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Author Topic: make a ttl cpu or a cmos?  (Read 1613 times)
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Quebec, Canada
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Hello guys! I'm working on a project and i need a little help to developp my idea! I want to make a homemade cpu only made of transistor and logic gates!

I know it's a bit complex but with reference and tutorial i will probably succeed. But my big question..do i use ttl chip or cmos one..i found that ttl chip a very very cheaper..but i little bit more complex to get to work( i'm talking about transistor, not logic gate). Because i'm working on the SRAM of the unit and i know power comsumption is less with cmos than ttl.

I want to create NAND memory ram..but with cmos i need 16 transistor  to create one bit and with ttl i need 4 ttransistor but more resistor...

I really think i will go with ttl logic gate..and not ttl transistor or mosfet..

What do you think?
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Quebec, Canada
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I think i will use those cmos 4 bit memory ic! They have a low price and contain much more memory! Lol
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Left Coast, CA (USA)
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It really doesn't make much sense using older pure TTL 74xx00 series chips these days as their current consumption will drive your costs and problems up beyond your expectations. There are TTL voltage compatible chips available such as the 74HCxx and 74HCTxx that make most all the useful logic functions available in very low power CMOS versions.

Lefty


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You'll find the surface mount versions (SOIC14, SOIC16 etc) are much cheaper these days
as they are used in quantity in new designs.

The 74HC family has the advantage of running on 2 to 6V and is widely available.  The
74HCT family only runs at 5V and is designed for interworking with TTL.

Definitely avoid TTL or even LS TTL, the power consumption will be huge for such
a project, CMOS will run off a small battery (well if there aren't too many LEDs).

There are CPU bitslice chips in the 74XX series if I remember right,
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Quote
What do you think?

For what you do, either will work.
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Ambitious project, there are one or two folks out there that have tackled similar projects, gotta admire 'em, e.g.:
http://www.homebrewcpu.com/
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I understand the fact that cmos is the modern standard of the industry..but for a little mcu made with a bunch of ic i really look pricing first. Power consumption is not a big problem because i will use a constant piwer source..

The only thing thag i dont like on ttl chip is the voltage drop..with many chip i dont know if it will drop enough to end with a high level signal to a low..between 2 volt (2 volt is the min high level for 74lsxx series, that i wasplanning to use).

Thanks everybody!
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What voltage drop are you worried about ?

If it is the overall voltage available from your power supply then you need one that will supply more current.
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yeah,i dont know if it will drop enough to end with a high level signal to a low.i agree with you,thank you
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yeah,i dont know if it will drop enough to end with a high level signal to a low.i agree with you,thank you

Your concern is a little confusing. A TTL output signal has a 'fan-out' rating, that is how many input pins it can drive while still maintaining legal TTL high and low voltage ratings. Most TTL output signals have a minimum fan-out rating of 10 and driver chips have even higher fan-out ratings. This has nothing to do with the Vcc voltage rating for the chips, old standard TTL chips usually had ratings of 4.75 to 5.25 although there were  military rated versions of the 7400 series that had a higher range of Vcc allowed.

Lefty

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Ok nice for you anwser everybody! Some anwser bring me to learn nore about cmos technology..and i realised learn that memory cells today were made from cmos gates because of the power consumption. So i found on the web cmos sram chip that cost only a couple bucks and the greastest thing is that most of them are ttl compatible. That is really nice! Many chip today are made of cmos and ttl. Because ttl is faster and cheaper ill probably use them for logic..

Now that i will use cmos sram..ill save power consumption and save money and space and i'll will have more memory smiley

There are few ttl sram ttl chip on the market and they get a 4 bit scale data bus.

If someine is interest in a cpu project tell me i would really love to create a community to share design and idea with people interest in it!

(And sorry for my english i am from quebec i my native language is french. :p
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Well the key TTL component to use in designing and building your own cpu is the four bit ALU chip. You can interconnect them to become a 8, 16, 24, or even 32 bit processor if you wish. This series of chips was used in most all the minicomputers built in the 70s by Dec, Data General, and all the rest. It really combines a lot of the complexity of a CPU using this chip.

http://susta.cz/fel/74/pdf/sn_74181.pdf

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I suggest these links for brainstorming on this project.  Schematics are available for all three.

http://www.homebrewcpu.com/
http://web.cecs.pdx.edu/~harry/Relay/
http://www.galacticelectronics.com/Simple4BitCPU.HTML
« Last Edit: January 08, 2013, 01:36:16 am by JoeN » Logged

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Nice for the 74181 chip! I ordered two of these! Thanks smiley
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I think you're confused about the difference between "74xx series logic" and "ttl  logic."  The 7400 series as a very popular logic series, originally implemented in TTL (which is a type of internal circuitry.)  Since then there have been quite a huge number of 74xxYY series that implement the same functions in the same pinout as the original 74YY chips, but with different internals.  74L00, 74S00, 74LS00, 74F00 were all variations of TTL internally, but 74hc00, 74hct00, 74ac00, 74lvc00, and MOST currently manufactured lines are based on CMOS circuirty internally.  When I look at a distributor like Mouser, at some popular function like a 74151 decoder, I find that the cheapest chips tend to be 74hc series, which is very similar in other characteristics (propagation delay, max clock speed, etc) to 74ls series chips.  But much lower in power.  (other sources may vary; I'd imagine that there's a fair amount of "real" TTL logic on the surplus market these days.)
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