Lecture 23 Logistics HW8 due today, HW9 is due Friday All lab must - PDF document

Lecture 23 � Logistics � HW8 due today, HW9 is due Friday � All lab must be done by 6/5 Thu 6pm � All lab must be done by 6/5 Thu 6pm. � Last lecture � State encoding � One-hot encoding � Output encoding � Today: � Optimizing FSMs � Optimizing FSMs � Pipelining � Retiming � Partitioning CSE370, Lecture 25 1 Example: Digital combination lock � An output-encoded FSM � Punch in 3 values in sequence and the door opens � If there is an error the lock must be reset � If there is an error the lock must be reset � After the door opens the lock must be reset � Inputs: sequence of number values, reset � Outputs: door open/close new value reset clock open/closed CSE370, Lecture 25 22 2

Design the datapath value i C1 i C2 i C3 i C1 C2 C3 4 4 4 mux mux control control control control multiplexer 4 value comparator equal 4 � Choose simple control � 3 wire mux for datapath � 3-wire mux for datapath � Control is 001, 010, 100 � Open/closed bit for lock state equal � Control is 0/1 CSE370, Lecture 25 22 3 Output encode the FSM � FSM outputs � Mux control is 100, 010, 001 � Lock control is 0/1 � Lock control is 0/1 � State are: S0, S1, S2, S3, or ERR � Can use 3, 4, or 5 bits to encode � Have 4 outputs, so choose 4 bits � Encode mux control and lock control in state bits � Lock control is first bit, mux control is last 3 bits S0 = 0001 (lock closed, mux first code) S1 S1 = 0010 (lock closed, mux second code) 0010 (l k l d d d ) S2 = 0100 (lock closed, mux third code) S3 = 1000 (lock open) ERR = 0000 (error, lock closed) CSE370, Lecture 25 22 4

Logic designed ERR closed not equal not equal not equal & new & not equal & new & new S0 S1 S2 S3 closed closed closed start open equal equal equal mux= C1 mux= C2 mux= C3 & new & new & new not new not new not new Preset 0 = start D 0 = Q 0 N’ Preset 1,2,3 = 0 D 1 = Q 0 EN + Q 1 N’ Reset 0 = start’E’N D 2 = Q 1 EN + Q 2 N’ Reset 1,2,3 = start + E’N D 3 = Q 2 EN + Q 3 CSE370, Lecture 25 22 5 D 0 = Q 0 N’ D 1 = Q 0 EN + Q 1 N’ D 1 = Q 0 EN + Q 1 N D 2 = Q 1 EN + Q 2 N’ D 3 = Q 2 EN + Q 3 Preset 0 = start Preset 1,2,3 = 0 Reset 0 = start’E’N Reset 1 2 3 = start + E’N Reset 1,2,3 start + EN E’ N CSE370, Lecture 25 22 6

Last topic: more FSM optimization techniques � Want to optimize FSM for many reasons beyond state minimization and efficient encoding � Additional techniques � Pipelining --- allows faster clock speed � Retiming --- can reduce registers or change delays � Partitioning --- can divide to multiple devices, simpler logic CSE370, Lecture 25 7 Pipelining related definitions � Latency: Time to perform a computation � Data input to data output � Throughput: Input or output data rate � Typically the clock rate � Combinational delays drive performance d ≡ delay through slowest combinational stage � Define n ≡ number of stages from input to output � Latency ∝ n * d (in sec) � Throughput ∝ 1/d (in Hz) Th h t 1/d (i H ) CSE370, Lecture 25 8

Pipelining � What? � Subdivide combinational logic Logic Reg � Add registers between logic � Add registers between logic � Why? � Trade latency for throughput � Increased throughput � Reduce logic delays � Increase clock speed Logic Reg Logic Reg � Increased latency � Increase circuit utilization � Simultaneous computations CSE370, Lecture 25 9 Reg Logic Reg Pipelining � When? � Need throughput more than latency � Signal processing � Signal processing � Logic delays > setup/hold times � Acyclic logic � Where? � At natural breaks in the combinational logic � Adding registers makes sense CSE370, Lecture 25 10

Retiming � Pipelining adds registers � To increase the clock speed � Retiming moves registers around � Reschedules computations to optimize performance � Change delay patterns � Reduce register count � Without altering functionality CSE370, Lecture 25 11 Retiming examples � Reduce register count a D Q a a D Q D Q x x b d d b D Q � Change output delays CSE370, Lecture 25 12

FSM partitioning � Break a large FSM into two or more smaller FSMs � Rationale � Rationale � Less states in each partition � Simpler minimization and state assignment � Smaller combinational logic � Shorter critical path � But more logic overall � Partitions are synchronous � Same clock!!! � Same clock!!! CSE370, Lecture 25 13 Example: Partition the machine � Partition into two halves C1 S1 S6 C2 C3 S2 S5 S3 C4 C5 S4 CSE370, Lecture 25 14

Introduce idle states � SA and SB handoff control between machines C1 S1 S1 S6 S6 C2 C3 S2 S5 S3 S4 C4 C5 C1 S6 S1 C1•S1 (C2•S6)’ (C2 S6) C2•S6 C2 (C1•S1+ C3•S2+ C3•S2+ SB S5 S2 SA C3+ C5 C4•S3 C4•S3+ C5•S2)’ C4 S3 C5•S2 S4 CSE370, Lecture 25 15 Partitioning rules Rule # 1: Source-state transformation Replace by transition to idle state (SA) Replace by transition to idle state (SA) C1 C1 S1 S6 S1 SA Rule # 2: Destination state transformation Replace with exit transition from idle state p C2 C2• S6 S1 S6 S1 SA CSE370, Lecture 25 16

Partitioning rules (con’t) Rule # 3: Multiple transitions with same source or destination Source ⇒ Replace by transitions to idle state (SA) Destination ⇒ Replace with exit transitions from idle state C3 C3+C5 C3•S2 + S2 S5 S5 S2 C4•S3 SA SB C4 C5 C4 S3 S4 S3 S4 C5•S2 Rule # 4: Hold condition for idle state OR exit conditions and invert C2•S6 C2•S6 S1 SA CSE370, Lecture 25 17 Mealy versus Moore partitions � Mealy machines undesirable � Inputs can affect outputs immediately � “output” can be a handoff to another machine!!! � output can be a handoff to another machine!!! � Moore machines desirable � Input-to-output path always broken by a flip-flop � But…may take several clocks for input to propagate to output CSE370, Lecture 25 18

Example: Six-state up/down counter � Break into 2 parts U ≡ count up U t D ≡ count down U S0 S5 U U D D D S1 S4 D D D D U U S2 S3 U CSE370, Lecture 25 19 Example: 6 state up/down counter (con’t) � Count sequence S 0 , S 1 , S 2 , S 3 , S 4 , S 5 � S 2 goes to S A and holds, leaves after S 5 � S goes to S and holds leaves after S � S 5 goes to S B and holds, leaves after S 2 � Down sequence is similar S0 S5 U•S5 U U U D D D•S0 D (D•S3 + (D•S0+ SA SA SB SB S1 S1 S4 S4 U•S5)’ U•S2)’ D•S3 D D D U U U•S2 U S2 S3 CSE370, Lecture 25 20

Minimize communication between partitions � Ideal world: Two machines handoff control � Separate I/O, states, etc. � Real world: Minimize handoffs and common I/O � Minimize number of state bits that cross boundary � Merge common outputs CSE370, Lecture 25 21 Done! CSE370, Lecture 25 22