Digital electronics use a finite number of states, unlike analog electronics that treat continuous signals. Digital logic is used to perform arithmetic operations with electric signals, and constitutes the base for building CPUs.
18
votes
4answers
2k views
Why are things like RESET/MCLR active low on most ICs?
Convention?
Easier to implement?
Another reason?
Is there a reason things like MCLR or RESET on microcontrollers are active-low, that is, you have to pull them down to reset the IC, and pull them ...
15
votes
4answers
663 views
Why old PMOS/NMOS logic needed multiple voltages?
Why does old PMOS/NMOS logic needed multiple voltages like +5, -5, and +12 volts?
For example, old Intel 8080 processors, old DRAMs, e.t.c...
I'm interested in the causes on the physical/layout ...
13
votes
8answers
1k views
Is it worth it learning how to use 7400 series “jelly bean” logic IC's or are they completely obsolete?
My road map for learning electronics included the 7400 series logic chips. I started in on electronics by following the labs in the "Art of Electronics" lab manual which includes labs with these ...
13
votes
2answers
588 views
transmitting 5v signal over a long cable
I'm seeking for help here as I need a reliable answer to this. I need to get an input signal (low frequency 5v digital pulse), to a micro-controller from a ...
11
votes
6answers
4k views
Why are NAND Gates cheap?
In my digital electronics labs and lectures we are told to try and make things from NAND gates, because they are the cheapest kind of gate available to buy. Why is this? Why isn't an OR/AND gate the ...
11
votes
6answers
645 views
What are the advantages of using FPGAs over TTL in intro computer architecture?
I teach the one and only computer architecture course at a liberal arts college. The course is required for the computer science major and minor. We do not have computer engineering, electrical ...
10
votes
2answers
722 views
Not sure how to name what this resistor is doing?
I have a basic circuit that uses a photoresistor powered by a five volt source. I had made this project to show my son about various sensors and had used a circuit I had found online. It looks ...
10
votes
2answers
33 views
Why do some pins have `~` signs next to their numbers?
I was looking at my Arduino Uno and I noticed that symbol by digital pins 11, 10, 9, ...
10
votes
1answer
892 views
Getting a large number of (~100) digital output signals from Arduino
I would like to be able to control around 100 independent LEDs using an Arduino. The problem is that the Arduino does not have nearly enough pins that can be configured for this. What would be a way ...
9
votes
2answers
497 views
What type of Boolean logic circuit is this?
I need help confirming some hunches. I'm trying to figure out what type of circuit this is:
Any ideas?
Also, I think the logic expression for this is:
...
9
votes
4answers
1k views
Calculating the square root of 8-bit binary number
I was looking for a way to calculate the square root of a given 8-bit number using only digital combination or sequential logic. Is that possible?
One way may be to just use a look up table since ...
8
votes
3answers
841 views
Need an XOR gate that works up to 2 to 3 GHz
I have encountered an unusual situation in which I require an XOR gate that will function reliably when presented with a square wave input with a frequency between 2 and 3 GHz. I know that desktop ...
8
votes
5answers
296 views
How is frequency chosen?
I am not a specialist in electronics. I'm just a programmer. I ask this question just for fun.
My question: How the frequency for a design of a digital circuit is chosen?
Is the frequency chosen ...
8
votes
2answers
3k views
Connecting AND Gate Chip to an Integrated Circuit
I am trying to teach myself how to make integrated circuits and I'm having trouble integrating an AND gate into the circuit though.
I took a picture of it. The voltage running across the power rails ...
8
votes
2answers
284 views
What logic family is best for general-purpose hobbyist stuff?
I need to buy a few logic ICs. Which family should I get? HC? HCT? Which kind is best to have lying around in a parts box, for maximum compatibility with unpredictable future projects? Wide supply ...
8
votes
2answers
906 views
When do I need to use a clock buffer IC?
I am designing a circuit and PCB for driving 7 DACs from an FPGA. (DAC is AD9762)
Would it be possible to drive the clock inputs on all 7 DACs with a single clock output (from a PLL output pin) of ...
7
votes
4answers
723 views
How are logic gates created electronically?
So we have AND, NOT, NAND, NOR, OR gates, but how are they created electronically/electrically?
For example, what makes NOT gate reverse the value?
7
votes
6answers
3k views
How is binary converted to electrical signals?
I'm just starting off in electronics and am already fascinated. Having played with an arduino these last few days, I'm aware that binary is a representation of different voltages - such as +5V ...
7
votes
3answers
494 views
Using SPST switch as digital input?
I am new to "electrical engineering" and I am working on a little arduino project. I want to have a SPST switch be used as a digital input on the board.
I have one of the switch leads connected to ...
7
votes
3answers
297 views
Circuit works on breadboard but not PCB
I have a fairly simple circuit that works perfectly on the breadboard, but I am having a lot of trouble transferring it to a PCB. I am seeing very strange behavior which lies outside of my current ...
7
votes
3answers
445 views
How do I implement Lights-Out game using logic gates or flip flops?
First, for those unfamiliar with the game, this is how the game works,
The goal of the game is to turn off all the lights, hence called "Lights Out", and each press of the button/light inverses its ...
7
votes
4answers
279 views
Can individual pins of different port of a microcontroller be mapped to a register and their values be changed when changing the register value?
Q: Can individual pins of different port of a micro-controller be mapped to a register and their values be changed when changing the register value?
Scenario: I have used up some pins from each ...
6
votes
4answers
433 views
Logic gate symbol vs IC symbol
When I'm using a logic gate (let's say AND) in a schematic, which symbol should I use: the AND symbol or the IC symbol (the 4081, for example)? Or in the case of flip-flops, should I use the flip-flop ...
6
votes
3answers
281 views
Why is S=1, R=1 state forbidden in RS flip flop?
I have come across the RS flip flop & I have tried implementing that on a simulator & using actual logic gates. But I'm still not sure whether I have correctly understood the unstable or the ...
6
votes
3answers
386 views
What is the beginning state for Q in a SR latch?
In this diagram
what would be the beginning state for Q? Since the first NOR for S and R rely on previous results, there must be something for the first iteration?
NOTE:
I'm in a first year ...
6
votes
3answers
443 views
Is there a name for “chips out of which one can build a CPU”?
Some people enjoy building "homebrew" CPUs out of simpler ICs.
Is there a name for "chips out of which one can build a CPU, if you have enough of them"?
Is there a name for the other chips, "chips ...
6
votes
5answers
239 views
How to understand the SR Latch
I can't wrap my head around how the SR Latch works. Seemingly, you plug an input line from R, and another from S, and you are supposed to get results in Q and Q′.
However, both R and S require input ...
6
votes
3answers
650 views
Most Simple Stoplight circuit
What would be the simplest way without using a microcontroller to make a stop light circuit. All it must do is go from Green -> Yellow -> Red.
6
votes
2answers
94 views
process timing on FPGA
I'm new to fpgas, and there are some timing subtleties that I'm not sure I understand: if all my synchronous processes are triggered on the same edge, then that means my inputs are 'captured' on one ...
6
votes
5answers
401 views
Why are sum-of-products implementations more popular than product-of-sums implementations?
In my book about circuit design (Fundamentals of Digital Logic with VHDL from Stephen Brown and Zvonko Vranesic), the writers always prefer a sum-of-product for representing and implementing simple ...
6
votes
1answer
169 views
Low Voltage Logic
I am having an issue where I have 8 logic lines where the "Voltage High" is only 1.9V. The device I am writing to requires at least 2.2V.
Is there a way to amplifier these signals. There are about 12 ...
6
votes
2answers
389 views
logical not circuit?
I don't have much experience with electronics, but I think what I want to do could be cobbled together from a Radioshack.
I have a Dig 5006-IP automatic sprinkler controller. It controls 24V DC ...
6
votes
3answers
453 views
Simplifying digital circuit
I know how to simplify digital circuits using Karnaugh maps. However, the resulting circuits always consist of only not, and and ...
6
votes
2answers
266 views
How to connect external switches to this circuit board
I am circuit bending a walkie-talkie to move some of the switches around and embed it in a shoe.
One part I'm not sure about is how to replace some momentary switches on the front with external ...
6
votes
4answers
235 views
How to find the earliest edge
______
A _____| |________
______
B ______| |_______
Which edge arrives first?
I have two digital pulses that arrive at almost the ...
6
votes
1answer
140 views
Are there any interesting present-day or historical uses of “resistor priority” logic?
In many digital logic applications, it's necessary to have an output which will be derived from one signal under certain conditions, and from another signal under other conditions, etc. Sometimes the ...
6
votes
2answers
404 views
Polynomial division LFSR
Here is the schematic for LFSR (Linear Feedback Shift Register) polynomial division by \$w(x) = n_3x^3 + n_2x^2 + n_1x + n_0\$
\$\oplus\$ denotes adder without carry logic, (nx) denotes ...
5
votes
6answers
666 views
Sources for cheap/free electronics for students? [closed]
I've just recently discovered an incredible interest in building my own circuits and my own logic. I'm in a computer architecture class and we're learning how to make ALUs based on a set of ...
5
votes
6answers
573 views
Binary coded decimal ICs
Is there any IC that can be use to convert 9 bit binary code to seven segment display?
EX:
100101100->300
I want to display 300 in seven segment display when 100101100 binary code applied to input.I ...
5
votes
4answers
287 views
OR gate from one AND, three NOT?
I'm just starting with digital-logic but I now face a little problem: I learned that AND, OR, NOT were basic gates and that one basic gate could not be made from a combination of two others. How comes ...
5
votes
3answers
310 views
Tolerating Logic Inputs in Excess of Vcc
Referencing this TI inverter as an example part: http://www.ti.com/lit/ds/symlink/sn74ac14.pdf
Specifically, the table at the bottom of page 2 (I would put a picture but I do not know where to upload ...
5
votes
4answers
267 views
Why do we clock Flip Flops?
I am trying to understand Flip Flops & Latches. I am reading from Digital Logic book by Morris Mano. One thing I am not able to understand is why do we clock flip-flops ?
I understand why do we ...
5
votes
3answers
211 views
How are LFSRs used in real applications as PRNGs?
I am programming a LFSR (Linear Feedback Shift Register) in software for learning purposes, and have encountered some limitations in its use as a pseudo-random number generator (PRNG).
If the seed ...
5
votes
3answers
539 views
Get number of set bits in digital logic
As an exercise, I am trying to design an implementation of Conway's Game of Life in simple digital logic. I could do the whole thing by minimizing a 9-Variable function, but I imagine that will still ...
5
votes
3answers
226 views
When do truth tables use the “Don't-care” term?
In which situation can be used the Don't-care term?
Suppose that I want to use 6 symbols: I need 3 bits, which in turn can generate 8 combinations. Since the last ...
5
votes
1answer
133 views
Need 1-4 pins controlling 16pins logic level
I have max four outputs coming from a PIC. I need these four outputs (or one serially) to simultaneously control 16 pins with a logic level. This is for a clock application.
A BCD could accomplish ...
5
votes
5answers
473 views
Can someone explain the following flip-flop circuit?
I'm having trouble understand what's going on in this circuit. The question asks "what is the value of Q if \$\bar{R} = 1\$ and \$\bar{S} = 0\$" (as shown).
In this case, how can I determine ...
5
votes
3answers
646 views
Is there a bidirectional level converter IC for multiple I/O channels?
I have two digital devices to connect, both use 20 data lines but one works at 3.3V and the other at 1.8V, and most of the lines are bidirectional. The operating frequency should be at least 5 MHz. I ...
5
votes
2answers
706 views
Multiplexing 2 digit 7Segment with one Bit
I want to control a 2 digit 7-segment LED with a MSP430 micro controller. The display has 8 input pins per digit (8th pin is the dot, which I do not need) and two ground pins (common cathode). The ...
5
votes
2answers
850 views
What is the difference between a nonretriggerable and retriggerable One Shot?
One shot is a monostable multivibrator, it is normally in a stable state but once triggered it changes to an unstable state.
But I can't understand the difference between a nonretriggerable and a ...
