HackChat topic: CMOS to TTL

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Tuesday, February 9, 2016 06:00 pm PST - Tuesday, February 9, 2016 06:45 pm PST Local time zone:
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Join us on the Hacker Channel to discuss all things 3.3V and 5V and related conversations Tuesday, Feb 9th, 2016 at 6pm PST/ 9pm EST. Time Zone cheat courtesy of the World Clock.

It was suggested in the Hacker Channel that we take an hour or so to discuss CMOS and TTL. Then someone said let's talk about fanout, and someone else said...YEAH it's 74-series chip chat!

This HackChat will be co-hosted by @Philip and @Sophi Kravitz and @Bil Herd and @Al Williams

@Philip is an FPGA, CPU, and RF hacker, and recently released a product: OSH Chip.

@Bil Herd is an engineer who is into analog circuits, ground loops, RFI/EMI, RF and high speed digital/FPGA. He writes for

@Al Williams is an engineer who has designed hardware and software systems ranging from tiny embedded sensors to mainframe build systems. He writes for

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  • Here's the transcript of what we talked about

    Sophi Kravitz 02/11/2016 at 17:16 1 comment

    Benchoff: Hey it's Bil! Bil Bil Bil Herd the computer nerd

    Al Williams: Howdy Brian

    Bil Herd: Just to be clear I dont get called that to my face.

    Sophi Kravitz: :: laughing

    Sophi Kravitz: alright: first Q: What are the differences between CMOS and TTL?

    Bil Herd: Tobasco drinking contest! It will have to be based on the honor system tho...

    jlbrian7: I had to switch to a pc, the tablet was killing me.

    Benchoff: oh so peppercon...

    Al Williams: Well truthfully, it isn't just CMOS and TTL -- there are tons of families with different characteristics

    Sophi Kravitz: is there a chart?

    Al Williams: And variations of even those

    jlbrian7: The one I was working on is 75T. I think... i was more worried about where the wire were going.

    Al Williams: Well there's two ways to look at it: inside and black box

    Bil Herd: Some info in the video posts where I also showed the progression of the families as far as speed, etc.


    Al Williams: from the black box perspective what you want to know is what level is high and what level is low (on input) and how much you can drive on the output

    Philip: The difference is the way the silicon is processed and how the chips are designed. Logically a 7404 (TTL) and a 74C04 (CMOS) do the same basic function

    esot.eric: I found a nice graph some where, from TI I think, easier to visualize than a table... bbiaf

    Benchoff: That picture above is very much incomplete...

    Al Williams: So a TTL gate might take anything below 2V as a zero and anything above 3V as a 1. On output it might output 0.7V and 4.3V. However, usually the amount of current on a "1" at TTL is very low compared to the amount of current it can "sink" when doing a zero

    Bil Herd: To me the family difference starts with wether its a current consuming device on its input or a voltage sensing... except that that is a simplification.

    Sophi Kravitz:

    Al Williams: CMOS is more fair... usually 1/2 threshold (e.g., 2.5V @5V) and usually pretty equal sink/source currents

    Al Williams: You can dig into the internals to find out why, but for most people the black box approach is better.

    Al Williams: The sink and source currents get important when you fan out (connect lots of inputs to an output) or try to drive an LED or something else directly

    Bil Herd:

    Al Williams: the threshold voltages get important when you have noise issues although they can play in other things too including interfacing between families

    jlbrian7: What is the difference between CMOS and BiCMOS, and how do you identify the two families?

    Bil Herd:

    Al Williams: Now you are in the internals

    Bil Herd: TTL Input sinks current from the transistors typically

    Bil Herd:

    Al Williams: The way I like to explain is... is think of this way... a logic gate output is like two switches... one to + and one to ground (except ECL... nevermind)... only one switch is on at one time.

    Bil Herd: CMOS usually means MOSFET which has an insulated gate that senses voltage and doesn't really have something like a sharing of current

    Al Williams: Now the families depend on what are the characteristics of those switches

    Bil Herd: Love ECL

    Sophi Kravitz: what kinds of characteristics are there

    Sophi Kravitz: ?

    Al Williams: Well look at some old logic... the switch to ground would be a transistor

    Bil Herd: And over time the line blurred between CMOS and TTL and the Fast TTL families that CMOS can do almost everything TTL can

    Al Williams: and the switch to + would be just a resistor

    Al Williams: so the current through the resistor had to be low... but the transistor could sink a lot

    Al Williams: Then as Bil says.... what turns the switch on

    Al Williams: a bipolar switch looks like a diode from base to emitter (or you have the emitter inputs like in that schematic above)... but a CMOS (or NMOS or PMOS)...

    Read more »

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Alper wrote 02/08/2016 at 21:51 point

Google calendar ("Add to my Calendar") starts it at 16:00 UTC whereas "Time Zone cheat" at 02:00 UTC. One of them seems to be wrong. I think 2 am UTC is correct, is it?

  Are you sure? yes | no

femptofarad wrote 02/08/2016 at 20:52 point

I had fun with the Hitachi 6309, it was a CMOS 6809 when Motorola was still only making NMOS 9's. Very fun chip, the assembler program we used did not recognize the mnemonics for the extra instructions that 6309 could do and I needed to use one, "XGDX" so I simply in midstream of the assembler source put "FCB $0D" or something, I don't really remember the op code. But the assembler was dumb enough to do exactly as intended to make the image to blast to the chip. I think a regular '9 treats most bad codes as NOPs, other than halt and catch fire.

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Jovan wrote 02/08/2016 at 18:32 point

I hope we will skip old S, slow LS and even slower HC/HCT and instead talk about much faster Bipolar "F", "AS/ASL", ABT or almost evenly fast CMOS AC/VHC family's.

"Use heardryer for hear not for baking IC", long live DIP :)

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Duncan wrote 02/11/2016 at 01:52 point

What's a "heardryer"???

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Benchoff wrote 02/01/2016 at 15:33 point

Oh this is fun. 74LS vs 74HC. 

  Are you sure? yes | no

Yann Guidon / YGDES wrote 02/01/2016 at 16:17 point

I'm slowly converting to LVC and LCX :-P

  Are you sure? yes | no

jaromir.sukuba wrote 02/01/2016 at 20:01 point

LS? That modern low power crap for kids? Classic 74xx FTW, because 5V/50A source is what we need.

  Are you sure? yes | no

Yann Guidon / YGDES wrote 02/01/2016 at 23:01 point

I'm not a fan...
(pun intended)

  Are you sure? yes | no

Jason Westervelt wrote 02/02/2016 at 20:37 point

Hellz yeah! 5V and giga-amps for all!

  Are you sure? yes | no

Yann Guidon / YGDES wrote 02/03/2016 at 02:08 point

Jason, I've had a 3.3V kiloamp power supply... it was ridiculously massive :D

  Are you sure? yes | no

Hacker404 wrote 02/09/2016 at 07:48 point

Yeah, no one will ever need a fanout of more than 2. 

  Are you sure? yes | no

Yann Guidon / YGDES wrote 02/01/2016 at 14:56 point

I see CMOS and TTL, I click.

  Are you sure? yes | no

Yann Guidon / YGDES wrote 02/01/2016 at 15:01 point

Damned font where C and L fuse to make a D... the above comment feels awkward now :-D dcldcldcldcldcldc

  Are you sure? yes | no

jaromir.sukuba wrote 02/01/2016 at 15:44 point

Fortunately, there is more of clicks like you ;-)

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