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Basic Electronics » Is Single-bit Memory Cell Diagram on Pg 14 Correct?
October 09, 2010 by sjb |
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! |
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October 09, 2010 by mongo |
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) |
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 |
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 |
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 |
I was confused by this as well. The explanations above cleared up most of my confusion. |
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