CMOS fan out capacity

[Mikmo]

I'm planning a Lunetta style system, and i wonder how many CMOS logic inputs can i drive from one CMOS logic output without buffering ?

[blue hell]

Fanout is not really an issue for CMOS, in the thousands typically. For instance a CD4001 wants less than 5 uA worst case on its input while an output can provide around 5 mA .. depending a bit on the supply voltage. For high frequencies capacitance will play a roll, but you're not likely to hit that problem for a Lunetta device.

[loss1234]

Blue hell

i know that TTL has always had more of an issue with fanout but can you tell me if this is a problem with the 74C models?

i ask because as recently i have been finding some great ideas in the TTL cookbook which are not available as standard CMOS (like the divide by 12 chip i asked about a few days ago)

i have been thinking about setting up a +5 volt system for my lunetta (instead of the +/- 15 i am now using) so that i can mix TTL and CMOS...is this a realistic or practical idea?

thanks

[blue hell]

It is not trivial to mix CMOS and TTL, you'll need to be able to read the data sheets for it. The trip levels of inputs are different and the construction of the output circuits differs as well, also it depends a bit on what exact chips you want to mix.

When, as you sort of suggested, you want to use chips from the 74C family you can mix those freely with CMOS as they are in fact CMOS and they will run from a 15 V supply as well. Some of the traditional CMOS 4xxx series are also available as 74C4xxx BTW.

The classical TTL prefix is 74, without any letters (avoid these altogether), but then there also is 74LS, 74HC, 74HCT, 74ALS .. maybe more even - these are not easily mixed with CMOS, and have to run at around 5V (typically 4.5 .. 5.5 V).

For 74C the fanout is comparable to the CD4xxx series, for 74HC and 74HCT this seems to be the case as well (but the latter as said need a Vcc of around 5 V and can not not trivially be mixed with CMOS).

Inter family fanout questions are hard to answer, one would have to look carefully at the data sheets.

[loss1234]

thanks for the good info.

i think the cmos cookbook gives a little bit of info on how to make TTL and cmos work together right?

[blue hell]

[richardc64]

Come on, we're not talking about "mission critical" or life-support applications here.

http://www.interfacebus.com/Design_Translation.html

In general, a TTL output with a pull-up resistor can drive several CMOS inputs. I wouldn't expect a CMOS output to drive more than 1 TTL input.

For more in-depth, see the last 2 pages http://www.fairchildsemi.com/an/AN/AN-77.pdf

[loss1234]

thanks guys

blue hell-YOU have never seen the cmos cookbook? wow. you sure have a lot of information though. did you go to school for this stuff? i got almost all of my cmos info from that book (And this forum of course)

it is a great book though. i got my copy CHEEAP on amazon.

[blue hell]

I did do electronics once at school yes, but mainly I read data sheets

[loss1234]

nothing against data sheets but the cmos cookbook told me tons of in depth stuff that i often dont grasp as well from datasheets. like how to even understand what gates do,etc.

i think don lancasters books are really complete. unlike the cmos sourcebook which is really just a collection of datasheets, the cmos cookbook has complete projects, scenarios,etc as well as good writing. if you ever see it cheap, i think you'd enjoy it too.

anyway....

[blue hell]

[Pehr]

Is it possible to drive LEDs with the outputs aswell, without buffering with a transistor?