Flip-Flops. Outline: 2. Timing noise

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1 Outline: 2. Timing noise Flip-Flops Signal races, glitches FPGA example ( assign bad) Synchronous circuits and memory Logic gate example 4. Flip-Flop memory RS-latch example D and JK flip-flops Flip-flops in FPGAs Synchronous circuit design with FPGAs FPGA example ( always good). Parallel circuit design with FPGAs.

2 Timing noise Amplitude Noise A digital circuit is very immune to amplitude noise, since it can only have two values (Low or High, True or False, 0 or 1). Digital electronics circuits typically have error rates smaller than 1 part in 10 9 (no error correction). Timing Noise Just like an analog circuit, a digital circuit can experience timing noise. Fortunately, good clocks are cheap and easily available, and a good design will eliminate the effects of timing noise. Timing issues/errors can easily produce amplitude noise (bit errors).

3 Signal Race The timing delays produced by wires and logic gates can produce unwanted (illogical) outputs. Example: 3-input NAND gate A B AB C Y A B C Time ideal Y

4 Signal Race The timing delays produced by wires and logic gates can produce unwanted (illogical) outputs. Example: 3-input NAND gate A B AB C Y A B C AB resulting Y 2x gate delay Time If gate delays are too long output pulse could disappear

5 Signal Race The timing delays produced by wires and logic gates can produce unwanted (illogical) outputs. Example: 3-input NAND gate A B AB C Y A B C AB 2x gate delay Time actual Y Pulse is shorter than expected and delayed

6 A Signal Race with Glitch AB XOR B A Y A L L B L H Y L H B AB [diagram courtesy of Altera Inc.] H H L H H L resulting resulting A B A B AB AB Inverter delay Inverter delay + component differences Time resulting Y [Figure adapted from Principles of Electronics: Analog & Digital by L. R. Fortney]

7 A Signal Race with Glitch AB XOR B A Y A L L B L H Y L H B AB [diagram courtesy of Altera Inc.] H H L H H L A B A B Time real real real AB AB Y [Figure adapted from Principles of Electronics: Analog & Digital by L. R. Fortney]

8 Glitches with FPGAs Quartus II will simulate glitches glitches

9 Asynchronous Design Asynchronous design requires very careful attention to signal delays to avoid producing glitches and other spurious signals. Glitches will produce false data and can produce very wrong results e.g. a glitch on the most-significant-bit will produce a factor of 2 error. Asynchronous design can produce very fast digital circuits, but is generally avoided due to more difficult design.

10 Synchronous Design The use of memory and a clock can eliminate signal races and glitches. A B AB flip flop clock C in flip flop out Y Basic flip-flop operation clock The flip-flop will record and output the value at the input if the clock is HIGH. If the clock goes LOW, then the flip-flop does not change its value or output. Glitches are eliminated if 1. The clock HIGH and LOW times are longer than any gate delays. 2. The inputs are synchronized to the clock.

11 A B C Synchronous Timing AB flip flop flip flop clock Y clock clock A B C Flip-flop AB Flip-flop C resulting Y 2x gate delay Time Guaranteed minimum signal pulse

12 D-type Edge-Triggered Flip-Flop Generally, the flip-flop changes state on a clock signal edge, not the level. The flip-flop takes the value just before the clock edge. clock D t s t h S or PRE Q input D Q output For 74LS74: minimum t s = 20 ns minimum t h = 5 ns clock Q R or CLR [Texas Instruments 74LS74 flip-flop datasheet] Note: A flip-flop saves information (i.e. 1 bit); it does not modify it.

13 D-type Edge-Triggered Flip-Flop Generally, the flip-flop changes state on a clock signal edge, not the level. The flip-flop takes the value just before the clock edge. clock D t s t h rising-edge trigger S or PRE Q input D Q output For 74LS74: minimum t s = 20 ns minimum t h = 5 ns clock Q R or CLR [Texas Instruments 74LS74 flip-flop datasheet] Note: A flip-flop saves information (i.e. 1 bit); it does not modify it.

14 A B C clock A B Synchronous Timing (revisited) AB flip flop flip flop clock clock Y C Flip-flop AB Time Flip-flop C resulting Y

15 How does a flip-flop work? Basic flip-flop: the SR latch Logic table Q 0 = value before S&R changes R = 0 & S = 0: S = 0 & assume Q = 0 Q = 1. S = 0 & assume Q = 1 Q = 1. R = 0 & assume Q = 0 Q = 1. R = 0 & assume Q = 1 Q = 1.

16 How does a flip-flop work? Basic flip-flop: the SR latch Logic table Q 0 = value before S&R changes R = 0 & S = 0: S = 0 & assume Q = 0 Q = 1. S = 0 & assume Q = 1 Q = 1. R = 0 & assume Q = 0 Q = 1. consistent R=0 & S=0 Q=1 & Q=1 R = 0 & assume Q = 1 Q = 1.

17 How does a flip-flop work? Basic flip-flop: the SR latch Logic table Q 0 = value before S&R changes R = 0 & S = 1: S = 1 & assume Q = 0 Q = 1. S = 1 & assume Q = 1 Q = 0. R = 0 & assume Q = 0 Q = 1. R = 0 & assume Q = 1 Q = 1.

18 How does a flip-flop work? Basic flip-flop: the SR latch Logic table Q 0 = value before S&R changes R = 0 & S = 1: S = 1 & assume Q = 0 Q = 1. S = 1 & assume Q = 1 Q = 0. R = 0 & assume Q = 0 Q = 1. consistent R=0 & S=1 Q=0 & Q=1 R = 0 & assume Q = 1 Q = 1.

19 How does a flip-flop work? Basic flip-flop: the SR latch Logic table Q 0 = value before S&R changes R = 1 & S = 0: The opposite of R = 0 & S = 1 by symmetry.

20 How does a flip-flop work? Basic flip-flop: the SR latch Logic table Q 0 = value before S&R changes R = 1 & S = 1: S = 1 & assume Q = 0 Q = 1. S = 1 & assume Q = 1 Q = 0. R = 1 & assume Q = 0 Q = 1. R = 1 & assume Q = 1 Q = 0.

21 How does a flip-flop work? Basic flip-flop: the SR latch Logic table Q 0 = value before S&R changes R = 1 & S = 1: S = 1 & assume Q = 0 Q = 1. S = 1 & assume Q = 1 Q = 0. R = 1 & assume Q = 0 Q = 1. consistent R=1 & S=1 Q=1 & Q=0 consistent R=1 & S=1 Q=0 & Q=1 R = 1 & assume Q = 1 Q = 0.

22 How does a flip-flop work? Basic flip-flop: the SR latch Logic table Q 0 = value before S&R changes R = 1 & S = 1: S = 1 & assume Q = 0 Q = 1. S = 1 & assume Q = 1 Q = 0. R = 1 & assume Q = 0 Q = 1. R = 1 & assume Q = 1 Q = 0. consistent R=1 & S=1 Q=1 & Q=0 consistent R=1 & S=1 Q=0 & Q=1 Two settings are possible i.e. flip-flop keeps its state.

23 SR Latch Switch Debouncer SR latch flip-flops are not used much for memory, but they are used for debouncing switches. Switch Bounce: When a switch is toggled it will not go smoothly from HIGH to LOW, or vice versa. Volts bouncing switch Volts debounced switch time time R +5V R

24 Clocked D-type Latch S Logic table R Clock Circuit Analysis: C = 1 & D = 1 S = 0 & R = 1. C = 1 & D = 0 S = 1 & R = 0. C = 0 & D = 1 S = 1 & R = 1. C = 0 & D = 0 S = 1 & R = 1.

25 Clocked D-type Latch S Logic table R Clock Circuit Analysis: C = 1 & D = 1 S = 0 & R = 1. C = 1 & D = 0 S = 1 & R = 0. C = 0 & D = 1 S = 1 & R = 1. C = 0 & D = 0 S = 1 & R = 1. Clock HIGH: D sets the flip-flop state Clock LOW: flip-flop state is locked

26 Clocked D-type Latch input D Q output Logic table clock Q Clock Circuit Analysis: C = 1 & D = 1 S = 0 & R = 1. C = 1 & D = 0 S = 1 & R = 0. C = 0 & D = 1 S = 1 & R = 1. C = 0 & D = 0 S = 1 & R = 1. Clock HIGH: D sets the flip-flop state Clock LOW: flip-flop state is locked

27 Master-Slave D-type Flip-Flop Note: The flip-flop triggers on a the falling edge of the clock.

28 74LS74 D-type edge-triggered flip-flop [Texas Instruments 74LS74 flip-flop datasheet] Both PRE and CLR behave like S and R inputs, respectively, on the SR latch. IMPORTANT: Both PRE and CLR must be high for normal D-type operation. Note: The flip-flop triggers on the rising edge of the clock.

29 74LS74 D-type edge-triggered flip-flop input D PRE Q output [Texas Instruments 74LS74 flip-flop datasheet] clock CLR Q Both PRE and CLR behave like S and R inputs, respectively, on the SR latch. IMPORTANT: Both PRE and CLR must be high for normal D-type operation. Note: The flip-flop triggers on the rising edge of the clock.

30 JK-type flip-flop Logic table for clock falling edge input J Q output J K Q n+1 clock input K C Q Q n 0 1 Q n JK-type flip-flops are used in counters.

31 Flip-flops in FPGAs Architecture of a single Logic Element inputs clock signals LUT CLOCK triggers Memory (a few bits) global local outputs feedback Frequently a D-type Flip-Flop FPGAs are already set-up for synchronous circuit designs

32 Flip-flops in FPGAs Architecture of a single Logic Element inputs clock signals LUT CLOCK triggers Memory (a few bits) global local outputs feedback Frequently a D-type Flip-Flop FPGAs are already set-up for synchronous circuit designs

33 Synchronous programming in Verilog (I)

34 Synchronous programming in Verilog (I) Clock variable output register (i.e. flip-flop memory )

35 Synchronous programming in Verilog (I) Clock variable output register (i.e. flip-flop memory ) Read as always at the positive clock edge do the following always is the core command for synchronous programming, it should be used as frequently as possible. assign should be used as little as possible. It is only useful for DC- type signals (signals that don t change).

36 Synchronous programming in Verilog (II) Quartus II circuit simulation

37 Synchronous programming in Verilog (II) Quartus II circuit simulation Clock Line No more glitches

38 How did the FPGA implement the circuit? Tools > Netlists > Technology Map Viewer

39 How did the FPGA implement the circuit? Tools > Netlists > Technology Map Viewer D-type edge-triggered flip-flops

40 Always use always _ A. Stummer, U. of Toronto.

41 Parallel programming in Verilog The always structure is used for exploiting the parallel processing features of the FPGA. Parallel processing must almost always be synchronous if several processes exchange data. Parallel and Sequential processing examples: Sequential (negedge clock) begin a = b; c = a; end Parallel (negedge clock) begin a <= b; c <= a; end

42 Parallel programming in Verilog The always structure is used for exploiting the parallel processing features of the FPGA. Parallel processing must almost always be synchronous if several processes exchange data. Parallel and Sequential processing examples: Sequential (negedge clock) begin a = b; c = a; end Parallel (negedge clock) begin a <= b; c <= a; end executed simultaneously c = b a = b c = a (previous value)

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