Prop to 74HC595 - Can't find schematic

Jonathan

Hi All,

I looked! I really did! I know there is a schematic floating around to use a '595 with the Prop, in fact I used it previously. I thought it was in the "Safely interface 5V" thread, but I can't locate it now.

Sorry for the trouble, but could someone point me to it?

Thanks!

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www.madlabs.info - Home of the Hydrogen Fuel Cell Robot

deSilva

There are many ways to do it one as simple as the other...
I remember Arealperson hs done something
http://forums.parallax.com/showthread.php?p=695531
but also can't find code from him or such...

Jonathan

DeSilva,

Not looking for a code snip, I have used the 595 with the Prop before. I ran into issues of incorrect pin states trying to run the 505 directly from the Prop. I found some araangement of pullups that worked fine, but I can't seem to locate it now. The previous project doesn't live with me, so I can't just pull it apart. I'm suprised that I can't find the circuit in my collection, I'm ususally pretty good about archiving such things.

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www.madlabs.info - Home of the Hydrogen Fuel Cell Robot

deSilva

There should be no need for anything you describe.
A 74HC595 plugs directly into the Propeller, the other side of course can be more tricky, depending on external details...

But no, it's all standard CMOS output 20 mA plus TS

Post Edited (deSilva) : 2/24/2008 7:13:53 PM GMT

Bob Lawrence (VE1RLL)

Here is one example

http://forums.parallax.com/attachment.php?attachmentid=49682


Another one with Keyboard Matrix:

http://forums.parallax.com/attachment.php?attachmentid=49661

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Aka: CosmicBob

Jonathan

Thanks for the relpies. I can see I am not making myself clear. I am running the 74HC595's from a Prop. At 3.3V, sometimes the bits get scrambled, and the 595's outputs are not always in the same state. Somewhere on these forums, there is a schematic that has some sort of pullup system which enables the prop the shift out the bits to the 595's correctly, using just a couple of resistors per line.

Thanks for the help, and my apologies for not providing enough detail.

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Mike Green

You are making yourself fairly clear. What you're not providing is a schematic or detailed description of the circuit that you're using.

What we're saying is that, the 74HC595 should work without anything special when it's powered by the same 3.3V supply as the Propeller and is connected directly to the Propeller's I/O pins. You need at least 3 Propeller I/O pins ... SH_CP for the shift clock, ST_CP for the clock to transfer from the shift register to the storage register, and DS for the shift register data. The output enable (OE) and master reset (MR) have to have pullups to the +3.3V power source, maybe 10K or need to be connected to other output pins.

If this setup is not working then there is something else wrong and you'd have to go into more detail about the output connections, what you're observing, and you'd have to post your code since that may be your problem.

Peter Jakacki

74HC operation can work and also can be marginal when interfaced to 3.3V logic but there is an easy solution, use 74HCT instead. If you use resistors then you are only fudging it IMO, do it properly.

*Peter*

Bob Lawrence (VE1RLL)

Maybe the stamp example is close enough: (5v)

http://www.parallax.com/dl/docs/books/sw/exp/sw23a.pdf

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Aka: CosmicBob

Mike Green

If you decide to run the 74HC595 from +5V, you will need a resistor in series with the I/O pins from the Propeller as described in the "Safely interface with 5V logic" thread. Typically, 1K resistors will do the job.

Jonathan

Mike,

The 595's are driving old 74141's, nixie tube drivers. They have to run at 5V, and there is no easy 3.3V solution to drive nixies that I am aware of. If there is, let me know by golly! In any case, I have a load of the 74141's and they have very few other purposes, so I might as well use 'em. As to the connections and programming of the 595, no problems there.

Peter,

You are right, using the HC part is marginal. However, I am not driving the 595's very fast and it has worked with the cheesy resistor solution before, and I swear I got the schematic from somewhere on these fourms. I suppose I should just order the HCT part, but I am working on the project today and live 5 miles from a town with a population of 399, so going and picking some up isnt an option.

Thanks again, and apologies again for not making myself clear.

Cheers!

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Chuck McManis

So the electrical problem here is that an HC part needs 3.8v to recognize a logic true level whereas an HCT part will recognize 2.4v as logic true even when running on a 5V supply. I suspect the "fix" you are envisioning is pulling up the outputs of the propeller to 5V, except that the propeller doesn't have open collector outputs so what that ends up doing is feeding current back into the propeller in an annoying way (well annoying to the propeller). If you have a 7407 or a 7417 (which is an open collector hex buffer/driver) in your junk box somewhere you can use that between the propeller and the HC part to drive it reliably. It might save you a trip into town or an order from the local distributor.

--Chuck

DavidM

HI,

This is what I have and do using shift registers..

I don't have the schematic, but I have made these pcbs and they work well for me,
It connects to 16 buttons/keys and drives 16 led's
I drive the leds shift register the 74HC595 with 5 volts, and its interfaced to the PROP with 1K resistors,
I have also attached the SPIN code I use for both devices.
The RED is the top layer tracks and the BLUE is the back layer ( in this case its also my ground plane).

The keypad circuit only need to run at 3.3 volts so there is no need for resistors here.


Hope this helps.


Thanks

Dave M

deSilva

(a) I am aware of the theoretically marginal situation wrt high input/5V Vdd/HC, though in my limited experience that had never been an issue, and I never heard of one....

(b) A common solution for similar (but different!) situations is

                            +- 10k  Pull-up to 5V +
Propeller Pin --- |< Diode -+
                            +-- 595 Input ---

You then drive the 595 through DIRA rather than OUTA!

Though I think this is overkill...

(c) A trick solution can be to slightly reduce the SUPPLY VOLTAGE for the 595 by a 4001 diode to 4.4 Volts, bringing its input into the safe region and leaving its (high) output in safe TTL.

---
Edit:
I had one of those typical upside-down typos in the drawing ad (b)
The cathode of the diode has to be to the Prop side of course!
Case LOW: Current from 5V - 10k - diode (0.6V) - Prop Low (0.3V) = Voltage for the 595 is around 1V
Case Quasi-TriState: Current from 5V - diode (0.6) - ESD (0.4V) - Vdd (3.3V) = Voltage for the 595 is 4.3 V

Post Edited (deSilva) : 2/25/2008 3:43:35 AM GMT