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Topic: some thought of sn74hc595 (Read 1 time) previous topic - next topic

winner10920

So im reading the datasheet and im slightly confused about the two registers and usage, am I right in thinking the first register(rclk) would cycle in the new data without disrupting the outputs, then I hit the srclk and the 8bit output would be updated with the status of the first register?

Grumpy_Mike

Yes that is how it works. But the other way round srclk clocks the serial data and rclk clocks the data latch register.

winner10920

Ah ok, so if I wanted to only change one bit id clock the updated byte with sclk then hit rclk to actually set it up

winner10920

Is it easily possible to use a shift register like this without an mcu? Like a few buttons, rclk,sclk,ser maybe a switch on oe
I guess a 1uf or .1uf cap as a hardware debounce mainly the clk buttons
also if I set an arduino pins to input on those control lines do I need to have pull downs?

Graynomad

#4
Jun 02, 2012, 07:49 am Last Edit: Jun 02, 2012, 07:52 am by Graynomad Reason: 1
Yes it doesn't matter what you use to toggle the pins, switches are just fine as long as they are debounced as you say.

Quote
if I set an arduino pins to input on those control lines do I need to have pull downs?

Not really, but at power up there may be transient pulses on the pins so you may have to do a write to the SR to ensure it is in a known state.

If this is a problem you could also control the OE pin and not allow any hard outputs on the SR until you have initialised it. You could also have pullup/dn resistors on the SR outputs to cause a known state at power up.

______
Rob
Rob Gray aka the GRAYnomad www.robgray.com

Grumpy_Mike

You also have the option of using the shift register clear input to put it into a known state after power up.


Grumpy_Mike

Quote
if I set an arduino pins to input on those control lines do I need to have pull downs

My advice is never to use pull down resistors only pull ups.

winner10920

Why is that? Is it more reliable or something?


winner10920


Docedison

There is one thought left... By All Means Use A Pull-Down, only when it is needed directly...

Doc
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"The solution of every problem is another problem." -Johann Wolfgang von Goethe
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winner10920


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