presclr.abl

FPGA的时钟设计,源代码,很有参考价值,希望对学习FPGA设计的朋友有参考意义.

MODULE PRESCLR

TITLE 'Prescaler function for Clock'


"Inputs
SYSCLK,  TEST              PIN ;


"Outputs
HZ128, HZ16, HZ8           PIN ISTYPE 'REG_D';
HZ1                        PIN ISTYPE 'COM';

"Counter nodes - HZ outputs make up missing 4 bits of counter.
BC19, BC18, BC15, BC14, BC13, BC11, BC10, BC9,
BC8, BC7, BC6, BC5, BC4, BC3, BC2, BC1, BC0    NODE ISTYPE 'REG_D';

"Cascade nodes
TCLO, TCMID   NODE;


"Counter group
CNTLOW = [BC7,BC6,BC5,BC4,BC3,BC2,BC1,BC0];
CNTMID = [BC15,BC14,BC13,HZ128,BC11,BC10,BC9,BC8];
CNTHI =  [BC19,BC18,HZ8,HZ16];

TERMCNT     = [1,1,1,1,1,1,1,1];   "Terminal (Highest) count value
PRELOAD_MID = [0,0,0,0,1,0,1,1];   "Middle Byte Counter preload value
PRELOAD_LO  = [1,1,0,1,1,0,1,1];   "Low Byte Counter preload value
TCHI        = [1,1,1,1];

Equations

        "Clock and Reset equations for all registers
	CNTLOW.CLK = SYSCLK;
        CNTMID.CLK = SYSCLK;
        CNTHI.CLK = SYSCLK;


	"Low Byte Counter equation. Count up if not at 16-bit terminal 
        "count, or load counter with preload value if at 16-bit 
        "terminal count.
	CNTLOW.D = (CNTLOW.Q + 1) & !TCMID
                 # (PRELOAD_LO)   &  TCMID;


        "Summing node for low-byte terminal count.
	TCLO = (CNTLOW.Q == TERMCNT);


        "Mid Byte Counter equation. Count up if not at 16-bit terminal
        "count, hold if not at low byte terminal count, load counter
        "with preload value if at 16-bit terminal count.
	CNTMID.D = ((CNTMID.Q + 1) & TCLO) & !TCMID
                 # ((CNTMID.Q) & !TCLO)    & !TCMID
                 #  (PRELOAD_MID)          &  TCMID;


        "Summing node for mid-byte terminal count. Test input forces
        "rollover state to occur immediately, effectively bypassing 
        "most of the prescaler's counters for simulation efficiency.
	TCMID = ((CNTMID.Q == TERMCNT) & TCLO) # TEST;	


	"High nibble counter equation. Count up one with each 16-bit
        "terminal count, hold in between.
	CNTHI.D = (CNTHI.Q + 1) & TCMID
                # (CNTHI.Q) & !TCMID;


	"Final, one-second pulse output occurs when counter high byte
        "reaches terminal count.
	HZ1 = (CNTHI.Q == TCHI);


END