Announcement. VHDL in Action. Review. Statements. Process Execution with no sensitivity list. Sequential Statements

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1 Annuncement Prject 2: Assigned tday, due 9/30 ning f class. VHDL in Actin Mdeling a binary/gray cunter. As always, start early Chapter 3 Sequential Statements 1 Review Statements Architecture bdy Cncurrent signal assignments Cmpnent instantiatins Prcesses Testbenches Prtable way t validate a mdel Stimulus nly, r stimulus + validatin Sequential statements Inside blcks Similar t "nrmal" prgramming Cncurrent statements Outside f blcks within an architecture bdy Cncurrent == all evaluated at the same time. 3 4 Sequential Statements Prcess Executin with n sensitivity list. Remember that statements within a PROCESS blck are executed sequentially. Because f this, PROCESS cde is similar t sftware (C/C++/Pascal/...) [( sensitivity_list )] declaratin_sectin _bdy 1. Declaratins elabrated 2. Bdy is executed 3. Repeat: return t BEGIN 4. Lp frever -- Variable Declaratins -- Sequential Statements 5 6 1

2 Prcess Executin with Sensitivity Lists Preview 1. Declaratins elabrated 2. Bdy is executed 3. Wait until signals in sensitivity list change. 4. Lp t BEGIN NO YES Did a,b, r c change? (A, B, C) -- Variable Declaratins -- Sequential Statements Sensitivity List wait if/then/else case next exit null 7 8 WAIT Statement WAIT Statement wait [n sensitivity_list] [until cnditin ] [fr time_expr]; Suspends the enclsing. variable A: Bit :='0'; wait fr 100 ns; A := nt A; 9 Time WAITs wait fr 100 ns; Signal WAITs wait n A,B,C; Cnditin WAITs wait until Z='0'; -- Z must change t 0 Cmb WAITs wait n A,B until Z='1'; wait n A,B,C fr 100 ns; wait n X,Y until Z='0' fr 100 ns; 10 Nte n WAITs Using WAITs Tw types f : with sensitivity list withut sensitivity list Prcess withut sensitivity list must have WAIT statement(s) t prevent infinite s Prcess with sensitivity list may NOT have WAIT statements

3 Cmparisn Relatinal Operatrs The fllwing are equivalent: ( S1, S2 ) X1 <= S1 r S2; X1 <= S1 r S2; wait n S1, S2; Express relatinships between signals r variables: = /= < <= > >= IF Statement IF Statement Usage very similar t C/C++ (A) if (A = '1') then B <= C +1; else C <= C - 1; end if; Nte: the ELSE clause is ptinal 15 if cnditin1 then sequential statement(s)1 elsif cnditin2 then sequential statement(s)2 elsif cnditin3 then sequential statement(s)3 else sequential statement(s)4 end if; 16 CASE Statement CASE Statement Similar t SWITCH/CASE in C/C++ ( LIGHT_COLOR ) case LIGHT_COLOR is when RED => TRAFFIC <= FALSE; when YELLOW => TRAFFIC <= TRUE; when GREEN => TRAFFIC <= TRUE; DONTWALK <= '1'; when thers => LIGHT_BROKEN <= TRUE; case discriminant is when chice1 => sequential_statements(1) ; when chice2 => sequential_statements(2) ; when chice3 => sequential_statements(3) ; when chicen => sequential_statements(n) ; NOTE: Chice cnditins must be mutually exclusive and must include all pssible values f discriminant

4 Similar t FOR and DO/WHILE statements in C/C++ wait until clk = '1' and clk'event; fr i in 0 t 9 a(i) <= a(i+1); NOTE: The variable i is an index variable, and is ne f the few instances in VHDL where a variable des nt need t be declared first. 19 fr parameter in discrete_range 20 Anther variatin: variable cunt : integer; sum <= 12; cunt := 0; while sum <= A wait until clken = '1'; cunt := cunt + 1; sum <= sum + 12; while blean_cnditin NEXT Statement General Lp Pssibly an infinite. NEXT statements can be used t mdify the executin f a LOOP statement. Terminates executin f a particular iteratin (skipping iteratins). Frms: 1. next; 2. next when cnditin ; 3. next label ; 4. next label when cnditin ;

5 NEXT Statement EXIT Statement FRED: fr i in 10 dwnt 0 sequential_statements1 next FRED when i = 3; sequential_statements2 25 EXIT statements can be used t terminate the executin f a LOOP statement. Terminates all remaining iteratins f a Frms: 1. exit; 2. exit when cnditin ; 3. exit label ; 4. exit label when cnditin ; 26 EXIT Statement NULL Statement SAM: while DONE = false -- sequential_statements1 exit when DONE = true; -- sequential_statements2 27 Des nthing -- filler ( SEL ) case SEL is when 0 => decut <= "00"; when 1 => decut <= "01"; when 2 => decut <= "10"; when 3 => decut <= "11"; when thers => null; 28 Summary Sequential cnstructs Lps: Fr/while Case If/then/elsif/else Prcess executin Sensitivity lists vs. wait statements Next time: Cncurrent cnstructs 29 5