Thanks Dag, you've light me !
regards, Vasile

On Fri, 1 Feb 2002, Dag Bakken wrote:

> //read byte from SPI (mode 3) as a master
> BYTE rdSPI(void)
> {
>    register BYTE BitCnt = 8;   // Do 8 times
>    register BYTE Shift;        // Input data register
>    do                          // Top of loop
>    {
>       SPI_SCK_LO();            // SPI clock output low
>       Shift <<= 1;             // C left shift one bit
>       if(IS_SPI_SDI_LO())      // Check level of data input
>          Shift &= ~1;          // Set bit zero to zero
>       else
>          Shift |= 1;           // Set bit zero to one
>       SPI_SCK_HI();            // SPI clock output high
>    } while (--BitCnt);         // Do all 8 bits
>    return(Shift);              // Return the input value
> }
>
> The &= operator is a bitwise and
> The |= operator is a bitwise or
> The ~ operator is a bitwise negate
>
> The last one will work differently on different variables depending on
> the bit-count.  In this example (8 bit), the expression ~1 is equal to
> 254.
> The code in itself looks like it's converted asm-code with the shift
> and test performed.  A smoother way would be like this:
>
> //read byte from SPI (mode 3) as a master
> BYTE rdSPI(void)
> {
>    register BYTE BitCnt = 8;   // Do 8 times
>    register BYTE Shift;        // Input data register
>    do                          // Top of loop
>    {
>       SPI_SCK_LO();            // SPI clock output low
>       Shift <<= 1;             // C left shift one bit
>       Shift.0=SPI_SDI_LEVEL(); // Set data
>       SPI_SCK_HI();            // SPI clock output high
>    } while (--BitCnt);         // Do all 8 bits
>    return(Shift);              // Return the input value
> }
>
> This is of course dependent on the compiler, if it supports the
> bit-addressing withing registers.
>
> Dag S
>
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