Combining TTL and CMOS

Thread Starter

RyanS

Joined Aug 15, 2007
4
I recently pulled out a couple old electronics learning kits of mine that came with CMOS 4000 series logic chips. I thought it might be cool to (try to) build a simple 4-bit processor with the combined parts.

I don't have enough parts with just the CMOS chips, and I don't really feel like spending the extra time and money to get more, but I have a couple TTL 7404 inverter chips and a bunch of 74S37 nand chips.

If I ran everything at 5 volts, would the CMOS work with the TTL, or would it ker-splode? If it would work, is there anything that I might have to take into account such as a difference in the number of inputs I could drive with one output, etc?

Any help would be appreciated.:)
 

Salgat

Joined Dec 23, 2006
221
Should work fine. CMOS, if I remember right, has a slightly higher voltage for TRUE logic(3.8V?), but if you are using the traditional 0V and 5V, they will work together fine. Just remember when using CMOS ICs to never leave any floating inputs, tie them to either one logic state(ground or 5v).
 

lightingman

Joined Apr 19, 2007
374
Also... remember that CMOS driving TTL will have a limited fan-out.... I.E. you may only be able to drive 1 TTL input with a CMOS output.... The fan-out being the number of outputs that can drive a number of inputs for a given logic family...Daniel.
 

beenthere

Joined Apr 20, 2004
15,819
It's coming from TTL to CMOS that may present the most trouble. CMOS likes the input to swing all the way to Vcc, wher some TTL is only able to go to around 4 volts. There are CMOS buffers (4049 comes to mind) that are able to handle lower input swings.
 

Thread Starter

RyanS

Joined Aug 15, 2007
4
I have a 4049 that I can use to buffer 6 lines, but I will have 8 lines at the TTL level. Will transistors work to buffer out the other lines?

[edit]
Something like the image I attached.
 

Attachments

Salgat

Joined Dec 23, 2006
221
It's coming from TTL to CMOS that may present the most trouble. CMOS likes the input to swing all the way to Vcc, wher some TTL is only able to go to around 4 volts. There are CMOS buffers (4049 comes to mind) that are able to handle lower input swings.
Assuming he's using the same 5V for both TTL and CMOS, there should be no issues correct? I've done this myself with no issues on my digital clock.
 

beenthere

Joined Apr 20, 2004
15,819
Depends on the TTL type. 74HC would be just fine. 74LS might not be satisfactory without buffering, because the output swing won't go past 4.4 volts, which is a bit too little for a CMOS input. Original TTL would absolutely have to be buffered coming and going.
 

bloguetronica

Joined Apr 27, 2007
1,371
When driving a CMOS gate with a TTL gate, remember to add a pull-up resistor to the output of the TTL gate. You can refer to here:
http://www.kpsec.freeuk.com/components/ic.htm

Depends on the TTL type. 74HC would be just fine. 74LS might not be satisfactory without buffering, because the output swing won't go past 4.4 volts, which is a bit too little for a CMOS input. Original TTL would absolutely have to be buffered coming and going.
The 74HC family is a CMOS family, not a TTL one. HC stands for high speed CMOS. There is also the 74HCT family, which is compatible with TTL. Never confuse the 74HC or 74HCT CMOS families with other families such as 74, 74S, 74L, 74LS and 74F (TTL families). A common mistake.
 

Thread Starter

RyanS

Joined Aug 15, 2007
4
I'm using the 74S series. I tested it and found that the outputs from the TTL ICs could drive the inputs of a CMOS IC just as they were. I'll try it without any buffering or pullup resistors and see what happens.
 

Salgat

Joined Dec 23, 2006
221
Depends on the TTL type. 74HC would be just fine. 74LS might not be satisfactory without buffering, because the output swing won't go past 4.4 volts, which is a bit too little for a CMOS input. Original TTL would absolutely have to be buffered coming and going.
Why would it have to be buffered?
 
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