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Help Using 74HC594 Shift Register (Please)

Discussion in 'General Electronics' started by [email protected], Feb 27, 2005.

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  1. Guest

    I've been trying to interface a SN74HC594 chip to an AVR 2313 (using
    and having a tough time with it.

    I have the following function, but it appears as if only the MSB get's
    every time it's called.

    void write7SegChar(char displayValue) {

    // Turn bit on PORTX |= BIT(x)
    // Turn bit off PORTX &= ~BIT(x)
    // Toggle bit PORTX ^= ~BIT(x)
    // Load PB0 with serial value (MSB First)
    // Pulse SCLK (PB1) with each value
    // Pulse RCLK (PB2) after all eight bits are gone
    // PB3 is RCLR
    // PB4 is SCLR

    PORTB &= ~(_BV(PB3) | _BV(PB4)); // bring RCLR & SCLR LOW for clear
    PORTB |= (_BV(PB3) | _BV(PB4)); // bring high again

    for (int i = 0; i < sizeof(char); i++) {
    if((displayValue << i) & 0x80) {
    PORTB |= _BV(PB0);
    } else {
    PORTB &= ~_BV(PB0);

    PORTB |= _BV(PB1); // strobe SCLK
    PORTB &= ~_BV(PB1);
    PORTB |= _BV(PB2); // strobe RCLK
    PORTB &= ~_BV(PB2);

    I have pull up resistors on all the clr and clock pins (PB1-PB4) so I
    think it's an open collector problem.

    VCC is 5VDC, and the uP clock is at 4MHz so I shouldn't need any delay
    when strobing clock or clear.

    I can't see what I'm doing wrong here. Any suggestions are welcome.


  2. Rich Webb

    Rich Webb Guest

    Presuming that it's
    #define BIT(a) (1 << (a))
    shouldn't the toggle be "PORTX ^= BIT(x)"?
    I wouldn't go overboard on "portable-izing" the code with the sizeof
    operator in an embedded app, since it's so wired to the underlying
    hardware anyway.

    In this case, the extra effort you took to make the code portable came
    back and bit you -- remember that, by definition, sizeof(char) is 1.
  3. Guest

    That was the problem. Thank you!

    The rub is I wasn't even trying to make it portable. That was
    simply force of habit and not thinking it thru.

    I really appreciate the second pair of eyes spotting the
    obvious problem!

    Thanks again.

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