Digilent Parallel Interface Model - Manual (Page 8)

Digilent Parallel Interface Model Digilent, Inc.
www.digilentinc.com Copyright Digilent, Inc. Page 8
regData3 when regEppAdr = "0011" else
regData4 when regEppAdr = "0100" else
regData5 when regEppAdr = "0101" else
regData6 when regEppAdr = "0110" else
regData7 when regEppAdr = "0111" else
rgSwt when regEppAdr = "1000" else
"000" & rgBtn when regEppAdr = "1001" else
"00000000";
------------------------------------------------------------------------
-- EPP Interface Control State Machine
------------------------------------------------------------------------
-- Map control signals from the current state
ctlEppWait <= stEppCur(0);
ctlEppDir <= stEppCur(1);
ctlEppAwr <= stEppCur(2);
ctlEppDwr <= stEppCur(3);
-- This process moves the state machine to the next state
-- on each clock cycle
process (clkMain)
begin
if clkMain = '1' and clkMain'Event then
stEppCur <= stEppNext;
end if;
end process;
-- This process determines the next state machine state based
-- on the current state and the state machine inputs.
process (stEppCur, stEppNext, ctlEppAstb, ctlEppDstb, ctlEppWr)
begin
case stEppCur is
-- Idle state waiting for the beginning of an EPP cycle
when stEppReady =>
if ctlEppAstb = '0' then
-- Address read or write cycle
if ctlEppWr = '0' then
stEppNext <= stEppAwrA;
else
stEppNext <= stEppArdA;
end if;
elsif ctlEppDstb = '0' then
-- Data read or write cycle
if ctlEppWr = '0' then
stEppNext <= stEppDwrA;
else
stEppNext <= stEppDrdA;
end if;
else
-- Remain in ready state
stEppNext <= stEppReady;
end if;
-- Write address register
when stEppAwrA =>