Difference between revisions of "Flip-flop"
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In modern terminology, the term 'flip-flop' is usually reserved for the first group; 'latch' is used for everything else. | In modern terminology, the term 'flip-flop' is usually reserved for the first group; 'latch' is used for everything else. | ||
| − | ==S-R latch== | + | ==Types== |
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| + | Names exist for several of the most common forms of flip-flop; these are mostly due to Montgomery Phister, from his book "Logical Design of Digital Computers" (John Wiley & Sons, New York, 1958). | ||
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| + | ===S-R latch=== | ||
The simplest is the S-R latch (called a 'S-R flip-flop' in the early period), which consists of just two NAND or NOR gates. The output of each gate, in addition to being an output from the latch, is also connected to one of the inputs of the other gate. | The simplest is the S-R latch (called a 'S-R flip-flop' in the early period), which consists of just two NAND or NOR gates. The output of each gate, in addition to being an output from the latch, is also connected to one of the inputs of the other gate. | ||
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The more complicated flip-flops are often made up of several S-R latches, along with other gates. | The more complicated flip-flops are often made up of several S-R latches, along with other gates. | ||
| − | ==Latch== | + | ===Latch=== |
A latch has a single data input, and a clock input. As long as the clock is asserted, the input data is passed through to the output(s). When the clock stops being asserted, it continues to output the data that was present at that time, and the input is thereafter ignored. | A latch has a single data input, and a clock input. As long as the clock is asserted, the input data is passed through to the output(s). When the clock stops being asserted, it continues to output the data that was present at that time, and the input is thereafter ignored. | ||
| − | ==D flip-flop== | + | ===D flip-flop=== |
A D flop-flop has a single data input, and a clock input. It samples the input on either the rising or falling edge of the clock (depending on the variety). Most D flip-flops also have 'preset' and 'clear' inputs, which force the output to 1 or 0. | A D flop-flop has a single data input, and a clock input. It samples the input on either the rising or falling edge of the clock (depending on the variety). Most D flip-flops also have 'preset' and 'clear' inputs, which force the output to 1 or 0. | ||
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It is constructed out of pair of S-R latches, one after the other, along with some additional gates. | It is constructed out of pair of S-R latches, one after the other, along with some additional gates. | ||
| − | ==J-K flip flop== | + | ===J-K flip flop=== |
A J-K flop-flop has two data/control inputs, and a clock input. It too samples the inputs on either the rising or falling edge of the clock; again, most also have preset and clear inputs. | A J-K flop-flop has two data/control inputs, and a clock input. It too samples the inputs on either the rising or falling edge of the clock; again, most also have preset and clear inputs. | ||
Revision as of 18:14, 14 March 2018
A flip-flop or latch (historically, the terms were somewhat interchangeably, but in contemporary usage they usually refer to disjoint sets of devices) is an extremely common element in digital logic; it is a store for one bit of state.
Unlike gates, which are necessarily made out of transistors, all forms of flip-flop and latch (there are quite a few) can be made out of gates. All involve feedback, from the outputs, to some of the inputs of the gates used to create them; they are thus subject to meta-stability issues.
They usually have one data or two control inputs, two data outputs (the data and its inversion, usually denoted 'Q' and 'Q-bar'), and optionally a clock input, which tells it when to store the current state of the input(s).
Of the many different varieties, the chief grouping is 'edge-triggered', which only change the output once, at either the rising or falling edge of the clock. Of the rest, some are 'transparent', which pass the input data through whenever the clock input is asserted, so that the output can move up and down during that period.
In modern terminology, the term 'flip-flop' is usually reserved for the first group; 'latch' is used for everything else.
Types
Names exist for several of the most common forms of flip-flop; these are mostly due to Montgomery Phister, from his book "Logical Design of Digital Computers" (John Wiley & Sons, New York, 1958).
S-R latch
The simplest is the S-R latch (called a 'S-R flip-flop' in the early period), which consists of just two NAND or NOR gates. The output of each gate, in addition to being an output from the latch, is also connected to one of the inputs of the other gate.
When constructed with NOR gates, the other two inputs (asserted high) are 'reset' (on the gate with the Q output), and 'set' (on the other). With NAND gates, the two inputs are asserted low, and the 'set' is on the gate providing the Q output.
The more complicated flip-flops are often made up of several S-R latches, along with other gates.
Latch
A latch has a single data input, and a clock input. As long as the clock is asserted, the input data is passed through to the output(s). When the clock stops being asserted, it continues to output the data that was present at that time, and the input is thereafter ignored.
D flip-flop
A D flop-flop has a single data input, and a clock input. It samples the input on either the rising or falling edge of the clock (depending on the variety). Most D flip-flops also have 'preset' and 'clear' inputs, which force the output to 1 or 0.
It is constructed out of pair of S-R latches, one after the other, along with some additional gates.
J-K flip flop
A J-K flop-flop has two data/control inputs, and a clock input. It too samples the inputs on either the rising or falling edge of the clock; again, most also have preset and clear inputs.
When the clock happens, with both data/control inputs low, it holds its previous state; with only J high, it sets; with only K high, it clears; with both high, it inverts its previous state.
