LATCH. Storage Bi-stability Latches. ECEn LATCH Page BYU

Transcription

1 LATCH torage Bi-stability Latches Page 23-28

2 This is THE END of Combinatorial Circuits Page

3 and the beginning of sequential circuits Page

4 equential Circuits The output of a Combinatorial Circuit depends only on the current inputs The output of a equential Circuit depends on the current and past inputs Page

5 equential Applications Do X, then do Y three times, then do Z Take the dot product of two vectors, input one element at a time Control a car wash inse oap inse Dry All of these require memory To remember where in the process they are Page

6 Bi-tability The Key to Memory This is a stable state it will sit like this forever This is also a stable state it will sit like this forever There are 2 stable states - a bi-stable circuit Page

7 Latch A Bi-table Circuit This is a stable state it will sit like this forever This is also a stable state it will sit like this forever Page

8 Latch Transition Page

9 A B C D Latch Timing A B C t t = t + 2 x t NO time D t = t + t NO Page

10 Latch Timing A B C A B C t t time t = t + 2 x t NO t = t + 2 x t NO t = t + t NO t = t + t NO Page 23-28

11 Latch Transition Table + Comment?? No change eset et The current state of the output The next state of the output (what it will change to) Page 23-28

12 Latch Transition Table +?? No change eset it et it The current state of the output The next state of the output (what it will change to) Page

13 Latch: == Is the latch ET??? no Is the latch EET??? no What is it? neither We avoid this input combination in normal usage, mainly because it makes no sense. Page

14 Latch Transition Table + N/A N/A No change eset it et it Unused Page

15 Latch Next tate Equation + X X X X + = + The next state of will be true any time: You ET it It is already true and you don t EET it Page

16 Latches o What? Illustrate simple notion of bi-stability Two stable states and inputs move latch between them A memory When = latch storing a When = latch storing a Will hold its value indefinitely As long as circuit is powered Page

17 Latches What Are They Used For? Mainly to explain simple storage in digital design textbooks imple latch not used by itself in designs very much Due to some timing issues we will learn later imple latch forms basis for most other kinds of storage elements we will study Page

18 ymbology Page

19 Gated Latches Page

20 Latches Always ampling Inputs An latch will always respond to / input changes ALWAY! ometimes we want to control when a storage element changes Use a gated latch Page

21 The Gated Latch G When GATE = G = G = Latch cannot be modified GATE G Latch When GATE = G =, G = Works like an latch The GATE signal allows us to control when the latch will be loaded with a new value Page

22 Gated Latch Timing GATE= GATE= GATE= Latch ignores inputs Latch responds to inputs Latch holds old value ignores inputs GATE time Page

23 Gated Latch Transition Table G How many inputs are in the transition table? G Can you draw it? GATE Page

24 Gated Latch Transition Table Gate + G G X No change possible GATE X Like an Latch X X Page

25 Gated Latch Next tate Equation GATE G G X X GATE X X + = GATE + + GATE Page

26 Gated Latch ometimes known as a loadable latch The GATE signal acts like a load input G G GATE Page

27 ymbology Gate GATE or Load Page

28 The Gated D Latch D GATE + = GATE D + GATE GATE D + When GATE = follows D (storage) When GATE = retains old value (retention) Page

29 Gated D Latch - Timing Hold last loaded value Follow D input Hold value GATE D time Page

30 Gated D Latches ometimes called a transparent latch When GATE =, the D input propogates to output Allows us to control when to store new data into latch D = data to be stored GATE = control signal Page

31 Gated D Latch Usage torage element Data to be stored D Indication of data stored GATE tore/load signal Page

32 ymbology D D Gate or GATE D Load Page

33 Gated D Latches - Timing D GATE If GATE = When D changes, output will change after: Why t NOT? Why 2 t NO? t D = t NOT + t AND + t NO + t NO = t NOT + t AND + 2 t NO Page

34 Gated D Latches - Timing D GATE If D is constant When GATE rises ( ), output will change after: t GATE = t AND + t NO + t NO = t AND + 2 t NO Why no t NOT? Page

35 An Example Gated D Latch Circuit D GATE When GATE = : Latch loads (toggles) or Is there another way to wire this circuit? Page

36 Toggle Circuit Timing Diagram D A GATE B GATE D oscillation Page 36 time 23-28

37 Toggle Circuit Problem As long as GATE =, latch will repeatedly load new values... Wouldn t it be nice if it just loaded once each time GATE went high? olution #: Make GATE = for a very short time Hard to do reliably olution #2: Build a new storage element out of gated D latches (the flip flop next lecture) Page