1ECE 274 - Digital LogicLecture 14 Lecture 14 – RTL Design Four Step RTL Design Method2RTL Design Method5.23Step 1: Create a High-Level State Machine Let’s consider each step of the RTL design process in more detail Step 1 Soda dispenser example Not an FSM because: Multi-bit (data) inputs aand s Local register tot Data operations tot=0, tot<s, tot=tot+a. Useful high-level state machine: Data types beyond just bits Local registers Arithmetic equations/expressionsInputs:c (bit),a (8 bits), s (8 bits)Outputs: d (bit)Localregisters:tot (8 bits)WaitDispInitd=0tot=0c’(tot<s)d=1ctot=tot+ac’(tot<s)’4Step 1 Example: Laser-Based Distance Measurer Example of how to create a high-level state machine to describe desired processor behavior Laser-based distance measurement – pulse laser, measure time T to sense reflection Laser light travels at speed of light, 3*108m/sec  Distance is thus D = T sec * 3*108m/sec / 2Object ofinterestD2D =T sec∗3⋅108m/secsensorlaserT (in seconds)5Step 1 Example: Laser-Based Distance Measurer Laser-based Distance Measurer Used in adaptive cruise control systems for automobiles Maintain particular “safe” distance between car in front May need to decrease cruise speed Need to measure the distance to the car in front6Step 1 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 L: 16-bit output, displays computed distance sensorlaserT (in seconds)Laser-baseddistancemeasurer16from buttonto displaySLDBto laserfrom sensor7Step 1 Example: Laser-Based Distance Measurer Step 1: Create high-level state machine Begin by declaring inputs and outputs Create initial state, name it S0 Initialize laser to off (L=0) Initialize displayed distance to 0 (D=0)Laser-baseddistancemeasurer16from buttonto displaySLDBto laserfrom sensorInputs :B, S(1 bit each)Outputs : L (bit), D (16 bits)S0?L = 0 (laser off )D = 0 (distance = 0)a8Step 1 Example: Laser-Based Distance Measurer Add another state, call S1, that waits for a button press B’ – stay in S1, keep waiting B – go to a new state S2Laser-baseddistancemeasurer16from buttonto displaySLDBto laserfrom sensorInputs: B, S(1 bit each)Outputs:L (bit), D (16 bits)S0L = 0D = 0S1?B‘(button not pressed)B(buttonpressed)S0Q: What should S2 do? A: Turn on the laseraa9Step 1 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 S3Laser-baseddistancemeasurer16from buttonto displaySLDBto laserfrom sensorInputs:B, S(1 bit each)Outputs:L (bit), D (16 bits)S0S1 S2L = 0D = 0L = 1(laser on)S3L = 0(laser off)B‘BaQ: What do next? A: Start timer, wait to sense reflectiona10Step 1 Example: Laser-Based Distance Measurer Stay in S3 until sense reflection (S) To measure time, count cycles for which we are in S3 To count, declare local register Dctr Increment Dctr each cycle in S3 Initialize Dctr to 0 in S1. S2 would have been O.K. tooLaser-baseddistancemeasurer16from buttonto displaySLDBto laserfrom sensorInputs:B, S(1 bit each) Outputs:L (bit), D (16 bits)LocalRegisters:Dctr (16 bits)S0S1 S2 S3L = 0D = 0L = 1 L = 0Dctr = Dctr + 1(count cycles)Dctr = 0(resetcyclecount)B‘BS'(noreflection)S(reflection)?a11Step 1 Example: Laser-Based Distance Measurer Once reflection detected (S), go to new state S4 Calculate distance  Assuming clock frequency is 3x108, Dctr holds number of meters, so D=Dctr/2 After S4, go back to S1 to wait for button againLaser-baseddistancemeasurer16from buttonto displaySLDBto laserfrom sensorInputs: B, S(1 bit each) Outputs: L (bit), D (16 bits)LocalRegisters: Dctr (16 bits)S0S1 S2 S3L = 0D = 0L = 1 L=0Dctr = Dctr+1Dctr = 0B‘S‘BSD =Dctr/2(calculate D)S4a12Step 2: Create a Datapath Datapath must Implement data storage Implement data computations Look at high-level state machine, do three substeps (a) Make data inputs/outputs be datapathinputs/outputs (b) Instantiate declared registers into the datapath (also instantiate a register for each data output) (c) Examine every state and transition, and instantiate datapath components and connections to implement any data computationsInstantiate: to introduce a new component into a design.13Step 2 Example: Laser-Based Distance Measurer(a) Make data inputs/outputs be datapathinputs/outputs(b) Instantiate declared registers into the datapath(also instantiate a register for each data output)(c) Examine every state and transition, and instantiate datapathcomponents and connections to implement any data computationsDatapathDreg_clrDctr_clrDctr_cntDreg_ldInputs: B, S(1 bit each) Outputs:L (bit), D (16 bits)LocalRegisters:Dctr (16 bits)S0S1 S2 S3L = 0D = 0L = 1 L=0Dct r = Dctr+ 1Dctr = 0B‘S‘BSD =Dctr/2(calculate D)S4loadQIDreg: 16-bitregisterQDctr: 16-bitup-counter16Dclearclearcounta14Step 2 Example: Laser-Based Distance Measurer(c) (continued) Examine every state and transition, and instantiate datapathcomponents and connections to implement any data computationsInputs: B, S(1 bit each) Outputs:L (bit), D (16 bits)LocalRegisters:Dctr (16 bits)S0S1 S2 S3L = 0D = 0L = 1 L=0Dct r = Dctr+ 1Dctr = 0B‘S‘BSD =Dctr/2(calculate D)S4clearcountclearloadQQIDctr: 16-bitup-counterDreg: 16-bitregister16DDatapathDreg_clrDctr_clrDctr_cntDreg_ld1616>>1a15Step 2 Example Showing Mux Use Introduce mux when one component input can come from more than one sourceT0T1R = E + FR = R + GE, F, G, R (16 bits)Lo c a lr eg i st e r s:(a)EFGAB+Radd_A_s0add_B_s02⋅1 2⋅1(d)××aEFGAB+R(b)EFGAB+R(c)16Step 3: Connecting the Datapath to a Controller Laser-based distance measurer example Easy – just connect all control signals between controller and datapath300 MHz ClockDBLS16to displayfrom but tonControllerto laserfrom sensorDatapathDreg_clrDreg_ldDctr_clrDctr_cntclearcountclearloadQQIDctr: 16-bitup-counterDreg: 16-bitregister16DDatapathDreg_clrDct r_ clrDctr_ cntDreg_ld17Step 4: Deriving the Controller’s FSM FSM has same structure as high-level state machine Inputs/outputs all bits now Replace data operations by bit operations using datapath300 MHz ClockDBLS16to disp layfrom buttonCont r o l l e rto lase rfrom sensorDatapathDreg_clrDreg_ldDct r _ c l rDct r _ cntInputs: B, SOutputs: L, Dreg_clr, Dreg_ld,Dctr_clr,Dctr_cntDreg_clr = 1Dreg_ld =