7-segment LED display

7-segment

A 7-segmented LED number-indicator is a display that consists of 7 LEDs positioned in the shape of the number 8. By lighting or switching off the corresponding LEDs (segments), it is possible to display numbers from 0 to nine as well as some letters.

Electrically, all anodes of the LEDs are connected to one anode pin ca. LEDs are lit by switching their cathodes (a, b, c…). There also exist reversed connections, where the indicators have a common cathode cc. Generally, several number-indicators are used for displaying multi-digit numbers - for this purpose, the indicators are equipped with a comma (point) segment dp. All in all, one indicator has 8 segments, but they are still called 7-segmented according to the number of segments.

Positioning of the LED indicator's segments and electrical scheme

The build-up of the LED driver's shift-index with corresponding segments of the indicator.

LED number-indicators are easy to use; they can be controlled directly from the pins of the microcontroller, but there are also special drivers, which are able to control number-indicators using fewer pins of the microcontroller. There are different colors of LED number indicators, which can be very bright and very large. For displaying the entire Latin alphabet, there exist indicators with extra segments. There are different drivers, but common drivers using a serial interface, which is similar to the SPI, where both the clock signal and the data signal are used. Different from SPI, the chip-select is not used there, and is replaced with a latch function. The above-mentioned three lines are connected to the controller pins.

• Latch-signal
• Clock-signal
• Data-signal

There is one 7-segment LED number-indicator on the Digital i/o module. It is controlled through a driver with serial interface. For displaying the numbers on the HomeLabs Digital i/o module indicator, the following functionality is written to the library of the HomeLab.

// Marking card. The bits are marking the segments.
const unsigned char __attribute__ ((weak)) segment_char_map[11] = 
{
	0b00111111,
	0b00000110,
	0b01011011,
	0b01001111,
	0b01100110,
	0b01101101,
	0b01111100,
	0b00000111,
	0b01111111,
	0b01100111,
	0b01111001   // E like Error
};
 
// Start-up of the 7-segment indicator
void segment_display_init(void)
{
	// Set latch, data out, and clock pins as output
	pin_setup_output(segment_display_latch);
	pin_setup_output(segment_display_data_out);
	pin_setup_output(segment_display_clock);
}
 
//Displaying a number on the 7-segment indicator
void segment_display_write(unsigned char digit)
{
	unsigned char map;
	signed char i;
 
	// Check-up of the number
	if (digit > 9)
	{
		digit = 10;
	}
 
	// Number as the card of the segments
	map = segment_char_map[digit];
 
	// Latch-signal off
	pin_clear(segment_display_latch);
 
	// Sending the bits. The higher ranking goes first
	for (i = 7; i >= 0; i--)
	{
		// Setting the pin according to the value of the bit of the card
		pin_set_to(segment_display_data_out, bit_is_set(map, i));
 
		// The clock-signal is high for a moment
		pin_set(segment_display_clock);
		_delay_us(1);
 
		// The clock signal is low
		pin_clear(segment_display_clock);
		_delay_us(1);
	}
 
	// Latch-signal on
	pin_set(segment_display_latch);
}

For displaying numbers and the letter “E”, a “weak” constant array segment_char_mapis created, where the lighting of all 8 segments is marked with bit 1 and switching off is marked with bit 0. The bits form lower to higher (from right to left in binary form) are marking segments A, B, C, D, E, F, G ja DP. The control interface of the driver is realized through software SPI, i.e. by using a software for controlling the data communication pins in the program. All three pins are set as output with segment_display_init function. segment_display_write is for displaying the function, which finds the segment-card of the mark from the array and transmits bit by bit all values of the segments to the driver. The frequency of the clock signal with the software delays is now approximately 500 kHz. When a user defines a variable segment_char_map in their own code, it is possible to create other characters on the screen (eg, text, etc.)

The following is a more concrete example of a program for using the number-indicator. The previously described function of the library is described in the program. The program counts numbers from 0 to 9 with approximate interval of 1 second and then displays letter E, because two-digit numbers is not possible to show on the one digit indicator.

// The example program of the 7-segment LED indicator of the HomeLab's
#include <homelab/module/segment_display.h>
#include <homelab/delay.h>
 
// Main program
int main(void)
{
	int counter = 0;
 
	// Set-up of the 7-segment indicator
	segment_display_init();
 
	// Endless loop
	while (true)
	{
		// Displaying the values of the counter
		segment_display_write(counter % 10);
 
		// Counting up to 10
		counter++;
		if (counter>19) counter=0;
 
		// Delay for 1 second
		sw_delay_ms(1000);
	}
}

1. Present numbers in the hexadecimal system randomly on the 7-segment display. The frequency is 1 Hz.
2. Light in circular sequence six outside segments on the 7-segment indicator with a period of 500 ms.