XC95108 VHDL Parallel to Serial Shift Registers and Arduino Example

A parallel to serial shift register is useful for microprocessor digital inputs extending. For instant a keypad input for printer for photocopier machine. There are many off-the-shelf parallel to serial shift register chips, especially the SN74HC165.

Hardware Testing

Using a programmable logic device we can design this kind of chip with customizable operations, and the numbers of I/O. This job could be done with a CPLD chip and its design tool, VHLD, Verilog code, or even schematic design tool.

Running Arduino Program and the XC95108

In this example, I use an XC95108 CPLD to design a parallel to serial converter chip. VHDL code is preferred for this design. This design contains,

• Clock (CLK) input,
• LOAD input to latch data from an 8-bit digital input to the internal register,
• Serial Data Out (DOUT) that shift data serially at the rising edge of input clock,
• Digital Input (D) is an 8-bit data input.

The overall process requires 9 clock cycles. At the first clock, data will be read and latched into an internal register, then the 8-bit serial data shifting out will be processed.

----------------------------------------------------------------------------------
-- Company: 
-- Engineer: 
-- 
-- Create Date:    08:31:41 02/03/2024 
-- Design Name: 
-- Module Name:    parallel_serial_2 - Behavioral 
-- Project Name: 
-- Target Devices: 
-- Tool versions: 
-- Description: 
--
-- Dependencies: 
--
-- Revision: 
-- Revision 0.01 - File Created
-- Additional Comments: 
--
----------------------------------------------------------------------------------
library IEEE;
use IEEE.STD_LOGIC_1164.ALL;
 
entity parallel_serial_2 is
    Port ( CLK : in  STD_LOGIC;
           LOAD : in  STD_LOGIC;
           DOUT : out  STD_LOGIC;
           D : in  STD_LOGIC_VECTOR (7 downto 0));
end parallel_serial_2;
 
architecture Behavioral of parallel_serial_2 is
	SIGNAL REG: STD_LOGIC_VECTOR(7 DOWNTO 0);
begin
	PROCESS(CLK,LOAD)
		BEGIN
			IF(CLK'EVENT AND CLK='1') THEN
				IF(LOAD='1') THEN REG<=D;
				ELSE REG<=REG(6 DOWNTO 0)&'0'; END IF;
			END IF;
	END PROCESS;
	DOUT<=REG(7);
end Behavioral;
 
 

You can use the FloorPlan I/O tool in the ISE Design Suite to assign its I/O pins. On the XC95108 CPLD Prototype Board, I set the I/O as follow.

#PACE: Start of Constraints generated by PACE
 
#PACE: Start of PACE I/O Pin Assignments
NET "CLK"  LOC = "P50"  ;
NET "D<0>"  LOC = "P48"  ;
NET "D<1>"  LOC = "P47"  ;
NET "D<2>"  LOC = "P46"  ;
NET "D<3>"  LOC = "P45"  ;
NET "D<4>"  LOC = "P44"  ;
NET "D<5>"  LOC = "P43"  ;
NET "D<6>"  LOC = "P41"  ;
NET "D<7>"  LOC = "P40"  ;
NET "DOUT"  LOC = "P52"  ;
NET "LOAD"  LOC = "P51"  ;
#PACE: Start of PACE Area Constraints
 
#PACE: Start of PACE Prohibit Constraints
 
#PACE: End of Constraints generated by PACE
 

The Arduino is very popular for most of electronics projects. I use an Arduino Uno to read the data from this chip.

 
 
const char clockPin= 13;
const char Enable = 10;
const char dataPin=12;
 
uint8_t incoming=0,oldData;
 
void setup() {
  Serial.begin(9600);
  Serial.println("Arduino ShiftIn And XC95108 Parallel To Serial Converter.");
  pinMode(clockPin,OUTPUT);
  pinMode(Enable,OUTPUT);
  pinMode(dataPin,INPUT);
  digitalWrite(clockPin,LOW);
  digitalWrite(Enable,LOW);
}
 
void loop() {
  digitalWrite(Enable,HIGH);
  digitalWrite(clockPin,LOW);
  digitalWrite(clockPin,HIGH);
  digitalWrite(Enable,LOW);
 
  for(uint8_t i=0;i<8;i++){
    digitalWrite(clockPin,LOW);
    if(digitalRead(dataPin)==1) incoming|=(1<<(7-i));
    else incoming&=~(1<<(7-i));   
    digitalWrite(clockPin,HIGH);
  }
  if(incoming!=oldData){
    Serial.println("Received 8-Bit HEX Data: 0x" + String(incoming,HEX));
    Serial.println("Received 8-Bit BIN Data: 0b" + String(incoming,BIN));
    oldData=incoming;
  }
  // Serial.println(incoming,HEX);
  //Serial.println(incoming,BIN);
  delay(1500);
}

Click here to download this example.