CPLD VHDL intro 3: Inverse LED toggle

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Contents

Overview

In tutorial 3 we build on the simple push button example in tutorial 2. We'll add logic that inverts the output of button 1 on LED D2.

Tutorial 3 truth table
Button PB LED D1LED D2
Pressed OFF ON
NOT pressed ONOFF

Schematic

Cpld-tutorial-external-circuit.png

In this demo we'll use both LEDs and the push button connected to the CPLD.

CPLD dev board connections
IO pin connection
LED D1 P39
LED D2 P38
Button BP P18
The XC2C64A CPLD has internal pullup resistors
R1 is unpopulated on the XC2C64A development board

VHDL

Cpld-demo3.png

library IEEE;
use IEEE.STD_LOGIC_1164.ALL;
use IEEE.STD_LOGIC_ARITH.ALL;
use IEEE.STD_LOGIC_UNSIGNED.ALL;
 
entity light_LED_PB is Port (BUTTON: in  STD_LOGIC:='1';
                             LED : out  STD_LOGIC:='0';
                                      LED_INV: out STD_LOGIC:='0');
end light_LED_PB;
 
architecture Behavioral of light_LED_PB is
begin
 
LED <= BUTTON;
LED_INV <= not (BUTTON);
 
end Behavioral;

LED_INV is set to be the opposite (not) of BUTTON.

UCF

XC9572 UCF

Cpld-demo3.png

#PIN MAP OF DANGEROUSPROTOTYPES.COM CPLD BREAKOUT BOARDS
#lICENSE: CC-0 (CREATIVE COMMONS 0)
#http://dangerousprototypes.com/docs/XC9500XL_CPLD_breakout_board
#http://dangerousprototypes.com/docs/CoolRunner-II_CPLD_breakout_board
NET "LED"            LOC =  "P39";
NET "BUTTON"        LOC =  "P18";
NET "LED_INV"        LOC =  "P38";

Input and outputs from the module are mapped to actual CPLD pin numbers in the UCF file. This is an example UCF file that defines the three IO connections on the development boards.

  • The Pxx numbers are the actual pin number on the CPLD. Easy.

We mapped the BUTTON input market to pin 18. The LED output marker connects to pin 39 (D1), and LED_INV connects to pin 38 (D2).

XC2C64A

Cpld-xc2c-ledinv.png

#PIN MAP OF DANGEROUSPROTOTYPES.COM CPLD BREAKOUT BOARDS
#lICENSE: CC-0 (CREATIVE COMMONS 0)
#http://dangerousprototypes.com/docs/XC9500XL_CPLD_breakout_board
#http://dangerousprototypes.com/docs/CoolRunner-II_CPLD_breakout_board
NET "LED"            LOC =  "P39";
NET "LED_INV"        LOC =  "P38";
NET "BUTTON"         LOC =  "P18";
NET "BUTTON"         PULLUP;

The XC2C64A version is the same, except we use the internal pull-up resistor instead of R1 on the development board.