Seven segment display

In this article I intend to explain the functionality and usage of the seven segment display, probably you have seen many projects with these type of displays, however the price drop of LCD’s tend to overtake the market, there are a still few applications for which these devices are more suited. For large numeric displays, like clock’s, railway station displays, low-cost measuring devices or very stressing environments the led based displays are better, and cheaper.  The most simple led display available is the seven segment display, it consists of 7 led stripes arranged forming the number 8, because of its simple construction it is very robust and can function in very low or high temperatures, can withstand vibrations,  mechanical shocks without problems, for what the LCD would fail to work or even get permanently damaged.

single sevene segment

If we look at one digit we can see 10pins each segment and the small dot are LED’s, each of them has one terminal connected to a common pin, from this comes the name common anode or common cathode, and the other terminal is connected to a standalone pin, since the common pin is doubled, we have the 10 pins for each digit.

seven segment config

Lets take as example the common anode type, to light the segments we need to connect the positive supply rail to the common pin, and pull to ground the segments, each segment depending on its size can handle a few miliamps, after all we are talking about LED’s not bulbs. That’s fine if you need just one number to display, but how can these digits be connected to form a multi-digit display? The first approach is to connect each segment to a micro-controller pin, this way for each digit you need 8 pins and isn’t elegant at all.

The other solution is to connect each corresponding segment from the digits to a common bus, and power the digits one at a time, thus multiplexing the data.

seven segment schematic

This multiplexing probably sounds more complicated than it really is, look at the next picture:

illustrated seven segment operation

Since the digits share the same data bus, each digit will have the same number displayed, like the wheel on the picture, to change the number the “data guy” rotates the wheel. So how can we display 1234 you might ask, well wee need another guy, the selector, which will leave only one digit to be seen, all the others are shut off, by synchronizing the “data guy” and the “selector guy ” so they operate at the same time, when the wheel is at the 1111 position, the selector opens the first window, when at 2222 it opens the second and so on. By changing the data and selecting the digits at many times per second the human eye will see a steady image with 1234, the display refresh rate should be above 50 times in 1 second, otherwise the image may flicker.

In the schematic the “selector guy” is implemented by the PNP(BC327) transistors and the “data guy” is the data bus (PORTB0..7). For the practical demonstration I used my own avr development board with ATmega88, four HD1131R type seven segment digits which I rescued from an old TV a few years ago, the connections are made on a prototyping pcb with scrap wires. The data port is PORTB, for selection the PD4..PD7 pins where used.

Like I mentioned earlier we need a periodic data update with digit selection to have a steady image, and since the entire display refresh should be above 50Hz and we have four digits we need to change the digits at frequencies greater than 200Hz. Since the amount of time which one digit is turned on is 1/digit count, as the display has more digits the light intensity gets weaker, in our case the HD1131R has 1.6V voltage drop, by powering from 5V and trough 330Ohm results 10mA trough each segment, but since we have 4 digits the average current received by each segment is 10/4=2.5mA, this will result in a very fade light. For the demo application I used 220Ohm values, with the on board 100Ohm resistors the average current is 2.5mA/segment, although I could have used only the on board resistors, thus having 8.5mA average current, but the peak would be 35mA, and the micro-controller can handle at maximum 25mA/pin, off course the pin won’t get blowed right away from 35-50mA, but you should be careful when designing similar circuits.

That’s enough about the hardware, let’s see the software, for the periodic refresh I used a timer interrupt with 244Hz, resulting in 61Hz refresh rate, in Europe we have 50Hz AC power, so that’s fine, but in the case of 60Hz AC power the refresh rate should be at 70Hz to avoid the stroboscopic effect.

For the display data I used 4 byte buffer from which the interrupt reads and sends to PORTB, the buffer index can be used also for digit selection(see void DisplayData(uint8_t* Display) ).

Because of the seven segment data format by directly writing 5 to PORTB, won’t result in displaying 5, so the numeric data must be converted first, because there isn’t any easy algorithm to mathematically make this conversion, we must use another method, for a one digit display you could make if-else or switch statements, but with 2 digits you already need 100 lines of code, for 3 digits 1000 lines and so on. A very handy solution is a look-up table:

const uint8_t DigitCharTable[] = { DigImage0, DigImage1, DigImage2, DigImage3, DigImage4,\
DigImage5, DigImage6, DigImage7, DigImage8, DigImage9, };

By indexing the table with the numeric value 0-9 we get the needed data format, example PORTB = DigitCharTable[5]; results in displaying 5;

The void NumToDispValue(int16_t Number, uint8_t* DataBuffer) function uses this method to convert 16bit signed value into display compatible string.

After the conversion made, the actual display buffer needs to be updated with the new value, here is a tricky part, by converting 1234 into display compatible data we will have [DigImage1, DigImage2, DigImage3, DigImage4], by having DigImage1 at the first position and my display has  D4,D3,D2,D1 physical configuration the data displayed will look like this: 4321, it gets reversed. This can be overcome by reversing the selection lines PD4..PD7, or by copying the reversed string into the display buffer(see void StrUpdateDisplay(uint8_t* DisplayBuffer, uint8_t* UpdateData, uint8_t InvDir)  ).

After the conversion, the result is a string(not ascii!), so the string manipulation techniques can be easily used, the dot can be added to any digit, simply by setting the 7-th bit in that byte(see macro WITHPOINT(a) in SSDisplay.h ). There is also possible to display a limited amount of characters like E,F,c,C.., (see SSDisplay.h) it won’t be pretty but it’s readable.

Last but not least, since the refresh is done in interrupt, the display buffer update operation should be made atomic, in other words first disable the refresh interrupt, and then update the buffer, otherwise the image will flicker, and don’t forget to re-enable the interrupt after!

If you use only the refresh interrupt, than it’s easier to just disable all interrupts globally, although if you have other interrupt sources too, then disable only the refresh interrupt, this way you won’t hold back the other incoming events.

For data source I used the ADC to measure the potentiometer output, you can also tweak the macro’s used to handle the ADC, the dot in the 2th digit is activated if the ADC value  is above 512.

The demo project was made using avr-gcc and avrstudio, some variable type notations may differ from the ANSI C standard since I use the avr-gcc typedefs, the entire project among with schematics, pdf’s is available for download.

In the next article I will explain the usage of the matrix keyboard.

Seven segment display explained: [download]