Re: DFF/clock necessity

Posted by cadet1620 on Jul 05, 2018; 2:06pm
URL: http://nand2tetris-questions-and-answers-forum.52.s1.nabble.com/DFF-clock-necessity-tp4032181p4032201.html

Charles Petzold wrote

... While you can certainly connect the output of a TTL inverter to the input, ...

Petzold's a good writer and I love Code and highly recommend it. In fact, I currently don't have a hard copy because I gave it away to a kid in my neighborhood who is showing an interest in hardware.

But he isn't a hardware guy. This is an error. If you connect a TTL inverter output directly to its input you get a steady output voltage of about 1 V.

Here's a 'scope shot of what actually happens at the moment the output is connected to the input while the chip is powered.

Upper trace: input Lower trace: output vert: 1V/div. horiz: 50ns/div.

It takes about 100ns for this "analog computer" to solve V_out = V_in.

Notes:
  The input has a pullup resistor to +5v so that it isn't floating before the out-to-in connection is made.
  All voltages displayed in the screen shot are 1/10 actual value since I am using 10:1 high impedance probes.

--Mark