Display Interfacing

Display Interfacing

AU : Dec.-04,05, May-06,18

Most of the microprocessor-controlled instruments and machines need to display letters of the alphabet and numbers to give directions or data values to users. This information can be displayed using CRT, LED or LCD displays. CRT displays are used when a large amount of data is to be displayed. In systems where only a small amount of data is to be displayed, simple LED and LCD displays are used.

 

1. Interfacing Static LED Display

Fig. 12.2.1 shows a circuit to drive a single, seven segment, common anode LED display. For common anode, when anode is connected to positive supply, a low voltage is applied to a cathode to turn it on. Here, BCD to seven segment decoder, IC 7447 is used to apply low voltages at cathodes according to BCD input applied to 7447. To limit the current through LED segments resistors are connected in series with the segments. This circuit connection is referred to as a static display because current is being passed through the display at all times.

The value of the resistor in series with the segment can be calculated as follows :

We know, V_CC - drop across LED segment - IR = 0

Drop across LED segment is nearly 1.5 V.

ஃ IR = V_CC - 1.5 V = 5 - 1.5 V

= 3.5 V

Each LED segment requires a current of between 5 and 30 mA to light. Let's assume that current through LED segment is 15 mA.

ஃ R = 3.5 V / 15 mA = 233 Ω

In practice, the voltage drop across the LED and the output of 7447 are not exactly predictable and the exact current through the LED is not critical as long as we don't exceed its maximum current rating. Therefore, a standard value 220 Ω can be used.

The static display circuits work well for driving just one or two LED digits. However, these circuits are not suitable for driving more LED digits, say 8 digits. When there are more number of digits, the first problem is power consumption. For worst-case calculations, assume that all eight digits with all segments are lit. Therefore, worst case current required is

1 = 8 (digits) × 7 (segment) × 15 mA (current per segment)

= 840 mA

A second problem of the static approach is that each display digit requires a separate BCD to 7 segment decoder.

 

2. Interfacing Multiplexed Display

To solve the problems of the static display approach, multiplexed display method is used. Fig. 12.2.2 shows the 4 seven segment displays connected using multiplexed method. Here, common anode seven segment LEDs are used.

Anodes are connected to +5 V through transistors. Cathodes of all seven segments are connected in parallel and then to the output of 7447 IC through resistors. Looking at the Fig. 12.2.2, the question may occur in our mind that,"Aren't all of the digits going to display the same number?" The answer is that they would show the same number only if all the digits are turned on at the same time. However, in multiplexed display the segment information is sent for all digits on the common lines (output lines of 7447), but only one display digit is turned on at a time. The PNP transistors connected in series with the common anode of each digit act as an ON and OFF switch for that digit. Here's how the multiplexing process works.

The BCD code for digit 1 is first output from port A, to the 7447. The 7447, BCD to seven segment decoder outputs the corresponding seven segment code on the segment bus lines. The transistor Q 1 connected to digit 1 is then turned on by outputting a low to that bit of port B. All of the rest of the bits of port B are made high to ensure no other digits are turned on. After 2 ms, digit 1 is turned OFF outputting all highs to port B. The BCD code for digit 2 is then output to the port A, and bit pattern to turn on digit 2 is output on port B. After 2 ms, digit 2 is turned off and the process is repeated for digit 3 and digit 4. After completion of turn for each digit, all the digits are lit again in turn.

With 4 digits and 2 ms per digit we get back to digit 1 every 8 ms or about 125 times a second. This refresh rate is fast enough that, to our eye and due to persistence of vision all digits will appear to be lit all the time.

In multiplexed display, the segment current is kept in between 40 mA to 60 mA so that they will appear as bright as they would if not multiplexed. Even with this increased segment current, multiplexing gives a large saving in power and hardware components.

 

Lab Experiment 12.2.1 : Hardware and software for interfacing 8-digit 7-segment display.

AU : May-18

Statement : Interface an 8-digit 7-segment LED display using 8255 to the 8085 microprocessor system and write an 8085 assembly language routine to display message on the display.

Hardware : Fig. 12.2.3 shows the multiplexed eight 7-segment display connected in the 8085 system using 8255. In this circuit port A and port B are used as simple latched output ports. Port A provides the segment data inputs to the display and port B provides a means of selecting a display position at a time for multiplexing the displays. A₀-A₇ lines are used to decode the addresses for 8255. For this circuit different addresses are :

PA = 00H   PC = 02H

PB = 01H   CR = 03H.

The register values are chosen in Fig. 12.2.3 such that the segment current is 80 mA. This current is required to produce an average of 10 mA per segment as the displays are multiplexed. In this type of display system, only one of the eight display position is 'ON' at any given instant. Only one digit is selected at a time by giving low signal on the corresponding control line. Maximum anode current is (7 - segments × 80 mA = 560 mA) but the average anode current is 70 mA.

Software : Before going to write the software we must know the control word to program 8255 according to hardware connections. For 8255 Port A and B are used as output ports.

; Software to initialize 8255

MVI A, 80H ; Load control word in AL

OUT CR ; Load control word in CR

; Subroutine to display message on multiplexed LED display

; set up registers for display

MVI B, 08H ; load count

MVI C, 7FH ; load select pattern

LXI H, 6000H ; starting address of message

; display message

DISP_1 : MOV A, C ; select digit

OUT PB ;

MOV A, M ; get data

OUT PA ; display data

CALL DELAY ; wait for some time

MOV A, C

RRC

MOV C, A ; adjust selection pattern

INX H

DCR B ; Decrement count

JNZ DISP_1         ; repeat 8 times

RET

Note This subroutine must be called continuously to display the 7-segment coded message stored in the memory from address 6000H.

Disadvantage of software approach

In the last example, we have seen the software approach to drive multiplexed displays. In software approach CPU has to look after digit selection in synchronism with the data for specific digit. In other words, CPU has to give digit data on one port and then the digit selection bit pattern on the another port in synchronism. The process of refreshing has to be repeated all the time, which puts an additional burden on the CPU. This is a major disadvantage of the software multiplexing approach. Another disadvantage of software multiplexing approach is that, if CPU gets involved in going some lengthy task which cannot be interrupted to refresh the display, only one digit of the display will be left lit.

An alternative approach to interface multiplexed displays to microprocessor systems is to use a dedicated keyboard and display controller 8279, designed by Intel. IC 8279 independently keeps display refresh and scans the matrix keyboard. In the next section, we will see the details of 8279 and its interfacing with 8085 microprocessor.

Review Questions

1. Explain the seven segment LED interface with microprocessor.

AU : Dec.-04, May-06, Marks 16

2. Design an interface circuit needed to connect DIP switch as an input device and display the value of the key pressed using a 7 segment LED display. Using 8085 system, write a program to implement the same.

AU : Dec.-05, Marks 16