Dual 4-digit seven segment LED display with SPI interface

Posted on June 26, 2012 by R-B 2 comments |

This project is an extension of my previous MAX7219 based SPI seven segment LED display module. The new display features eight 7-segment displays arranged in two rows of four digits. The on-board MAX7219 driver enables you to easily add eight 7-segment LED displays to your project using only 3 I/O pins of microcontroller. The major advantage of using this board is the time-division multiplexing operations required for continuous refreshing of the display digits are performed by the MAX7219 chip, thereby keeping the microcontroller free for doing other pressing tasks. It is suitable for displaying two variable values simultaneously in a project, such as displaying temperature and humidity, or current and voltage, etc.

8-digit seven segment LED display with serial interface

Key Features:

based on MAX7219 display driver
SPI interface (3 pins)
operates at +5V supply
individual control of all digits and decimal points
display brightness control through software
dimensions 1.95? x 1.95? (50mm x 50mm)
two 0.36? 4-digit seven segment LED display

The circuit diagram of this project is very simple (shown below) and derived from the datasheet of the MAX7219.

Circuit diagram as described in the datasheet of MAX7219

Assembled display board

In the above board, digits from right to left in the first row are DIG0 through DIG3, whereas in the second row, the digits from right to left are DIG4-DIG7. The following example code is written in C and compiled with mikroC Pro for PIC compiler to illustrate how to interface this display module with the PIC12F project board. The program is for a dual 4-digit decimal counter. The first row of the display will be an up counter counting from 0000 to 9999 with one second duration. Meanwhile, the second row will count in reverse order from 9999 to 0000. The microcontroller used in the PIC12F board is PIC12F683. The details of MAX7219 registers and their initialization is described in my previous article Serial 4-digit seven segment display module.

/*
  Project: 4-digit Up/Down counter using SPI 7-segment LED display
  MCU: PIC12F683 running at 4.0MHz internal clock, MCLR disabled
  Date: June 26, 2012
*/
 
// Define Soft-SPI connections
#define CS_Pin       GP0_bit
#define MOSI_Pin     GP1_bit
#define CLK_Pin      GP2_bit
 
void SPI_Write_Byte(unsigned short num){
 unsigned short t, Mask, Flag;
 CLK_Pin = 0;
 Mask = 128;
 for (t=0; t&lt;8; t++){
  Flag = num &amp; Mask;
  if(Flag == 0) MOSI_Pin = 0;
  else MOSI_Pin = 1;
  CLK_Pin = 1;
  CLK_Pin = 0;
  Mask = Mask &gt;&gt; 1;
 }
}
 
void MAX7219_INIT() {
  // Disable Shutdown mode
  CS_Pin = 0;              // CS pin is pulled LOW
  SPI_Write_Byte(0x0C);    // Select Shutdown register
  SPI_Write_Byte(0x01);    // Set D0 bit to return to normal operation
  CS_Pin = 1;              // CS pin is pulled HIGH
 
  // Set BCD decode mode for digits DIG0-DIG3
  CS_Pin = 0;              // CS pin is pulled LOW
  SPI_Write_Byte(0x09);    // Select Decode Mode register
  SPI_Write_Byte(0xFF);    // Select BCD mode for digits DIG0-DIG7
  CS_Pin = 1;              // CS pin is pulled HIGH
 
  // Set display brighness
  CS_Pin = 0;              // CS pin is pulled LOW
  SPI_Write_Byte(0x0A);    // Select Intensity register
  SPI_Write_Byte(0x0C);    // Set brightness
  CS_Pin = 1;              // CS pin is pulled HIGH
 
   // Set display refresh
  CS_Pin = 0;              // CS pin is pulled LOW
  SPI_Write_Byte(0x0B);    // Select Scan-Limit register
  SPI_Write_Byte(0x07);    // Select digits DIG0-DIG3
  CS_Pin = 1;              // CS pin is pulled HIGH
 
 // Enable Display-Test
  CS_Pin = 0;              // CS pin is pulled LOW
  SPI_Write_Byte(0x0F);    // Select Display-Test register
  SPI_Write_Byte(0x01);    // Enable Display-Test
  CS_Pin = 1;              // CS pin is pulled HIGH
 
  Delay_ms(1000);
 // Disable Display-Test
  CS_Pin = 0;              // CS pin is pulled LOW
  SPI_Write_Byte(0x0F);    // Select Display-Test register
  SPI_Write_Byte(0x00);    // Disable Display-Test
  CS_Pin = 1;              // CS pin is pulled HIGH
 
}
 
 void Display_Counter(unsigned int j, unsigned short k){
  CS_Pin = 0;                    // CS pin is pulled LOW
  SPI_Write_Byte(k);             // Send thousands digit
  SPI_Write_Byte((j/1000)%10);
  CS_Pin = 1;                    // CS pin is pulled HIGH
 
  CS_Pin = 0;                    // CS pin is pulled LOW
  SPI_Write_Byte(k-1);             // Send hundreds digit
  SPI_Write_Byte((j/100)%10);
  CS_Pin = 1;                    // CS pin is pulled HIGH
 
  CS_Pin = 0;                    // CS pin is pulled LOW
  SPI_Write_Byte(k-2);             // Send tens digit
  SPI_Write_Byte((j/10)%10);
  CS_Pin = 1;                    // CS pin is pulled HIGH
 
  CS_Pin = 0;                    // CS pin is pulled LOW
  SPI_Write_Byte(k-3);             // Send ones digit
  SPI_Write_Byte(j%10);
  CS_Pin = 1;                    // CS pin is pulled HIGH
 }
 
unsigned short i;
unsigned int counter = 0;
 
void main() {
  TRISIO=0b00001000;        // GP3 is input only
  CMCON0 = 0x07;
  ANSEL = 0x00;
  MAX7219_INIT();              // initialize  max7219
  do{
   for (counter=0; counter&lt;10000; counter++) {
    Display_Counter(counter, 4);
    Display_Counter(9999-counter, 8);
    Delay_ms(1000);
   }
  }while(1);
}

Download mikroC source and HEX files

Interfacing with the PIC12F683 microcontroller to make a dual counter

The kit consists of:

printed circuit board (1)
MAX7219 (1)
0.36? four-digit seven segment LED display (2)
1N4148 diode (1)
18K resistor (1)
100nF capacitor (1)
5-pin male header

Complete kit of 8-digit SPI seven segment LED display

While mounting the parts on PCB, solder low profile components (resistor, capacitor, and diode) first.

Placement of resistor, capacitor, diode and MAX7219 on board

The actual circuit of the display is shown below.

8-digit SPI display circuit

You can purchase this display module as kit for $12.00 on Tindie. Here’s the buying link: SPI7SEGDISP8 on Tindie.

Update (12/17/2012):

The new PCBs for this display board has an additional 10uF electrolytic capacitor between VCC and Ground, as shown below. You will receive the new version if you order it now.