Tutorial 18: Routing I/O Signals in VHDL

Created on: 13 March 2013

Another easy thing to do in VHDL is to route I/O pins from a microcontroller through a CPLD to connect the pins to peripherals.

In this tutorial, a switch and two LEDs that are interfaced to a CPLD are connected to a microcontroller that is also connected to the CPLD. VHDL is used to connect the switch and LEDs to the microcontroller.

The microcontroller runs software to read the switch and control the LEDs.

Hardware Setup

Connections between an ATtiny2313 microcontroller and CPLD are made as shown in the diagram below.

Pins used to connect the microcontroller to CPLD
Pins used in CPLD to Microcontroller Connections

On the home made Xilinx CPLD board, the above connections are made by connecting wires between the connector on the microcontroller and the connector on the CPLD. The photo below shows the three green wires connecting microcontroller to the CPLD.

CPLD to microcontroller connections on the home built Xilinx CPLD board
CPLD to Microcontroller Connections of the Home Built Xilinx CPLD Board

VHDL Code for Routing Microcontroller Pins

The VHDL code for connecting the switch and two LEDs to the microcontroller pins is shown here.

library IEEE;

entity IO_redirect_top is
    Port ( LED    : out STD_LOGIC_VECTOR (1 downto 0); -- LEDs on CPLD
           BUTTON : in  STD_LOGIC;                     -- button on CPLD
           U_LED  : in  STD_LOGIC_VECTOR (1 downto 0); -- connection to uC
           U_SW   : out STD_LOGIC);                    -- connection to uC
end IO_redirect_top;

architecture Behavioral of IO_redirect_top is


    -- connect microcontroller pins that drive LEDs to LEDs
    LED <= U_LED;
    -- connect push button switch to microcontroller input
    U_SW <= BUTTON;

end Behavioral;

This code is pretty straight forward and is nothing new if you have been following the VHDL CPLD course. The LEDs and push button switch that are interfaced to the CPLD are named LED and BUTTON. The microcontroller pins that will connect to the LEDs are called U_LED and the microcontroller pin that will connect to the push button switch is called U_SW.

The VHDL code simply connects the signals from the LEDs to the microcontroller LED signals. The push button switch signal is connected to the microcontroller switch signal.

Microcontroller Software

The software used on the ATtiny2313 microcontroller to read the switch and control the LEDs is listed below.

#include <avr/io.h>

void Delay(volatile unsigned int);
void ChangeDelay(volatile unsigned int*);

int main(void)
    volatile unsigned int del_val = 40000;
    // LEDs are redirected to pins PB2 and PB3
    DDRB |= ((1 << PB2) | (1 << PB3));
    // push button switch redirected to PB1
    DDRB &= ~(1 << PB1);
        // switch one LED on / other LED off
        PORTB &= ~(1 << PB2);           // switch PD0 LED off
        PORTB |=  (1 << PB3);           // switch PD5 LED on
        PORTB &= ~(1 << PB3);           // switch PD5 LED off
        PORTB |=  (1 << PB2);           // switch PD0 LED on

void Delay(volatile unsigned int del)

void ChangeDelay(volatile unsigned int *del)
    if (PINB & (1 << PB1)) {    // check switch state
        *del = 10000;
    else {
        *del = 40000;

The program reads the push button switch and then changes the speed at which the LEDs flash, depending on if the switch is open or closed.