module cnt600;  
title '4 bit toggle flip flop counter
Michael Holley   Data I/O  21 Feb 1987'

	cnt600	device	'e0600';

" Pin 1 clock for pins 3..10, pin 13 clock for pins 15..22
" input pins 2,11,14,23
" programmable i/o pins 3..10 and 15..22
"
" Reset nodes labeled with .RE
" Async clock nodes labeled with .C
"
	Clk1,Clk2	pin   1,13;
	Dir,Ena,Clr	pin   2,3,11;	
	Q3,Q2,Q1,Q0	pin  15,16,17,18;

	Q3,Q2,Q1,Q0	istype 'reg_T,pos';

	Count		= [Q3,Q2,Q1,Q0];
	X,Z,H,L		= .X.,.Z.,1,0;

equations   "4 bit toggle flip flop counter

"Count Up
	Q0.t =  Dir;
	Q1.t =  Dir &  Q0.fb;
	Q2.t =  Dir &  Q0.fb &  Q1.fb;
	Q3.t =  Dir &  Q0.fb &  Q1.fb &  Q2.fb;

"Count Down
	Q0.t = !Dir;
	Q1.t = !Dir & !Q0.fb;
	Q2.t = !Dir & !Q0.fb & !Q1.fb;
	Q3.t = !Dir & !Q0.fb & !Q1.fb & !Q2.fb;

"Reset
	[Q3,Q2,Q1,Q0].AR  = !Clr;
	[Q3,Q2,Q1,Q0].CLK = Clk2;
	[Q3,Q2,Q1,Q0].OE  = Ena;

@message 'Use the -e flag to get an expanded listing of test vectors';
test_vectors 'Count Up'
	([Clk2,Ena,Dir,Clr] -> Count)
	 [ L  , 1 , H , L ] ->   0;
@const n = 0; @repeat 16 {
	 [ L  , 1 , H , H ] ->   @expr n; ; @const n = n+1;
	 [ H  , 1 , H , H ] ->   @expr n; ;}

	 [ L  , 1 , H , H ] ->   @expr n; ;

test_vectors 'Count Down'
	([Clk2,Ena,Dir,Clr] -> Count)
@repeat 6 {
	 [ L  , 1 , L , H ] ->   @expr n; ; @const n = n-1;
	 [ H  , 1 , L , H ] ->   @expr n; ;}

	 [ L  , 1 , L , H ] ->   @expr n; ;
	 [ L  , 1 , L , L ] ->   0;
end