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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.

winner10920


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?

Grumpy_Mike


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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winner10920


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