Help Using 74HC594 Shift Register (Please)

kccnospam_at_glenevin.com
Date: 02/27/05 

Date: 26 Feb 2005 22:49:30 -0800

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

I have the following function, but it appears as if only the MSB get's
set
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);
        return;
}

I have pull up resistors on all the clr and clock pins (PB1-PB4) so I
don't
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.

Thanks,

Kevin