How Combinational Logic Devices Work - The Learning Circuit

the following program is brought to you by element14 the electronics community where you can connect and collaborate with top engineers from around the world join now at element14.com slash presents hi and welcome back to the learning circuit today we're continuing to talk about digital logic and learning about combinational logic devices logic device is more complex than individual logic aids can be described as combinational logic or sequential logic in combinational logic the output is a product of the current states of the inputs in sequential logic the output is dependent on the current state of its inputs but also the previous state of its inputs in a way so Cuenco logic has memory while combinational logic does not we'll get into sequential logic devices in a future episode the most common combinational logic devices are multiplexers demultiplexers encoders and decoders multiplexers sometimes called data selectors or muxes act as digitally controlled switches this 7 for LS 1 5 7 is a 4-channel 2 to 1 data selector multiplexer you can see that there are 4 sets or channels that each have two inputs yielding one output at the bottom you can see pin 1 is select and pin 15 is enable let's talk about those there are effectively four things that affect the output on each Channel inputs a and B can be high one or low zero the select pin digitally it flips the switch between inputs a is selected when pin 1 is high B is selected when pin 1 is low the enable pin pin 15 can effectively connect or disconnect the outputs from the inputs the output is enabled if pin 15 is low or disabled when pin 15 is high so let's say pins 2 5 6 10 and 13 were all high and the other inputs are low if pin1 the select pen was high and the enable pin pin 15 was low all the A's would be selected and the output would be enabled pin 4 would be high pin 7 would be high pin 9 would be low and pin 12 would be low now if pin 1 was low and pin 15 was low all the B's would be selected and the output is still enabled pin 4 would be low pin 7 would be high pin 9 would be high and pin 12 would be high at any point if pin 15 goes high the outputs are disabled for this chip disabled means all the outputs go low no signal gets through from any input here's the logic diagram for this chip you can see the four different channels this MUX is 2 2 1 so 2 inputs go to one output another type is the 4 to 1 multiplexer like you can see here each channel has four inputs one enable pin and one output the two selector pins affected both channels lastly there are 8 to 1 muxes this one has 3 selectors that can focus on the state of a single input to determine the output while the state of the other inputs don't matter this MUX has two outputs one is the normal output Y the other is called not Y and gives you the option of the inverse of the first output demultiplexers or d Moxa's work like multiplexers in Reverse while a MUX takes multiple inputs and routes them to a single output D muxes take a single input and route them to multiple outputs muxes and D muxes are 2 to the N devices where n equals the number of select lines 2 to the first would be a 2 to 1 MUX or 1 to 2 D MUX and each has one selector line a 2 to the second which equals 4 is a 4 to 1 MUX or a 1 to 4 D MUX and each have 2 selector lines a two to the third which equals eight is an 8 to 1 MUX or a 1 to 8 D MUX and each have three selector lines in a demultiplexer the selector pins are used to determine which output is selected in this one to eight active Lodi MUX the states of the three selector lines determines which of the eight outputs is enabled in this one to two D max pin one is the selector s and pin three is the input a if the selector is low output y zero is enabled if the selector is high output y 1 is enabled muxes and D Max's transmit data from single inputs to single outputs and that data is typically logic level multiplexers and demultiplexers are used for more simple signal mapping for more specific applications encoders and decoders are used to translate one form of data into another binary encoders take the data at their inputs figure out what number it represents and then output that data as a binary code you can see here that input 0 yields an output of binary 0 input 1 an output of binary 1 input to an output of binary to an input 3 and output of binary 3 however if inputs 1 into we're both active their outputs could combine and appear the same as if just input 3 were active that could be problematic that problem can be solved by using a priority encoder in a priority encoder input pins have a priority range from highest to lowest this is an active low chip so you can see in the left eye I enable input pin column that the pin must be set low for the inputs to affect the output pin 7 has the highest priority and is active low so if it is set to low all the lower priority inputs are ignored as represented by the X's X means high or low because they don't matter now if input 5 was set low it wouldn't matter if pin 0 through 4 were high or low but pin 6 & 7 have to be set high effectively off if either input 6 or 7 were to go low they would become the new priority and this state of 5 would no longer matter so the priority encoder takes the number at the input and outputs that number in binary since this is an active load device a binary one is represented as low not high when input 0 is low the output is high high high which is a binary 0 when input 1 is low the output is high high-low binary 1 when input 2 is low the output is high low high binary 2 and so on as long as one of the inputs is low that number is output in binary if no input is active then output is binary 0 priority encoders can be used in keyboards for positional control like in robot arms or a ship navigation or detecting interrupts when working with micro processors while binary encoders take data and translate it to its binary equivalent binary decoders take binary and translate it to other forms of data like decimal the combinations of low and high at the inputs determine which output line is supplied a signal sound familiar it should they're basically D multiplexers and you'll often find IC is labeled with both another common type of decoder translates BCD in order to control seven segment displays seven segment displays are used to display numbers by the use of seven LED lit segments each segment has one pin and can be sent signals high or low to turn them on or off in order to create each number you can see that to display the number is zero requires six segments to be on while the number one requires only two segments to be active binary is a base to as opposed to our common base-10 system so 5:00 in base 10 is 1 0 1 in binary BCD or binary coded decimal is similar to binary except it is broken down into four digit binary you can see this on the chart where binary would typically get to a fifth digit like four decimal number ten one zero one zero is not a valid BCD number any binary number above that are unused in BCD instead the number is carried over to the next set of four digit binary the first four digits resets to zero then begins counting up again so BCD to seven-segment decoders take four inputs of BCD data and translate it to seven outputs that control the segments of the display lastly there are also BCD to decimal decoders decimal is base 10 counting so those decoders have ten outputs one for each numeral zero through nine one simple characteristic to note is that encoders have more inputs and outputs and decoders have more outputs than inputs think of it in terms of zip files encoders take the data and make it smaller Legg zipping a file decoders take the data and make it bigger like unzipping a file hopefully you understand these combinational logic devices better now and I didn't just confuse you further but if you do have questions or if you'd like to share more about muxes des muxes encoders and decoders post on the 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Keywords: electronics, hardware, gaming, hacking, mods, weekly, element14, maker, engineering, element14presents, multiplexer, mux, demultiplexer, demux, encoder, decoder, selector, enable, BCD, 7-segment, binary, binary coded decimal, logic level

Id: gsmB7J72Tgc

Length: 10min 37sec (637 seconds)

Published: Wed Aug 14 2019