595 shiftout capacitor

I'm using this tutorial to use my shift registers. The text says to use a 0.1"f capacitor on the latch pin and the schematic image says 1uf. I've been searching for the past hour or two for another example that includes a capacitor but I can't find anything. What am I supposed to use?

thanks

Neither is correct. The value is the text, 0.1"f. is obviously not correct since " is not a valid prefix. The value on the diagram, 1uf, seems pretty large and it will probably distort the latch signal. Most likely the value should have been 0.1uF. Note the leading 0 which is there to call attention to the decimal point which might otherwise be lost in the editing/publication process (as probably happened in this diagram in this case). Also note the capital 'F' since this is the name of a unit (and a person).

You would think that someone would have noticed this previously, it's only been five years since the article was written and just over a year since it was last edited.

Don

[Edit] "[u]Re: 959 shiftout capacitor[/u]" - You should fix the subject to reflect the proper part number.

My guess is that capacitor was put there to fix sloppy wiring. It really shouldn't be there at all. Adding capacitors directly to Arduino output pins stresses them because there is a big burst of current (that only lasts for a short time, but it's still bad) whenever the pin goes high or low. Over time this could destroy the pin driver and the pin will never go high or low again.

Keep the wiring as short and tight as possible. If you get "erratic" behavior (which the original designer probably observed and tried to fix with a capacitor) try adding some resistance in series with the signal, like 100 ohms to 300 ohms. But if you keep the wiring short and tight it shouldn't be necessary.

-- The Ruggeduino: compatible with Arduino UNO, 24V operation, all I/O's fused and protected

Yet again someone falls over that cap on the tute, can nobody fix the schematic?


Rob

Thanks guys... Can I ask then, is this tutorial wrong also about the two capacitors on the oscillator pins of the atmega?

http://arduino.cc/en/Main/Standalone

That tutorial is correct about the need for capacitors with the crystal and about the placement of the crystal and the capacitors. The tutorial, however, omits the required bypass capacitors (0.1uF) between each of the power pins and the adjacent GND pin.

Don

Yet again someone falls over that cap on the tute, can nobody fix the schematic?

I'm with you on this.

There is just no need for this cap unless you have some really screwy wiring. My opinion is that is can and should be eliminated and and only added back if you can prove that you have poor signal in that pin and you are having troubles latching data for some reason.

Here is a complex 595 scenario... I see no caps of data lines. http://softsolder.files.wordpress.com/2009/07/arduino-hardware-spi-schematic.png

There is no need for this capacitor under any circumstance. Its inclusion was probably due to lack of power supply decoupling. Adding the cap appered to solve the problem but it was done by someone with little grasp of electronics.