TTL IN MEGA128

Go To Last Post
27 posts / 0 new
Author
Message
#1
  • 1
  • 2
  • 3
  • 4
  • 5
Total votes: 0

Hello
I want to get an ATTEGA128 TTL output What should I do?

  • 1
  • 2
  • 3
  • 4
  • 5
Total votes: 0

In other words, if I want to say, I want to get the3 TTL output from mega128. Now what should I do?
Thank you for your guide

  • 1
  • 2
  • 3
  • 4
  • 5
Total votes: 0

Explain what you mean by "TTL output".  IIRC, TTL outputs go from about 3.5V hi to 0.4V lo.  That means run your AVR at between maybe 4V and 5V and you'll have TTL-friendly output levels.

  • 1
  • 2
  • 3
  • 4
  • 5
Total votes: 0

I did not really understand your point

  • 1
  • 2
  • 3
  • 4
  • 5
Total votes: 0

I want to get 3 ttl outputs and 8 digital outputs from mega128 but I do not know what to do.

  • 1
  • 2
  • 3
  • 4
  • 5
Total votes: 0

How does a ttl output differ from a digital output? TTL is just a voltage level specification, so kk6gm's comment stands. 

 

  • 1
  • 2
  • 3
  • 4
  • 5
Total votes: 0

     From Wikipedia (Link):

A TTL input signal is defined as "low" when between 0 V and 0.8 V with respect to the ground terminal, and "high" when between 2 V and VCC (5 V)

If Vcc is 2 volts or more, the AVR output port is TTL compatible.

 

Are you asking how to configure AVR ports as outputs and inputs?

 

 

 

  • 1
  • 2
  • 3
  • 4
  • 5
Total votes: 0

I'm really confused.
I have a board  with my MEGA128 processor and telling me 8 digital PINS and 3 TTL PINS give us as well protocol for communicating with other hardware.
I do not know now

Last Edited: Sat. Sep 23, 2017 - 06:31 AM
  • 1
  • 2
  • 3
  • 4
  • 5
Total votes: 0

What is the board (who from and what board name)? Do you have a schematic of the board? If you do, please add it as a picture (use the picture icon).

 

There ARE physical differences between CMOS and TTL outputs. HOWEVER, they are generally compatible if the micro is run at 5V power supply.

 

Jim

Jim Wagner Oregon Research Electronics, Consulting Div. Tangent, OR, USA http://www.orelectronics.net

  • 1
  • 2
  • 3
  • 4
  • 5
Total votes: 0

bravehamid wrote:
I'm really confused.

You're not the only one!

 

The question you asked has been clearly answered several times - it looks there must be a language barrier.

 

You seem to be using "TTL" in a different way to everyone else - hence your question makes no sense to us, and our answers make no sense to you.

 

So be precise, and say exactly what you mean by "TTL output" in this context.

 

I have a board  with my MEGA128 processor and telling me 8 digital PINS and 3 TTL PINS

Is this a board that you have designed yourself?

If so:

  • What do you mean by "digital" pins?
  • What do you mean by "TTL" pins?
  • What do you think is the difference between "digital" and "TTL" here?

 

Or, is this a board that you have obtained (or intend to obtain) from some supplier?

If so, give a link to the board details so that we can see what you are talking about.

 

Last Edited: Sat. Sep 23, 2017 - 07:32 AM
  • 1
  • 2
  • 3
  • 4
  • 5
Total votes: 0

Chuck99 wrote:

     From Wikipedia (Link):

A TTL input signal is defined as "low" when between 0 V and 0.8 V with respect to the ground terminal, and "high" when between 2 V and VCC (5 V)

If Vcc is 2 volts or more, the AVR output port is TTL compatible.

 

But this would leave no noise margin on the high side.  Personally I'd choose 3V minimum Vcc for good noise margins.

 

Problem here is ambiguity in the phrase "TTL outputs".  Does this mean outputs that have typical TTL output range?  Or outputs that meet minimum TTL input requirements?

  • 1
  • 2
  • 3
  • 4
  • 5
Total votes: 0

TTL does not just define the voltage levels but also the dirve capabilities

 

A TTL output should be capable of driving a load of at least 10 TTL inputs

 

This needs to be taken into account when designing circuits - especially if you are mixing logic families (I recall one product I worked on in the late 80's where an 74LS04 gate was needed as a buffer between a cmos chip & a 74XX series IC so as to be able to drive an LED, the cmos 4xxx series could not directly drive the 74xx) otherwise unpredictable results can occur

 

  • 1
  • 2
  • 3
  • 4
  • 5
Total votes: 0

IPguru IS correct. That is why I asked for a model number for the board and a schematic. 

 

It MIGHT be that three outputs are buffered with a standard TTL buffer logic device though that seems a bit bizarre in this day.  Perhaps the board was designed for a special application that requires this? We simply have no way of knowing until more information is provided.

 

Jim

Jim Wagner Oregon Research Electronics, Consulting Div. Tangent, OR, USA http://www.orelectronics.net

  • 1
  • 2
  • 3
  • 4
  • 5
Total votes: 0

The OP seems to be drawing some distinction between "digital" and "TTL" ...

 

  • 1
  • 2
  • 3
  • 4
  • 5
Total votes: 0

I suspect that this is on the board or in the documentation. That was one of the reasons for requesting more information. There may be some language translation "issues" somewhere along the line.

 

Jim

Jim Wagner Oregon Research Electronics, Consulting Div. Tangent, OR, USA http://www.orelectronics.net

  • 1
  • 2
  • 3
  • 4
  • 5
Total votes: 0

IPguru... wrote:
I recall one product I worked on in the late 80's where an 74LS04 gate was needed as a buffer between a cmos chip & a 74XX series IC so as to be able to drive an LED, the cmos 4xxx series could not directly drive the 74xx)....

If a bit of propagation delay was o.k., adding a 1K pull down resistor to a 4xxx B-series output would have worked for a single TTL input.

- John

Last Edited: Sat. Sep 23, 2017 - 08:14 PM
  • 1
  • 2
  • 3
  • 4
  • 5
Total votes: 0

Just run your AVR system at +5V.  Ignore everything above.  Too many notes, Herr Mozart, too many notes.

  • 1
  • 2
  • 3
  • 4
  • 5
Total votes: 0

Simonetta's recommendation results in (more or less) "TTL Compatible" operation. It does not result in something that satisfies TTL specs, including driving some number of true TTL inputs. The logic threshold is also off, resulting in reduced noise immunity in the logic-high state. 

 

So, yes, it will usually work in simple (single TTL gate load) situations. Usually. As an engineer, usually is not usually good enough. Just like over-clocking!

 

Jim

Jim Wagner Oregon Research Electronics, Consulting Div. Tangent, OR, USA http://www.orelectronics.net

  • 1
  • 2
  • 3
  • 4
  • 5
Total votes: 0

Driving TTL chips using AVR do need series resistor. Even it's only tens of ohms. Same with cmos driving ttl.
But for one who don't care as long as it "works" then one can drive few TTL chips directly from AVR.
.
MG

I don't know why I'm still doing this hobby

  • 1
  • 2
  • 3
  • 4
  • 5
Total votes: 0

A series resistor is not required to interface an AVR running off 5V with TTL. 

 

It does require a pull-up when driving AVR from TTL as the hi-level may not meet the Vih of AVR.  The AVR has a worst case requirement of 3.5V (0.7 of VCC) for its high-level input, TTL only guarantees 2.4V. A pull-up can make this work but speed will be limited.

 

A series resistor may be needed to avoid ringing on the line, but that is not related to interfacing between CMOS and TTL.

 

kevin

 

  • 1
  • 2
  • 3
  • 4
  • 5
Total votes: 0

ka7ehk wrote:
There may be some language translation "issues" somewhere along the line.

Indeed - see #10.

 

This may be one of those cases where the OP really needs someone local - or, at least, with the same native language ...

  • 1
  • 2
  • 3
  • 4
  • 5
Total votes: 0

I was thinking less of the OP than of the board maker. I see several possibilities:

 

1. The board really has some TTL buffers. Seems strange but stranger things are out there.

 

2. The three "TTL" are intended as output only, while the "digital" really are GPIO.

 

3. I think that the original posting has changed. #2 now reads "I want to get the3 TTL output from mega128". If this is really what the OP wants, then the advice above is sufficient.

 

Jim

Jim Wagner Oregon Research Electronics, Consulting Div. Tangent, OR, USA http://www.orelectronics.net

  • 1
  • 2
  • 3
  • 4
  • 5
Total votes: 0

TTL could mean 'time to live'. Since we have no idea of the board involved, it is all conjecture.

  • 1
  • 2
  • 3
  • 4
  • 5
Total votes: 0

Anyone seen OP recently?

  • 1
  • 2
  • 3
  • 4
  • 5
Total votes: 0

clawson wrote:
Anyone seen OP recently?

+1

Thinking the same thing frown

 

JIm

If you want a career with a known path - become an undertaker. Dead people don't sue! - Kartman

Please Read: Code-of-Conduct

Atmel Studio6.2/AS7, DipTrace, Quartus, MPLAB user

  • 1
  • 2
  • 3
  • 4
  • 5
Total votes: 0

Anyone seen OP recently?

 

Perhap's he/she will be back to school on Monday? 

 

Greg Muth

Portland, OR, US

Xplained Boards mostly

Atmel Studio 7.0 on Windows 10

 

  • 1
  • 2
  • 3
  • 4
  • 5
Total votes: 0

2.5V, 3.3V, and 5V CMOS (mega128) can drive 5V TTL.

5V TTL can drive 5V CMOS through HCT or ACT buffers.

Mixed voltages are possible by other means.

 

Reference :

The Art of Electronics, third edition, pages 798 and 799

Figure 12.9. Logic family interconnections. See text for narrative.

http://www.cambridge.org/us/academic/subjects/physics/electronics-physicists/art-electronics-3rd-edition?format=HB

 

"Dare to be naïve." - Buckminster Fuller