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Keil Variable declaration help

Roy Thomas

Actually on the external interrupt from port,I want to read the timer 2 value into a variable.(TH2 and TL2).
suppose I declare a variable
unsigned int timer2_val;
How can I move TL2 and TH2 into lower and upper nibble of timer2_val, so that I can treat Timer2 value as a single variable.
Weather keil has any declaration for assigning the lower byte or higher byte of a unsigned int to any hardware register or something like that.
I am a beginner in C.
Cygnalroy

  • 0

    timer2_val = (TH2*256)+TL2
    or
    timer2_val = (TH2<<8)|TL2

    You didn't realy mean 'Nibble' as that's 4 bits.

  • 0 in reply to clever dick

    hi,

    timer2_val = (TH2*256)+TL2
or
timer2_val = (TH2<<8)|TL2

    Well, but just remember that you should stop the timer before read these registers otherwise you may obtain wrong values. It is because reading of integer value is split into two reading of bytes (chars) due 8051 8-bit architecture. And if timer run then there is a chance that TL2 and TH2 registers change their values between these two readings.

    Regards,
    Oleg

  • 0

    This is pure, standard 'C' - nothing specifically to do with Keil.

    See this thread - the principles involved are the same:

    http://www.keil.com/forum/docs/thread5275.asp

    Note that using pointers or unions may be more efficient, but is non-portable - since it relies upon the specific compiler's byte ordering.
    Using shifts (or multiplications) is portable, but may be less efficient.

  • 0 in reply to Andy Neil

    The SFR keyword can't be used within a struct or union.

    Nor can you do something like

    timer2_val = *(unsigned int*)&TH2;
    because TH2 must be a direct byte transfer otherwise the data will come out of idata, and it won't compile.

    On the cypress FX2 TH2 and TL2 are at 0xcd and 0xcc, which can be declared as:
    sfr16 T2 = 0xcc;
    This is straight out of the cx51 manual under sfr16, page 100.

    However, Timer 0 and Timer 1 are not as conveniant and this will work:
    union word_byte {
    unsigned int word;
    unsigned char byte[2];
};
    y.byte[0] = gTH1;
    y.byte[1] = gTL1;
    x = TIMER_1_RELOAD + y.word;
    which will compile a little tighter than
    x = ((unsigned int)gTH1 << 8 ) + gTL1 + TIMER_1_RELOAD;

    :-)
    Frodak

  • 0 in reply to Oleg Sergeev

    Well, but just remember that you should stop the timer before read these registers otherwise you may obtain wrong values. It is because reading of integer value is split into two reading of bytes (chars) due 8051 8-bit architecture. And if timer run then there is a chance that TL2 and TH2 registers change their values between these two readings.

    Sage advice!