Chapter 11: System Design Methodology Digital System Designs and Practices Using Verilog HDL and 2008, John Wiley11-1 Ders 9: RTL Design

Chapter 11: System Design Methodology Digital System Designs and Practices Using Verilog HDL and 2008, John Wiley Introduction  Ders-1,2,3,4,5  Capture Comb. behavior: Equations, truth tables  Convert to circuit: AND + OR + NOT  Comb. logic  Ders-6,7,8  Capture sequential behavior: FSMs  Convert to circuit: Register + Comb. logic  Controller  Ders-9  Capture behavior: High-level state machine  Convert to circuit: Controller + Datapath  Processor  Known as “RTL” (register-transfer level) design Note: Slides with animation are denoted with a small red "a" near the animated items Transistor level Logic level Register- transfer level (RTL) Levels of digital design abstraction Higher levels Processors: Programmable (microprocessor) Custom 5.1

Chapter 11: System Design Methodology Digital System Designs and Practices Using Verilog HDL and 2008, John Wiley11-3 Register-Transfer-Level Design  Features of register-transfer-level (RTL) designs  are sequential machines.  are structural.  concentrate on functionality, not details of logic design.  Two types of register-transfer-level design are  High Level Finite State Machine (HFSM)  ASM (algorithmic-state machine) chart  Datapath and controller

Chapter 11: System Design Methodology Digital System Designs and Practices Using Verilog HDL and 2008, John Wiley11-4 High-Level State Machines (HLSMs)  Some behaviors too complex for equations, truth tables, or FSMs  Ex: Soda dispenser  c: bit input, 1 when coin deposited  a: 8-bit input having value of deposited coin  s: 8-bit input having cost of a soda  d: bit output, processor sets to 1 when total value of deposited coins equals or exceeds cost of a soda  FSM can’t represent…  8-bit input/output  Storage of current total  Addition (e.g., ) as c d Soda dispenser processor tot: 25 tot: 50 a as c d Soda dispenser processor

Chapter 11: System Design Methodology Digital System Designs and Practices Using Verilog HDL and 2008, John Wiley11-5 HLSMs  High-level state machine (HLSM) extends FSM with:  Multi-bit input/output  Local storage  Arithmetic operations Inputs:c(bit),a (8 bits), s (8 bits) Outputs:d (bit) // '1' dispenses soda Local storage : tot (8 bits) Wait Disp Init d:='0' tot:=0 c’*(tot<s)’ c'  ( tot<s ) d:='1' c tot:=tot+a SodaDispenser 88 as c d Soda dispenser processor Conventions –Numbers: Single-bit: '0' (single quotes) Integer: 0 (no quotes) Multi-bit: “0000” (double quotes) –== for equal, := for assignment –Multi-bit outputs must be registered via local storage –// precedes a comment

Chapter 11: System Design Methodology Digital System Designs and Practices Using Verilog HDL and 2008, John Wiley11-6 Ex: Cycles-High Counter  P = total number (in binary) of cycles that m is 1  Capture behavior as HLSM  Preg required (multibit outputs must be registered) Use to hold count P m CountHigh clk 32 S_Clr S_Wt m' S_Inc m m m' Preg := 0 Preg := Preg + 1 // Clear Preg to 0s // Wait for m == '1' // Increment Preg CountHigh Inputs:m(bit) Outputs:P (32 bits) Local storage: Preg (c) S_Clr Preg := 0 // Clear Preg to 0s CountHigh Inputs:m(bit) Outputs:P (32 bits) Local storage: Preg (a) ? S_Clr S_Wt m' m Preg := 0 // Clear Preg to 0s // Wait for m == '1' CountHigh Inputs:m(bit) Outputs:P (32 bits) Local storage: Preg (b) ? Preg

Chapter 11: System Design Methodology Digital System Designs and Practices Using Verilog HDL and 2008, John Wiley11-7 Example: Laser-Based Distance Measurer  Laser-based distance measurement – pulse laser, measure time T to sense reflection  Laser light travels at speed of light, 3*10 8 m/sec  Distance is thus D = (T sec * 3*10 8 m/sec) / 2 Object of interest D 2D = T sec * 3*10 8 m/sec sensor laser T (in seconds)

Chapter 11: System Design Methodology Digital System Designs and Practices Using Verilog HDL and 2008, John Wiley11-8 Example: Laser-Based Distance Measurer  Inputs/outputs  B: bit input, from button, to begin measurement  L: bit output, activates laser  S: bit input, senses laser reflection  D: 16-bit output, to display computed distance sensor laser T (in seconds) Laser-based distance measurer 16 from button to display S L D B to laser from sensor

Chapter 11: System Design Methodology Digital System Designs and Practices Using Verilog HDL and 2008, John Wiley11-9 Example: Laser-Based Distance Measurer  Declare inputs, outputs, and local storage  Dreg required for multi-bit output  Create initial state, name it S0  Initialize laser to off (L:='0')  Initialize displayed distance to 0 (Dreg:=0) Laser- based distance measurer 16 from button to display S L D B to laser from sensor a Inputs:B (bit), S (bit) Outputs:L (bit), D (16 bits) Local storage:Dreg(16) S0 ? L:= '0' // laser off Dreg := 0 // distance is 0 DistanceMeasurer (required) (first state usually initializes the system) Recall: '0' means single bit, 0 means integer

Chapter 11: System Design Methodology Digital System Designs and Practices Using Verilog HDL and 2008, John Wiley11-10 Example: Laser-Based Distance Measurer  Add another state, S1, that waits for a button press  B' – stay in S1, keep waiting  B – go to a new state S2 Q: What should S2 do?A: Turn on the laser a Laser- based distance measurer 16 from button to display S L D B to laser from sensor S0 L := '0' Dreg := 0 S1? B'// button not pressed B // button pressed S0 DistanceMeasurer...

Chapter 11: System Design Methodology Digital System Designs and Practices Using Verilog HDL and 2008, John Wiley11-11 Example: Laser-Based Distance Measurer  Add a state S2 that turns on the laser (L:='1')  Then turn off laser (L:='0') in a state S3 Q: What do next?A: Start timer, wait to sense reflection Laser- based distance measurer 16 from button to display S L D B to laser from sensor DistanceMeasurer... S0S1 L := '0' Dreg := 0 S2 L := '1' // laser on S3 L := '0' // laser off B' B

Chapter 11: System Design Methodology Digital System Designs and Practices Using Verilog HDL and 2008, John Wiley11-12 Example: Laser-Based Distance Measurer  Stay in S3 until sense reflection (S)  To measure time, count cycles while in S3  To count, declare local storage Dctr  Initialize Dctr to 0 in S1. In S2 would have been O.K. too. Don't forget to initialize local storage—common mistake  Increment Dctr each cycle in S3 Laser-based distance measurer 16 from button to display S L D B to laser from sensor a S0S1S2S3 L := '0' Dreg := 0 L := '1'L := '0' Dctr := Dctr + 1 // count cycles Dctr := 0 // reset cycle count B' S' // no reflection B S//reflection ? Inputs:B (bit), S (bit)Outputs:L (bit), D (16 bits) Local storage:Dreg, Dctr (16 bits) DistanceMeasurer

Chapter 11: System Design Methodology Digital System Designs and Practices Using Verilog HDL and 2008, John Wiley11-13 Example: Laser-Based Distance Measurer  Once reflection detected (S), go to new state S4  Calculate distance  Assuming clock frequency is 3x10 8, Dctr holds number of meters, so Dreg:=Dctr/2  After S4, go back to S1 to wait for button again a S0S1S2S3 L := '0' Dreg := 0 L := '1'L := '0' Dctr := Dctr+1 Dreg := Dctr/2 // calculate D Dctr := 0 B' S' BS S4 Inputs:B (bit), S (bit)Outputs:L (bit), D (16 bits)DistanceMeasurer Local storage: Dreg, Dctr (16 bits) Laser- based distance measurer 16 from button to display S L D B to laser from sensor

Chapter 11: System Design Methodology Digital System Designs and Practices Using Verilog HDL and 2008, John Wiley11-14 ASM Charts  ASM (algorithmic state machine) charts  specify RTL operations on the per-cycle basis.  show clearly the flow of control from state to state.  are very suitable for data path-controller architectures.  An ASM chart is composed of  State block  Decision block  Conditional output block

Chapter 11: System Design Methodology Digital System Designs and Practices Using Verilog HDL and 2008, John Wiley11-15 ASM State Blocks  A state block  specifies a machine state and a set of unconditional RTL operations associated with the state.  may execute as many actions as you want and all actions in a state block occur in parallel.  occupies a clock period along with its related operations.

Chapter 11: System Design Methodology Digital System Designs and Practices Using Verilog HDL and 2008, John Wiley11-16 Decision Blocks  A decision block  describes the condition under which ASM will execute specific actions.  selects the next state based on the value of primary input or present state.  can be drawn in either two-way selection or multi-way selection. Two-way selectionMulti-way selection

Chapter 11: System Design Methodology Digital System Designs and Practices Using Verilog HDL and 2008, John Wiley11-17 Conditional Output Blocks  A conditional output block  describes the RTL operations executed under conditions specified by one or more decision blocks.  receives signals from the output of a decision block or the other conditional output block.  can only evaluate present state or primary input value on present cycle.

Chapter 11: System Design Methodology Digital System Designs and Practices Using Verilog HDL and 2008, John Wiley11-18 ASM Blocks  An ASM block has exactly one entrance path and one or more exit paths. Serial testingParallel testing

Chapter 11: System Design Methodology Digital System Designs and Practices Using Verilog HDL and 2008, John Wiley11-19 ASM Charts --- ASM Blocks  ASM blocks  An ASM block contains one state block and a serial- parallel network of decision blocks and conditional output blocks.  Each ASM block describes the operations executed in one state.  The basic rules for constructing an ASM chart  Each state and its associated set of conditions must define a unique next state.  Every path of the network of conditions must terminate at a next state.  There cannot exist any loop in the network of conditions.

Chapter 11: System Design Methodology Digital System Designs and Practices Using Verilog HDL and 2008, John Wiley11-20 Invalid ASM Blocks  Undefined next state: why? Try to explain it! Undefined exit path: why? Try to explain it!

Chapter 11: System Design Methodology Digital System Designs and Practices Using Verilog HDL and 2008, John Wiley11-21 ASM Modeling Styles  Style 1a: one state register and one always block. (Not a good style!!!)  part 1: initialize, determine, update the state register, and determine RTL operations. clk or negedge start_n) if (!start_n) state <= A; else state <= …;  Style 1b: one state register and two always blocks  part 1: initialize, determine, and update the state register. clk or negedge start_n) if (!start_n) state <= A; else state <= …;  part2: determine RTL operations clk) case (state ) … or assignment statements

Chapter 11: System Design Methodology Digital System Designs and Practices Using Verilog HDL and 2008, John Wiley11-22 ASM Modeling Styles  Style 2: two state registers and three always blocks (the best style !!!!)  part 1: initialize and update the state register clk or negedge start_n) if (!start_n) present_state <= A; else present_state <= next_state;  part 2: determine next state or x) case (present_state ) … or assignment statements  part 3: determine RTL operations clk) case (present_state ) … Or assignment statements

Chapter 11: System Design Methodology Digital System Designs and Practices Using Verilog HDL and 2008, John Wiley11-23 dp+cu Architecture Design  A digital system can be considered as a system composed of three major parts:  data path performs all operations that are required in the system.  Memory temporarily stores the data used and generated by data path unit.  control unit controls and schedules all operations performed by the data path unit.

Chapter 11: System Design Methodology Digital System Designs and Practices Using Verilog HDL and 2008, John Wiley11-24 A Three-Step Paradigm 1.Model the design (usually described by an ASM chart) using any modeling style described above as a single module. 2.Extract the datapath from the module and construct it as an independent module. 3.Extract control-unit module and construct top module.  Add a top module that instantiates both datapath and control-unit

Chapter 11: System Design Methodology Digital System Designs and Practices Using Verilog HDL and 2008, John Wiley11-25 Realization Options of RTL Design  The rationale behind these options is a tradeoff among performance (throughput, operating frequency), space (area, hardware cost), and power consumption.  Single cycle uses combinational logic only to realize the required functions.  It may require a quite long propagation time to finish a computation of required functions.

Chapter 11: System Design Methodology Digital System Designs and Practices Using Verilog HDL and 2008, John Wiley11-26 Realization Options of RTL Design  Multiple cycle executes the required functions in consecutive clock cycles.  Linear structure performs straightforward the required functions without sharing resources.

Chapter 11: System Design Methodology Digital System Designs and Practices Using Verilog HDL and 2008, John Wiley11-27 Realization Options of RTL Design  Nonlinear (feedback or feed-forward) structure performs the required functions with sharing resources by using feedback or feed-forward connection. Single-stage nonlinear structure Multiple-stage nonlinear structure

Chapter 11: System Design Methodology Digital System Designs and Practices Using Verilog HDL and 2008, John Wiley11-28 Realization Options of RTL Design  Pipeline is also a multiple cycle structure in which a new data can be fed into the structure at each clock cycle. Hence, it may output a result per clock cycle after the pipeline is fully filled.

Chapter 11: System Design Methodology Digital System Designs and Practices Using Verilog HDL and 2008, John Wiley11-29 A Single-Cycle Example // a single-cycle example module single_cycle_example(clk, data_a, data_b, data_c, total); parameter N = 8; input clk; input [N-1:0] data_a, data_b, data_c; output reg [N-1:0] total; // compute total = data_c - (data_a + data_b) clk) begin total <= data_c - (data_a + data_b); end endmodule

Chapter 11: System Design Methodology Digital System Designs and Practices Using Verilog HDL and 2008, John Wiley11-30 A Multiple-Cycle Example // a multiple cycle example --- an implicit FSM module multiple_cycle_example(clk, data_a, data_b, data_c, total); parameter N = 8; input clk; input [N-1:0] data_a, data_b, data_c; output reg [N-1:0] total; reg [N-1:0] qout_a, qout_b; // compute total = data_c - (data_a + data_b) clk) begin qout_a <= clk) qout_b <= qout_a + clk) total <= data_c - qout_b; end endmodule

Chapter 11: System Design Methodology Digital System Designs and Practices Using Verilog HDL and 2008, John Wiley11-31 Pipeline Principles  Pipelining: a pipeline is composed of several stages with each stage consisting of a combinational logic circuit and a register, called pipeline register.  Pipeline latency: the number of cycles between the presentation of an input value and the appearance of its associated output.

Chapter 11: System Design Methodology Digital System Designs and Practices Using Verilog HDL and 2008, John Wiley11-32 Pipelined Systems  The pipelined clock f pipe is set by the slowest stage and is larger than the frequency used in the original cascaded system.  For the ith-stage, the smallest allowable clock period T i is determined by the following condition:  The pipelined clock period for an m-stage pipeline is chosen to be:

Chapter 11: System Design Methodology Digital System Designs and Practices Using Verilog HDL and 2008, John Wiley11-33 A Simple Pipeline Example --- Not a Good One // a simple pipeline example --- Not a good one module simple_pipeline(clk, data_a, data_b, data_c, total); parameter N = 8; input clk; input [N-1:0] data_a, data_b, data_c; output reg [N-1:0] total; reg [N-1:0] qout_a, qout_b; // compute total = data_c - (data_a + data_b) clk) begin qout_a <= data_a; qout_b <= qout_a + data_b; total <= data_c - qout_b; end endmodule

Chapter 11: System Design Methodology Digital System Designs and Practices Using Verilog HDL and 2008, John Wiley11-34 A Simple Pipeline Example // a simple pipeline example module pipeline_example(clk, data_a, data_b, data_c, total); parameter N = 8; input clk; input [N-1:0] data_a, data_b, data_c; output reg [N-1:0] total; reg [N-1:0] qout_a, qout_b, qout_c, qout_d, qout_e; // compute total = data_c - (data_a + data_b) clk) begin qout_a <= data_a; qout_b <= data_b; qout_c <= data_c; qout_d <= qout_a + qout_b; qout_e <= qout_c; total <= qout_e - qout_d; end endmodule