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Topic: The 74HC595 and 0.1uf Capacitor (Read 3963 times) previous topic - next topic

lRaHl

What is the 0.1uf Capacitor for?
Do you need one? Or in my case 9?

http://www.arduino.cc/en/Tutorial/ShiftOut



Did you notice the 512 3mm Diffused Blue LEDs in the background? :)

CrossRoads

Yes, you should have a 0.1uF from each part's VCC pin to ground.
This helps with damping switching noise on signal lines, and also acts as source of localized current as the logic within the chip switches.

I'm not seeign 3mm LEDs clearly in the picture.
Designing & building electrical circuits for over 25 years.  Screw Shield for Mega/Due/Uno,  Bobuino with ATMega1284P, & other '328P & '1284P creations & offerings at  my website.

lRaHl


Yes, you should have a 0.1uF from each part's VCC pin to ground.
This helps with damping switching noise on signal lines, and also acts as source of localized current as the logic within the chip switches.

I'm not seeign 3mm LEDs clearly in the picture.


The shiftOut tutorial has the Cap on the ST_CP to GND. Should it be there or Vcc to GND?

CrossRoads

Vcc to Gnd.
Putting a cap across a clock signal will just screw up that signal.
Designing & building electrical circuits for over 25 years.  Screw Shield for Mega/Due/Uno,  Bobuino with ATMega1284P, & other '328P & '1284P creations & offerings at  my website.

Grumpy_Mike

The tutorial is wrong, we have been trying to get it changed for years.

Beauty

A few days ago somebody told me that the ShiftOut tutorial has the capacitor problem.
This could be one reason why I had problems in my last project where I used shift registers.

Also some pins of the Circuit Diagram are at the wrong position.
And I miss an important note:
Each output pin allows 35mA, but the current amount of all pins together has a maximum of 70mA. Very important!!
Additonally it would be useful to have a "See also" section, e.g. with links to similar ICs which are usable for higher currents.

Who is the administrator? Who can we ask for an update?
An other solution would be to create a copy in the wiki. There everybody can improve it.
I'm active in the Ogre 3D (game engine) community. There we have good experience with tutorials in the wiki. Several people made useful edits. For the Arduino tutorials we could go the same way.
My suggestion:
Create a copy of all tutorials in the wiki. And at the "old tutorial URLs" add a link or auto forwarding to the related wiki pages.

CrossRoads

"My suggestion:
Create a copy of all tutorials in the wiki. And at the "old tutorial URLs" add a link or auto forwarding to the related wiki pages."

I like that idea! Then we're not dependent on only the 5 founders to make the updates.
Designing & building electrical circuits for over 25 years.  Screw Shield for Mega/Due/Uno,  Bobuino with ATMega1284P, & other '328P & '1284P creations & offerings at  my website.

Grumpy_Mike

I like the idea too but beginners will often go to that page as they see it as official, so ideally we will need a link off that page.

floresta

I was once able to get an error in the LiquidCrystal tutorial corrected.  It was a simple editing error that slipped in during a wholesale update of the tutorials but it took several private messages to one of the '5' to even get an admission that the code in the tutorial bore no relation to the topic.    Once I got that point across he was very receptive to my suggested changes. 

A lot of the website needs some selective trimming to remove obsolete and incorrect information.  As I have said elsewhere Arduino tutorials are like the 'Roach Motel', articles go in but they never come out.


Don

floresta

Quote
Also some pins of the Circuit Diagram are at the wrong position.


My perception may be outdated but as I learned (and taught) there is a difference between a schematic diagram, a wiring diagram, and a lot of the diagrams we see around here now.  In a true circuit diagram the pins should be positioned to make the signal flow as straightforward as possible.  In a wiring diagram the pins should be positioned as they are on the actual device.  Most of the diagrams on the Arduino website are a bastardized combination of these two techniques as are the so called schematic diagrams produced by Eagle software.

Don

Beauty

Thanks floresta,
I didn't know this.
For me it was a little bit confusing and I think newcomers could use the wrong pins, because of this picture.
Now I understand the general sense of different pin positions.

hellonearthis

Is there a link to an updated ShiftOut tutorial?

Grumpy_Mike

No, most of it is fine just ignore the capacitor on the latch line bit and put it from the power pin to the ground.

pwillard

I agree with Don... "real" schematics need to be the goal as they are the common language of Electronics Engineers.

Here is my analogy for the day:
While I understand that it might be easier to hand a newcomer a paint by numbers using a wiring diagram...  if they are really serious... they should learn how to paint the way the experts do.

Only a little extra time is needed to understand true schematics.  Honest, it's not hard and it helps overcome the fear of data sheets that seems so prevalent.

Here is an idea... never take a design seriously until you download data sheets for the parts used.

http://www.datasheetcatalog.com/ can help.  Nowhere in the data sheet do they that say you should add passive parts to the "signal" lines on the 74x595

Grumpy_Mike

Quote
Nowhere in the data sheet do they that say you should add passive parts to the "signal" lines on the 74x595

No but in the ATmega data sheet it says absolute maximum output current should not be allowed to exceed 40mA. An uncharged capacitor looks like a short circuit and so you are feeding an output into nothing. From my tests with LEDs you can get at least 250mA peak current from an output pin so you are causing damage to your arduino using this capacitor.

The reason why it appeared to work was that the capacitor was providing some much needed power supply decoupling, parasitically through the signal leads. This again is not a good thing to do to a chip.

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