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程序一:
; v+ E; ?/ U# G# z l) G3 wentity latch is
: D# q7 \& S6 e3 [ Port ( D : in STD_LOGIC;
2 M4 B' o/ m9 p# p# X. | ENA : in STD_LOGIC;! I# a4 V1 R' V8 ? { H( `
Q : out STD_LOGIC);
5 z. x* l# m0 y7 C$ n1 f0 `$ H4 oend latch;
^( S" U8 o& }! karchitecture Behavioral of latch is
' C% d/ `- K- Z% s$ J SIGNAL sig_save :STD_LOGIC;! l f! z$ o# J- t0 r" m$ X
begin" i J9 u0 u4 i
PROCESS(D,ENA)
* r+ S8 U0 L& }& D begin0 D L1 v& `1 H0 |8 m( P: P
IF(ENA='1') THEN% b; l( a+ E- a5 k6 _
SIG_SAVE<=d;; N$ f, x k/ x, o& P& O$ C
END IF;
" b: `# t; W4 X9 r Q<=SIG_SAVE;$ {6 [0 d ]" l" z/ }- {5 Y
2 k3 i+ M7 r! n4 i7 L0 a* ^' |
END PROCESS;: }. t4 t' h/ ^0 [6 S3 Z" c
w0 U3 }$ m- x4 E; g Aend Behavioral;
& Q$ j5 T, p1 N+ e2 y4 I: z程序二:
. |/ t) i+ F& e9 _; r/ `, Z% Y! p Rentity latch is/ _5 O* S: x/ O- f8 w$ @' Z
Port ( D : in STD_LOGIC;0 n* o" Y. P+ i3 I
ENA : in STD_LOGIC;* r. q3 l* e: ^
Q : out STD_LOGIC);
7 T" p1 o- K* |0 {2 }end latch;
7 _6 S$ F! k2 P; Garchitecture Behavioral of latch is8 L; N/ e Y. R k, _' }/ a
--SIGNAL sig_save :STD_LOGIC;8 y. r0 [# @$ o x, c% g0 n
begin
0 N+ W9 _; r3 A, M* f) @+ \ PROCESS(D,ENA)
' d4 c) z4 s3 d begin
K5 v; Z; d1 k7 e IF(ENA='1') THEN
# s# S R3 |' ~. P Q<=D;
" N% @" {* G8 l) z: W- p0 p -- SIG_SAVE<=d;& v( k# {! Z- G/ P6 N
END IF; * s* v- a! Y F9 i- E M' n: i4 M9 G
--Q<=SIG_SAVE;
# Z( r/ }. O1 y, o' u
2 O$ u/ c9 }5 \: K# J END PROCESS;' |1 p: E* h7 A; U4 s: ]8 X: N" S
! I6 G9 ~( Q/ c I0 t0 q
end Behavioral;
! J/ d; T- S- h' R% m两个程序变动很少,第一个程序,Q与D的前一个状态一致,即锁存D的前一个状态,而程序二Q与D的当前值一致,为什么?? |
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