7-segment LED display

Necessary knowledge: [HW] User Interface Module, [LIB] Delay, [LIB] 7-segment LED Display, [PRT] Light-emitting Diode

7-segmented LED number-indicator is a display which consists of 7 LEDs positioned in the shape of 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.

Positioning of the LED indicator's segments and electrical scheme

Electrically all anodes of the LEDs are connected to one anode pin ca. LEDs are lit by switching their cathodes (a, b, c…). Exists also 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 coma (point) segment dp. All in all one indicator has 8 segments, but they are still called 7-segmented according to the number of number-segments.

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 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 exist indicators with extra segments.

There is one 7-segment LED number-indicator on the Digital i/o module. It is controlled through a driver with serial interface A6275. The serial interface of the driver is similar to SPI, where both clock signal and data signal are used. Different from SPI the chip-select is not used there, and is replaced with latch function. The above mentioned three lines are connected to the ATmega128 as follows:

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

• Latch-signal (latch) - PG2
• Clock-signal (clock) - PC7
• Data-signal (data) - PC6

The data is delivered by bits through the data pin. Every time the clock signal goes high, the contents of the shift-index is shifted to the right and the bit from the data-pin is read to the left most cell. By this 8-bits are loaded to the shift-index. If the latch signal is set high, the value of the shift-index is loaded to the latch-index, where it remains until new loading. Each bit of the latch-index is connected through the current switching to one number-indicator’s segment. The segment is lit during the high bit-rate.

For displaying the numbers on the HomeLabs Digital i/o module indicator, the following functionality is written to the library of the HomeLab.

//
// Set-up of the pins
//
static pin segment_display_latch =    PIN(G, 2);
static pin segment_display_data_out = PIN(C, 6);
static pin segment_display_clock =    PIN(C, 7);
 
//
// Marking card.
// The bits are marking the segments. Lower ranking is A, higher ranking is DP.
//
static const unsigned char 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 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 he bits. 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 as high for a moment.
		pin_set(segment_display_clock);
		_delay_us(1);
 
		// The clock-signal as low.
		pin_clear(segment_display_clock);
		_delay_us(1);
	}
 
	// Latch-signal on.
	pin_set(segment_display_latch);
}

For displaying numbers and the letter “E”, is created a constant array segment_char_map, where lighting of all 8 segments is marked with bit 1 and switch off is market with bit 0. The bits form higher to lower (from left to right in binary form) are marking segments DP, G, F, E, D, C, B, A. 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.

The following is a more concrete example of a program for using the number-indicator. 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. Such counting is achieved by taking a module from a much larger number.

//
// The example program of the 7-segment LED indicator of the HomeLab's
// input-output module.
//
#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 ones values of the counter.
		segment_display_write(counter);
 
		// Counting very long.
		counter++;
 
		// Delay for 1 second.
		sw_delay_ms(1000);
	}
}