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Basic Electronics » Is Single-bit Memory Cell Diagram on Pg 14 Correct?

October 09, 2010
by sjb
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On page 14 of the current NerdKits Guide (the page with the heading that reads: "Circuits with Memory"):

Why is it that if A is 1 B must be 0 and vice versa.

If A is 1, then the right side of the circuit is closed (so current is flowing through it), and hence there is a voltage at B with respect to the ground. And if there is a voltage at B (if this voltage is high enough) then isn't B 1 as well (completing the left side of the circuit as well)?

Should the n-channel transistors actually be p-channel transistors. B/c if that were the case then:

If A was 1, then the right side of the circuit would be open and hence the voltage at B would be zero, so B would be zero, and vice versa.

Thanks in advance!

October 09, 2010
by mongo
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The diagram is a basic as it can get. Both transistors (FETs) are correct. The are connected in a way that they are diametrically opposed, so if one is 'on' the other is 'off'. At the the resistor/transistor in both circuits act as your data input and output. Their states can be toggled by momentarily grounding one junction or the other, like a set/reset function. If one is forced into conduction, it in turn feeds back to the other to turn it off.

Most memory is a little more complicated to prevent getting the signals crossed or shorted input. Typically, a cell is a latching flip-flop. It has a single data input and a gate input. Whatever state the input happens to be at the time when the gate signal is received. The cells are a lot like the circuit but there is supporting circuitry to prevent the damage I mentioned.

October 10, 2010
by hevans
(NerdKits Staff)

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Hi sjb,

I think where you are getting confused is when you say "(so current is flowing through it), and hence there is a voltage at B with respect to the ground." The first part of your thinking is correct, with A at a logical 1 the n-channel mosfet on the rights side is closed (allowing current to flow). If you consider the mosfet to be like a voltage controlled switch (which in this case that approximation is very close) then B is connected directly to GND forcing it to 0V. The voltage is all dropped across the resistor. Which coincidentally allows you to figure out how much current is flowing.

Since B is at 0V, that means the left hand mosfet is an open switch, so A is not being dragged down to 0V. Instead since no current is flowing across the resistor, no voltage is being dropped across it, so A is at 5V.

Like mongo mentioned above there is generally more circuitry involved in practice, especially when you are talking about changing the state of the cell.

Humberto

October 12, 2010
by sjb
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Thank you Mongo and Mr. Humberto. Both explanations helped. And Mr. Humberto's explanation hit the nail on the head, the circuit diagram makes perfect sense now. Thanks!

December 05, 2010
by rpeiffer
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SJB - I was confused by the EXACT same thing. The explanations in this thread have cleared up the confusion. Thanks again to everyone that responded!

January 09, 2011
by pacnwdude
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I was confused by this as well. The explanations above cleared up most of my confusion.

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