits.LATD2 = LATDbits.LATD2 ? 0:1;

}

 

Observations:

from case2 ON-OFF time period = 3.3us

https://drive.google.com/open?id=0B1ujJMA_1UnydzhQVzRwMTZONWc

 

Case3:

while(1){

   LATDbits.LATD2 = ~ LATDbits.LATD2;            //toggle bit

}

 

Observations:

from case3 ON-OFF time period = 3.9us

https://drive.google.com/open?id=0B1ujJMA_1UnyaXEtX3BLZDNKLTQ

 

Case4: (Using macros)

while(1)

{

LED_OFF();   
LED_ON();
LED_OFF();
LED_ON();

}

 

Observations:

Time period = 500ns

same as case1.

https://drive.google.com/open?id=0B1ujJMA_1UnySUdKMi1JeVhMUWM

 

Please correct me if i am wrong..

Thank u...image

  • I'm not sure that I understand what is puzzling you:

     

    The problem seems to be that you are looking at code in C and attempting to predict what the processor will do - you need to look at the assembler code that actually runs on the processor. In the case of your first example the while loop will generate code that uses a jump instruction of some kind after the 4th assignment instruction.

     

    If you want to know what the processor is doing at this very low level then look at the assembler output of the C compiler or use your debugging tools to look at the code on the processor.

     

    It's very difficult to do precise timing by sequences of code - that's why modern micro controllers have timers which are often much easier to manage. You have lots of timers on this processor.

     

    MK