eda技术与vhdl.ppt

文档介绍：[|选择值]单个普通数值,如6。数值选择范围,如(2TO4),表示取值为2、3或4。并列数值,如35,表示取值为3或者5。混合方式,以上三种方式的混合。【例7-1】LIBRARYIEEE;;ENTITYmux41ISPORT(s4,s3,s2,s1:INSTD_LOGIC;z4,z3,z2,z1:OUTSTD_LOGIC);ENDmux41;ARCHITECTUREactivOFmux41ISSIGNALsel:INTEGERRANGE0TO15;BEGINPROCESS(sel,s4,s3,s2,s1)BEGINsel<=0;--输入初始值IF(s1='1')THENsel<=sel+1;ELSIF(s2='1')THENsel<=sel+2;ELSIF(s3='1')THENsel<=sel+4;ELSIF(s4='1')THENsel<=sel+8;ELSENULL;--注意,这里使用了空操作语句ENDIF;z1<='0';z2<='0';z3<='0';z4<='0';--输入初始值CASEselISWHEN0=>z1<='1';--当sel=0时选中WHEN13=>z2<='1';--当sel为1或3时选中WHEN4To72=>z3<='1';--当sel为2、4、5、6或7时选中WHENOTHERS=>z4<='1';--当sel为8~15中任一值时选中ENDCASE;ENDPROCESS;ENDactiv;【例7-2】SIGNALvalue:INTEGERRANGE0TO15;SIGNALout1:STD_LOGIC;...CASEvalueIS--缺少以WHEN引导的条件句ENDCASE;...CASEvalueISWHEN0=>out1<='1';--value2~15的值未包括进去WHEN1=>out1<='0';ENDCASE...CASEvalueISWHEN0TO10=>out1<='1';--选择值中5~10的值有重叠WHEN5TO15=>out1<='0';ENDCASE;【例7-3】LIBRARYIEEE;;;ENTITYaluISPORT(a,b:INSTD_LOGIC_VECTOR(7DOWNTO0);opcode:INSTD_LOGIC_VECTOR(1DOWNTO0);result:OUTSTD_LOGIC_VECTOR(7DOWNTO0));ENDalu;ARCHITECTUREbehaveOFaluISCONSTANTplus:STD_LOGIC_VECTOR(1DOWNTO0):=b"00";CONSTANTminus:STD_LOGIC_VECTOR(1DOWNTO0):=b"01";CONSTANTequal:STD_LOGIC_VECTOR(1DOWNTO0):=b"10";CONSTANTnot_equal:STD_LOGIC_VECTOR(1DOWNTO0):=b"11";BEGINPROCESS(opcode,a,b)BEGINCASEopcodeISWHENplus=>result<=a+b;--a、b相加WHENminus=>result<=a-b;--a、b相减WHENequal=>--a、b相等IF(a=b)THENresult<=x"01";ELSEresult<=x"00";ENDIF;WHENnot_equal=>--a、b不相等IF(a/=b)THENresult<=x"01";ELSEresult<=x"00";ENDIF;ENDCASE;ENDPROCESS;ENDbehave;(1)单个LOOP语句,其语法格式如下:[LOOP标号:]LOOP顺序语句ENDLOOP[LOOP标号];...L2:LOOPa:=a+1;EXITL2WHENa>10;--当a大于10时跳出循环ENDLOOPL2;...(2)FOR_LOOP语句,语法格式如下:[LOOP标号:]FOR循环变量,IN循环次数范围LOOP顺序语句ENDLOOP[LOOP标号];【例7-4】LIBRARYIEEE;;ENTITYp_checkISPORT(a:INSTD_LOGIC_VE