Hardware Design Clarification

Hi All,

I want to understand this, if I go for new hardware design should I make the following decisions at the beginning of the design

1. The design should be CMOS based or TTL based in the sense that all the ICs on the board should be supporting either CMOS or TTL?

2. If I go for CMOS based all the ICs I purchase should be of CMOS family and I need to manually check from the IC number whether it belongs to that family and similarly for TTL based design I should buy all the TTL ICs?

3. In case if I buy ICs in which some are CMOS and some are TTL then I should take care of the interfacing circuitry? (as i believe CMOS and TTL are not directly compatible)

4. And finally for every IC will it be mentioned that it is either from CMOS or TTL family?

These questions are giving me lot of confusion.

Thanks in advance,
Regards,
Satya
 
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39
We still use TTL stuff? Last design I did was all LV CMOS 6 years ago already!!!!
 
We still use TTL stuff? Last design I did was all LV CMOS 6 years ago already!!!!
I am sorry, I did not understand what you are trying to say. Are you trying to say that TTL family is no more available and we are using only CMOS?

Thanks and regards,
satya
 
825
1
Your main components will be the technology they are - sort of out of your control, and you pick those components for other reasons than their logic level. For example, I would pick a microcontroller based on all of its features, cost, package and almost never consider its logic level (unless it comes with different options or is non standard).

Once that is established, you will have to check every time you interface one IC to another. You need to look at what kind of logic they use and acceptable voltages between one chip's Voh/Vol and another chip's Vih/Vil. A lot of times you can choose the less important ICs to be the same logic as the ICs you have to use. If they do not satisfy the datasheets, then you will need some kind of voltage translator, buffer, or open-collector intermediate circuit to translate the voltage from one level to another. This is just part of the whole process which also includes satisfying timing diagrams or making sure propagation delays are in tolerance.
 
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