Re: 74HC299 resetting when I clock in a 0 bit (Help, I'm at my wits end here)

From: Anthony Fremont (spam_at_anywhere.com)
Date: 03/03/05 

Date: Thu, 03 Mar 2005 01:03:26 GMT

"Robert Monsen" wrote
> Anthony Fremont wrote:
> > Hello all,
> >
> > I'm tinkering with a 74HC299 and I'm having a bit of trouble. I am
just
> > using it as a serial-in/parallel-out shift register. Whenever I
clock
> > in 1 bits, all is fine. As soon as I clock in a single 0 bit, all
the
> > output registers reset to 0 simultaneously as if I had driven the
reset
> > pin low.

<snipped more of my whining>

> I'd guess that your clock is bouncing for some reason. Are you using a
> button to clock it? You are probably getting hundreds of clock
> transitions, so the single 0 you tried to clock in gets clocked into
> every cell.

I'm using the SQW output (1Hz square wave) of a Dallas 1307 real time
clock chip. That signal is also driving a PIC RB0 interrupt line and
that is all working fine generating 1 interrupt per second, so I suspect
the clock signal is ok. Plus, clocking in 1 bits results in the correct
operation of the first output turning on followed by the next on the
next clock tick etc... But as soon as I apply a 0 bit and clock it in,
the outputs all reset. It's too weird, I know it's something dumb, but
I've checked and rechecked every pin (all 20) I am manually switching
the data input pin (it has a weak pullup resistor) by attaching a wire
to ground, but the clock signal should be good.

> If you have a schmitt trigger buffer (or inverter) try using that
> between your clock and your shift register. Or, just debounce the
button
> like this:
>
> VCC
> .--o--.
> | |
> | .-.
> --- | | 100k
> 0.1uF--- | |
> N/O | '-'
> Button | |
> | |
> | | |
> --- | |
> .---o o-------o-----o--------- Out
> |
> ===
> GND
> (created by AACircuit v1.28 beta 10/06/04 www.tech-chat.de)
>
> The other thing to check is that you have Vcc connected. You can power
> cmos through inputs, which has confused me before. Seems to work until
> you change a data input.

BTDT ;-)) I thought there were ghosts in my PIC. But this does
describe the precise behavior that I am seeing. Right now I'm using a
pull-down on the data input pin. When I put a jumper wire from the
input pin to Vcc, 1 LED lights each second in the proper sequence
exactly as it should. As soon as I remove the jumper letting the pull
down do it's job, the outputs all reset. Vcc is verified at +5V (pin
20) and Gnd (pin 10) is at ground potential. I can tell that they all
reset precisely when the rising clock occurs clocking in the 0, not when
the data changes.

Thanks for the help.

Quantcast