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author: Oldrich Pecak
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# Latches, flip-flops

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## bistable circuits

- until now we worked with stable circuits
- astable - do not have stable state (vibrators, clock generators)
- monostable - one stable state (pulse generators)
- bistable - two stable states -> very useful
- two types of bistable circuits we use:
    - latches - sensitive to level of input
    - flip-flops - sensitive to edges of input

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## Basic SR latch

- most basic latch
- uses feedback from output
- transparent output - immediate change of output
- AND-OR construction

|S|R|Q-next|
|-|-|----:|
|0|0|Q|
|0|1|0|
|1|0|1|
|1|1|X|

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## NAND SR latch

- inverted SR latch
- has double feedback
- complementary output
- let's build it

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## Gated SR latch

- adds enable pin to SR latch
- can be built using two more NAND gates
- makes the output opaque/non-transparent
- still not a flip-flop
- let's modify the SR latch we built

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## D flip-flop

- basic flip-flop
- commonly used for memory, shift registers, delays

|Edge|D|Q-next|
|----|-|-----:|
|↑|0|0|
|↑|1|1|
|↓|X|Q|

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## T flip-flop

- toggles the output Q on positive edge
- can be used for clock dividing
- not common

|T|Q-next|
|-|-----:|
|0|Q|
|1|!Q|

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## JK flip-flop

- universal flip-flop, it can simulate D and T flip-flops
- very commonly used due to this
- pros: very common in 74xxxx logic
- cons: slower and more complex

|J|K|Q-next|
|-|-|-----:|
|0|0|Q|
|0|1|0|
|1|0|1|
|1|1|!Q|

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## Shift registers

- memory that shifts the contents on clock input
- let's build a 2 bit shift register