Interfacing level-shifting IC

Other Hardware General Electronics
1

Hello,

So I am trying to interface a 74LVC245 level-shifter. I have an Arduino DUE, which I want to use to communicate with a 5v (pre-existing) circuit.

I have the 3v3 from the Arduino hooked up to the Vcc-pin on the IC; and GND on the Arduino hooked up to the OE, DIR and GND pins on the IC.

The strange thing is that all output-pins are at this point high (no input pins connected). Surely this can't be right? Or am I misinterpreting something?

If I instead move the DIR pin to 3v3, the direction changes as you would expect, but the output-pins are still high. Do I need a resistor somewhere? :slight_smile: Horribly confused... =)

Spec here: http://www.adafruit.com/datasheets/sn74lvc245a.pdf

Thank you.
Pelle

2

If I instead move the DIR pin to 3v3, the direction changes as you would expect, but the output-pins are still high. Do I need a resistor somewhere? :slight_smile: Horribly confused IGNORANT... =)

Of course it's correct. A floating logic input floats high. Anyone with more than 10 minutes of TTL logic experience knows that. If you want a LOW output, ground the input. If don't know anything about TTL you shouldn't be messing with this chip until you do a little basic study of logic circuits. Who trained you ?

3

raschemmel:
Of course it's correct. A floating logic input floats high. Anyone with more than 10 minutes of TTL logic experience knows that. If you want a LOW output, ground the input. If don't know anything about TTL you shouldn't be messing with this chip until you do a little basic study of logic circuits. Who trained you ?

Dude I literally started 15 minutes ago. No one trained me, and I would not use ignorant (especially all caps) to describe my state of mind.

What you are saying seems true, and kinda makes sense; everything is working now. So thanks for that.

4

and I would not use ignorant

Dude I literally started 15 minutes ago

Ok, I was off by 5 minutes...

Why are you using a 7400 series chip if you've never heard of TTL ?
Aren't you skipping some steps ?

5

raschemmel:
Ok, I was off by 5 minutes...

Why are you using a 7400 series chip if you've never heard of TTL ?
Aren't you skipping some steps ?

They seemed easy to use, which later turned out to be an incorrect assumption.

Yes. Most likely a ton of steps.

However, in figuring out what this TTL stuff is I will (or at least I imagine I will) learn stuff. Sooner or later, boom - Absolute wizard in this subject too.

6

LVC is not the same as TTL. It is CMOS technology. You can't leave the inputs floating.

7

LVCmos logic levels

However, in figuring out what this TTL stuff is I will (or at least I imagine I will) learn stuff. Sooner or later, boom - Absolute wizard in this subject too.

Just for the sake of discussion, did you just start in electronics 15 minutes ago too ?

8

raschemmel:
Of course it's correct. A floating logic input floats high. Anyone with more than 10 minutes of TTL logic experience knows that.

There's no TTL involved, this is all CMOS if you cared to read the posting...

9

I find the tone of your post raschemmel to be very unpleasant

I find vitamins and some medicines to be very unpleasant but they are beneficial or people wouldn't be taking them. I can tell you for a fact that the USAF boot camp was very unpleasant but I found it to be quite rewarding and I think I was almost starting to like it when I graduated. They were never very nice to us in boot camp, but you know what , we totally understood why.

My apologies for the tone, but I was trying to make a point, that being: "Do your due diligence of researching your topic using Google before posting on the forum.

There's no TTL involved,

That isn't really true is it ? It's a TTL to CMOS converter chip. That's why it's called a level shifting ic.

So I am trying to interface a 74LVC245 level-shifter. I have an Arduino DUE, which I want to use to communicate with a 5v (pre-existing) circuit.

TTL integrated circuits were manufactured by several semiconductor companies, with the 7400 series (also called 74xx) by Texas Instruments becoming particularly popular. TTL manufacturers offered a wide range of logic gate, flip-flops, counters, and other circuits.

TTL

**74**LVC245

Supports Mixed-Mode Signal Operation on All Ports (5-V Input/Output Voltage With 3.3-V VCC)

Inputs can be driven from either 3.3-V or 5-V devices. This feature allows the use of this device as a translator in
a mixed 3.3-V/5-V system environment.

To say there's no TTL involved when discussing a 7400 series chip , the sole purpose of which is to interface TTL family ICs to CMOS family ICs doesn't make a whole lot of sense to me.

TTL or CMOS, there are still basics that should be learned before using it. The datasheet was linked yet evidently not read. I think it's not too much to ask to read the datasheet you are linking.

If the OP had started his post with:

"Hi, I have never used logic chips before (like the 7400 series) . I want to use the 74LVC245 but I don't even know what LVC means or how to use it. I have the link for the datasheet but don't understand what it says. Can someone please help me learn how to use this chip ?"

do you think my reply would have been the same ? (obviously not)

There should be some effort on the part of the OP to research his topic before posting. If you believe it is not nice to say that then I'm sorry.

What is the point of linking a datasheet you haven't read ?

page-4 of 74LVC245 datasheet

(1) All unused inputs of the device must be held at VCC or GND to ensure proper device operation. Refer to the TI application report,
Implications of Slow or Floating CMOS Inputs, literature number SCBA004.

We are here to help. I think I am doing that. (in my own way)(Think of it like chugging two raw eggs with a tablespoon of Cod Liver Oil)

I think a little effort on the part of the OP is not too much to ask.

10

raschemmel:
My apologies for the tone, but I was trying to make a point, that being: "Do your due diligence of researching your topic using Google before posting on the forum.

[...]

I think a little effort on the part of the OP is not too much to ask.

I am amazed at how butt-hurt you are from my low experience with electronics.

I did google. I spent a lot of time googling. Rebuilt the circuit using 3 different ICs (assuming a bad production batch or something) in accordance with various schematics I found online. Strangely, they were all more or less identical to how I had imagined interfacing the IC from its manual. I had probably been trying to figure this out for 2-3 hrs before asking here, and there are only so many configurations to try on an IC with 4+16 pins, so I absolutely feel I gave it my best before coming here.

This was of course before I had any idea that floating a pin != grounding the pin, so all my attempts were made moving a +5v cable around on the input pins thinking that floating == grounding. Sloppy, possibly, but again, I got the hardware in the mail today and as I am sure you have learned by now, I lack prior knowledge. What I am trying to say is, google'ing for concepts that you don't know exist is exceedingly hard.

So you will tell me to buy a book.

I do not desire absolute understanding enough to read a book on the subject. This is the exact reason why I bought the Arduino. Trial-and-error-quick-results-hacking.

Using only google, and this one question which I feel is inches away from having my mom insulted, I managed to record my controller inputs when playing on my Nintendo 8-bit, then on reset, have the Arduino play it back. Excellent fun, excellent progress. Lots of fatal crashes on the NES, occasionally it smelt kinda strange, but good fun. I learned a ton.

If I had aspired to become a professional on the subject, this is probably not route I would have taken, but I don't. I just want to have fun and learn quickly (possibly (probably) incompletely) with equally quick results.

11

I couldn't tell from your answer but, before wasting 2-3 hours with trial and error did you spend 30 minutes reading the datasheet ?

There is a special hell reserved for people who don't read datasheets. It's place where nothing works and everything smokes when you apply power.

The datasheet isn't there to help you become a professional. It's there to help you understand how to use the IC.

And I wasn't hurt. But if you read my post you saw that the datasheet did mention the floating inputs.

(If I can get you to read one datasheet from start to finish I will feel like I have made a difference)

Trial-and-error-quick-results-hacking.

We're quite familiar with your school of learning... ;D

Yes , we'll be seeing a lot more of you ...

What I am trying to say is, google'ing for concepts that you don't know exist is exceedingly hard.

Not that it matters now but I am a little curious exactly how you did Google it .(what search phrase you used)
We don't expect you to know anything. What we expect is honesty. If you don't have a clue, we expect you to state that right up front so we can start at the beginning. That's all we ask. For example, like asking a question such as "Is there anything in particular I need to know to use logic circuits ?)
(keep in mind, some of us have been using the 7400 series since the 1970s)

BTW, you never told us WHAT the "pre-existing 5V circuit" was OR whether the it was the receiver or the sender of the data. Do you understand what determines the output voltage of the chip and how you would wire it if you were sending data TO the DUE vs if you were sending the data FROM the DUE ?

12

raschemmel:
Yes , we'll be seeing a lot more of you ...

I highly doubt that. Too many raging elitists here.

13

I highly doubt that

You'll be back because people come here when they want the truth, the whole truth and nothing but the truth. They come here when they want answers.

Too many raging elitists here.

"A professional by any other name..." That's why you posted here in the first place and that's why you'll be back. When you get tired of wasting time with your"

Trial-and-error-quick-results-hacking.

you'll come here for the truth. And we'll be waiting, 24/7/365.
That's what we do here.

14

Well, That certainly proves that Raschemmel can piss further than anybody else.
Pellsson, don't let that deter you. He treats everybody that way and you can, at times, learn from his rants.
He really isn't such a bad fellow, he just gets a burr up his ...... sometimes.

May have bruised your feelings some, but you did learn a couple of things, didn't you?