module Z74XS3 
title '4-bit Binary Counter' 

" Input pins 
CLK, !LD, !CLR, ENP, ENT        pin; 
A, B, C, D                      pin; 

" Output pins 
QA, QB, QC, QD                  pin istype 'reg';
RCO                             pin istype 'com';

" Set definitions 
INPUT = [D, C, B, A]; 
COUNT = [QD, QC, QB, QA];

equations

COUNT.CLK = CLK;

COUNT := !CLR & ( LD & INPUT
       # !LD & (ENT & ENP) & 
           ((COUNT==12) & 3) # ((COUNT!=12) & (COUNT + 1))
        # !LD & !(ENT & ENP) & COUNT);
        
RCO = (COUNT == 12) & ENT;

end Z74XS3