Author Topic: Logic Gates and their Datasheets.....  (Read 2734 times)


Logic Gates and their Datasheets.....
« on: March 25, 2007, 10:56:54 PM »
Hoping someone can point me in the right direction here....

I've looked all through this datasheet but I didn't really know what to look for.

How do I determine how many gate-inputs can be driven by another gate's outputs before needing a buffer?

There are several tables and charts with current values but I'm not sure which ones apply.


Re: Logic Gates and their Datasheets.....
« Reply #1 on: March 26, 2007, 11:36:58 AM »
The problem is, the number is variable, not like the old TTL fan in and fan out specs.

A CMOS input is a capacitor. The outputs are current limited. How many capacitors can you drive with 1.3728ma of drive current?

Somewhere between 0 and infinity, depending on how long you can afford to wait.

The fan out is frequency dependent. You have to guarantee that all the input capacitors - and other junk if you put things other than CMOS inputs on the net - will charge to above/below the input thresholds by the time the next clock pulse comes along.

So the answer can be computed by knowing that I = C dv/dt. We know I from the current graphs which list it as a variable depending on the power supply voltage. We know dv, the difference between the long-term resting voltage of the output and the furthest threshold voltage, and we know dt as the time between clock pulses. We can compute the C that is the max capacitance that can be driven in that amount of time. We subtract the wire capacitance from that, divide by the input capacitance of a gate, and that's the number of gates we can drive without a buffer.

Sorry - it's not easy.

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