XC95108 16-bit Serial In Parallel Out Shift Registers VHDL Example

A Serial-In-Parallel-Out shift register IC is very popular among micro-controller programmers. It's commonly use for I/O extending, LED driving, relay driving, etc. The SN75C595N or the SN74HC164 are commonly used with micro-controllers to expand their outputs.

Hardware test on bread-board with Arduino Uno SPI

Using a CPLD/FPGA we can create any digital circuit including this type shift registers chip. In this example, I use the XC95108 CPLD to create a 16-bit serial in parallel out shift registers chip. The chosen language is VHDL.

The designed chip include reset(RST), serial clock(CLK), and serial data in (DIN). Its output is an 16-bit output port connects to LED.

----------------------------------------------------------------------------------
-- Company: 
-- Engineer: 
-- 
-- Create Date:    10:34:09 01/29/2024 
-- Design Name: 
-- Module Name:    shift_regs_16_LED - 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;
 
-- Uncomment the following library declaration if using
-- arithmetic functions with Signed or Unsigned values
--use IEEE.NUMERIC_STD.ALL;
 
-- Uncomment the following library declaration if instantiating
-- any Xilinx primitives in this code.
--library UNISIM;
--use UNISIM.VComponents.all;
 
entity shift_regs_16_LED is
    Port ( CLK : in  STD_LOGIC;
           RST : in  STD_LOGIC;
           DIN : in  STD_LOGIC;
           LEDS : out  STD_LOGIC_VECTOR (15 downto 0));
end shift_regs_16_LED;
 
architecture Behavioral of shift_regs_16_LED is
	SIGNAL REGS: STD_LOGIC_VECTOR(15 DOWNTO 0);
begin
PROCESS(CLK,RST)
	BEGIN
		IF(RST='0') THEN REGS<=x"0000";
		ELSIF(CLK'EVENT AND CLK='1') THEN
			REGS(0)<=DIN;
			REGS(1)<=REGS(0);
			REGS(2)<=REGS(1);
			REGS(3)<=REGS(2);
			REGS(4)<=REGS(3);
			REGS(5)<=REGS(4);
			REGS(6)<=REGS(5);
			REGS(7)<=REGS(6);
			REGS(8)<=REGS(7);
			REGS(9)<=REGS(8);
			REGS(10)<=REGS(9);
			REGS(11)<=REGS(10);
			REGS(12)<=REGS(11);
			REGS(13)<=REGS(12);
			REGS(14)<=REGS(13);
			REGS(15)<=REGS(14);
		END IF;
		LEDS<=REGS;
END PROCESS;
 
end Behavioral;
 
 

Its internal circuit doesn't need additional clock input to synchronize input data. The data is synchronized with serial clock pin from external device.

Its I/O pins are assigned as follow using the Floorplan I/O tool.

#PACE: Start of Constraints generated by PACE
 
#PACE: Start of PACE I/O Pin Assignments
NET "CLK"  LOC = "P48"  ;
NET "DIN"  LOC = "P50"  ;
NET "LEDS<0>"  LOC = "P63"  ;
NET "LEDS<10>"  LOC = "P67"  ;
NET "LEDS<11>"  LOC = "P68"  ;
NET "LEDS<12>"  LOC = "P69"  ;
NET "LEDS<13>"  LOC = "P70"  ;
NET "LEDS<14>"  LOC = "P71"  ;
NET "LEDS<15>"  LOC = "P72"  ;
NET "LEDS<1>"  LOC = "P62"  ;
NET "LEDS<2>"  LOC = "P61"  ;
NET "LEDS<3>"  LOC = "P58"  ;
NET "LEDS<4>"  LOC = "P57"  ;
NET "LEDS<5>"  LOC = "P56"  ;
NET "LEDS<6>"  LOC = "P55"  ;
NET "LEDS<7>"  LOC = "P54"  ;
NET "LEDS<8>"  LOC = "P65"  ;
NET "LEDS<9>"  LOC = "P66"  ;
NET "RST"  LOC = "P47"  ;
 
#PACE: Start of PACE Area Constraints
 
#PACE: Start of PACE Prohibit Constraints
 
#PACE: End of Constraints generated by PACE
 

After the design and programming process are completed, we can test this circuit manually with a few on-board input switches. For convenience we can use a popular Arduino Uno as an SPI master device transferring a 16-bit data to this shift registers chip.

#include<SPI.h>
 
void setup() {
  SPI.begin();
}
 
char data[2];
void loop() {
  data[0]=0xF0;
  data[1]=0x0F;
  SPI.transfer(data,2);
  delay(1000);
  data[0]=0x0F;
  data[1]=0xF0;
  SPI.transfer(data,2);
  delay(1000);
}

The circuit connections between the Arduino Uno and the XC95108 CPLD are:

Arduino Uno            XC95108 CPLD Prototype Board

GND                        GND

PIN11 (MOSI)            P50

PIN13 (SCK)               P48

I use the Arduino SPI module and library without bit-banging. Optionally we can use the on-board 7-Segment display instead of the LED.

Click here to download its source file.