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ECE 545—Digital System Design with VHDL Lecture 1

ECE 545—Digital System Design with VHDL Lecture 1. Digital Logic Refresher Part B – Sequential Logic Building Blocks. Lecture Roadmap – Sequential Logic. Sequential Logic Building Blocks Flip-Flops, Latches Registers, Shift Registers Counters RAM. Textbook References.

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ECE 545—Digital System Design with VHDL Lecture 1

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  1. ECE 545—Digital System Design with VHDLLecture 1 Digital Logic Refresher Part B – Sequential Logic Building Blocks

  2. Lecture Roadmap – Sequential Logic • Sequential Logic Building Blocks • Flip-Flops, Latches • Registers, Shift Registers • Counters • RAM

  3. Textbook References • Sequential Logic Review • Stephen Brown and Zvonko Vranesic, Fundamentals of Digital Logic with VHDL Design, 2nd or 3rd Edition • Chapter 7 Flip-flops, Registers, Counters, and a Simple Processors (7.3-7.4, 7.8-7.11 only) • ORyour undergraduate digital logic textbook (chapters on sequential logic)

  4. Sequential Logic Building Blocks some slides modified from:Brown and Vranesic, “Fundamentals of Digital Logic with VHDL Design, 2nd Edition”S. Dandamudi, “Fundamentals of Computer Organization and Design”

  5. Introduction to Sequential Logic • Output depends on the current input and the internal state • Past inputs effects the internal state • Sequential circuits consist typically of • Storage elements (flip-flop, latch, register, RAM, etc.) • Combinational logic

  6. Introduction (cont’d) Main components of a typical synchronous sequential circuit (synchronous = uses a clock to keep circuits in lock step) INPUT COMBINATIONAL LOGIC OUTPUT NEXT STATE S(t+1) PRESENT STATE S(t) STATE-HOLDING STORAGE ELEMENTS (i.e. FLIP-FLOPS) CLOCK

  7. State-Holding Memory Elements • Latch versus Flip Flop • Latches are level-sensitive: whenever clock is high, latch is transparent • Flip-flops are edge-sensitive: data passes through (i.e. data is sampled) only on a rising (or falling) edge of the clock • Latches cheaper to implement than flip-flops • Flip-flops are easier to design with than latches • In this course, primarily use D flip-flops

  8. D Latch vs. D Flip-Flop D D Q CLK CLK Q Latch transparent when clock is high D CLK D Q CLK Q “Samples” D on rising edge of clock

  9. Q D Clock D latch Truth table Graphical symbol Q(t+1) Clock D Q(t) – 0 0 1 0 1 1 1 Timing diagram t t t t 1 2 3 4 Clock D Q Time

  10. Q D Clock D flip-flop Truth table Graphical symbol Q(t+1) Clk D 0  0 1  1 – Q(t) 0 Q(t) 1 – Timing diagram t t t t 1 2 3 4 Clock D Q Time

  11. Bubble on the symbol means “active-low” When Set = 0, set Q to 1 When Set = 1, do nothing When Reset = 0, set Q to 0 When Reset = 1, do nothing “Set” and “Reset” also known as “Preset” and “Clear” respectively In this circuit, Set and Reset are asynchronous Q changes immediately when preset or clear are active, regardless of clock D Flip-Flop with Asynchronous Set and Reset Set Q D Q Reset

  12. D Flip-Flop with Synchronous Reset • Asynchronous active-low Reset: Q immediately clears to 0 • Synchronous active-low Reset: Q clears to 0 on rising-edge of clock D Reset CLK Reset Q(asynchronous Reset) Q(synchronous Reset)

  13. 4 4 D Q Clock Register • In typical nomenclature, a register is a name for a collection of flip-flops used to hold a bus • All flip-flops of a register share the same clock and control signals D(3) Q(3) D Q CLK D(2) Q(2) D Q CLK D(1) Q(1) D Q CLK D(0) Q(0) D Q CLK

  14. Q Q Q Q 3 2 1 0 Sin Q Q Q Q Q D D D D Clk (a) Circuit Q Q Q Q = Q Sin 3 2 1 0 t 1 0 0 0 0 0 t 0 1 0 0 0 1 t 1 0 1 0 0 2 t 1 1 0 1 0 3 t 1 1 1 0 1 4 t 0 1 1 1 0 5 t 0 0 1 1 1 6 t 0 0 0 1 1 7 Shift Register SHIFTREGISTER Clk Sin Q

  15. 4 4 4 4 Enable Enable D D Q Q Load Sin Sin Clock Clock 4-bit Shift Registers: symbols a) with Enable b) with Enable and Parallel Load

  16. D D D D Q Q Q Q Shift Register with Enable: internal structure Q(1) Q(0) Q(2) Q(3) Sin Clock Enable

  17. D(0) D D D D Q Q Q Q Shift Register with Parallel Load: internal structure Load D(3) D(1) D(2) Sin Clock Enable Q(3) Q(2) Q(1) Q(0)

  18. Synchronous Up Counter • Enable (synchronous): when high enables the counter, when low counter holds its value • Load (synchronous) : when load = 1, load the desired value into the counter • Output carry: indicates when the counter “rolls over” • D3 downto D0, Q3 downto Q0 is how to interpret MSB to LSB enable load carry D0 Q0 D1 Q1 D2 Q2 D3 Q3 clock

  19. Random Access Memory (RAM) • More efficient than registers for storing large amounts of data • Can read and write to RAM • Addressable memory • RAM dimensions are: • (number of words) x (bits per word) • Address is m bits, data is n bits • 2m x n-bit RAM • Example: address is 5 bits, data is 8 bits • 32 x 8 RAM • Write Enable (WE) • When set, writing takes place at the next rising edge of the clock RAM DIN DOUT n n ADDR m WE CLK

  20. Dual-Port RAM • Two sets of input ports {DINA, ADDRA, WEA} {DINB, ADDRB, WEB} • Two corresponding outputs DOUTA DOUTB • One memory matrix • Possible operations: • Read from two memory locations • Write to two different memory locations • Read from a memory location and write to a memory location (different or the same) RAM DINA DOUTA n n ADDRA m WEA DINB DOUTB n n ADDRB m WEB CLK

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