Tuesday, November 22, 2011

Truth Tables Help Us Understand Cmos Functions

 Sometimes the information on datasheets can be overwhelming to even those of us that have worked with them for a while.  For our needs, the Truth Table will often "decode" the jargon.  Let's take a look at the 4512's truth table found on the first page in this instance.

By studying the table above, we see that depending on which combination of AB&C are High (1) or Low (0), one of the eight channels will be "heard"; that is, present its voltage to pin 14.  If Inhibit is High, we disrupt this process, bringing the output Low.  Unless I want to run the Inhibit out to a jack to gate the output, I tend to tie it to ground.  The 3 State Disable is also brought to ground in my usage to make the chip play its tune.

Here's a truth table for a bigger cmos ic, the 4043 shift register.

Here's an example:  This ic can be configured in many ways but I want it to recirculate the data I put in.  Five steps down I see my answer!  There are 4 controllers that will dictate what state the chip is in (A Enable, P/S, A/B, A/S).  For recirculation that is in sync with the clock, I tie A Enable Low, P/S High, A/B High and A/S Low.  This is a fairly complicated chip but it shows you can use it in a lunetta with a little truth finding ;).


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