Ôóíêöèîíàëüíîå îïèñàíèå öèôðîâûõ óñòðîéñòâ - ÷àñòü 9

+ OUT1  OUT2  OUT3

+  IO_MODEL

+  TRISTATE:

+              ENABLE  LO = ENA

+              OUT1  =  {

+                          CASE (

+                                     CHANGED (REF1, 0) & TRN_LH, DELAY(. . .),

+                                     CHANGED (REF2, 0)  DELAY(. . .),

+                                     TRN_ZL,        DELAY(. . .),

+                                     .   .   .

+                                     )

+                          }

+              OUT3 = {

+                          CASE (

+                                     TRN_LZ, DELAY(. . .),

+                                     TRN_HZ, DELAY(. . .),

+                                     .   .   .

+                                     )

+                          }

+  PINDLY:

+              OUT2 = {

+                          CASE (

+                                     CHANGED (REF1, 0), DELAY(. . .),

+                                     .   .   .

+                                     )

+                          }

Ðèñ. 6.18. Óïðàâëåíèå çàäåðæêàìè ñ ïîìîùüþ óçëîâ ðàçðåøåíèÿ

Ñòðóêòóðà óïðàâëåíèÿ çàäåðæêàìè â äàííîì ïðèìåðå ïîêàçàíà íà ðèñ. 6.18.

Ôóíêöèîíèðîâàíèå ïðèìèòèâà PINDLY. Ñîñòîÿíèÿ âûõîäíûõ óçëîâ ïðèìèòèâà PINDLY èçìåíÿþòñÿ ïðè èçìåíåíèè ñîñòîÿíèé ëþáîãî âõîäíîãî óçëà èëè óçëà ðàçðåøåíèÿ. Êàæäîìó âõîäíîìó óçëó ñîîòâåòñòâóåò âûõîäíîé óçåë. Ñíà÷àëà îïðåäåëÿþòñÿ ñîñòîÿíèÿ âíóòðåííèõ ïåðåìåííûõ â ñåêöèè BOOLEAN, çàòåì âû÷èñëÿþòñÿ <âûðàæåíèÿ äëÿ çàäåðæåê> â  ñåêöèÿõ PINDLY èëè  TRISTATE. Ïîñëå ýòîãî èçìåíèâøååñÿ ñîñòîÿíèå âõîäíîãî óçëà ïðèñâàèâàåòñÿ ñîîòâåòñòâóþùåìó åìó âûõîäíîìó óçëó ñ çàäåðæêîé.

Ïðèâåäåì ïðèìåð ðåàëüíîãî ñ÷åò÷èêà 74LS160A:

ULS160ADLY   PINDLY (5,0,10)   DPWR   DGND

+ RCO   QA   QB   QC   QD                                                   ; Âõîäû

+ CLK   LOADBAR   ENT   CLRBAR                                  ; Âíóòðåííèå óçëû

+ RCO_O   QA_O   QB_O   QC_O   QD_O                          ; Âûõîäû