1. Embedded Systems Design 1 Lecture Set B Interfacing the MCS-51 to: –7 and 16 segment displays –a Multiplexed 7-Segment Display –a Keypad –an LCD

2. Embedded Systems Design 2 The 7-Segment Display The 7-segment display is nothing but 7 LEDs arranged to form the figure 8. –By turning on and off the appropriate segments (LEDs), different combinations can be produced. –The 7-segment display is useful for displaying the digits 0 through 9, and some characters. However, it would be difficult to distinguish between things like a D and a 0. a b c f e g d

3. Embedded Systems Design 3 The 7-segment Display (Contd.) 7-segment displays come in 2 configurations: Common AnodeCommon Cathode As we have seen when we discussed interfacing the basic LED, it would be preferable to connect the cathode of each diode to the output pin. –Therefore, the common anode variety would be better for our interfacing needs.

4. Embedded Systems Design 4 Interfacing a 7-segment display to the 8051 Also, as seen with interfacing the LED, a resistor will be needed to control the current flowing through the diode. –This leaves two possibilities: Case 2 would be more appropriate as case 1 will produce different brightness depending on the number of LEDs turned on.

5. Embedded Systems Design 5 The 16-segment Display Similar to the 7-segment display except that it contains 16 LEDs. –With the 16 segments, it is possible to represent all digits and letters in the alphabet (English). –With 16 LEDs, the 16-segment display requires 16 pins and 16 bits to determine the exact character. All 16 values must be present at the same time. –Therefore, interfacing to a 16-segment display will require: »Either external hardware such as a latch. »Using two of the 8051 ports for connecting to a single display.

6. Embedded Systems Design 6 Multiplexed Displays 7-segment displays require a lot of power (~10 mA per LED) and a large number of pins (8 per LED). –Suppose that a battery-powered system has 10 displays. If the system is displaying “8888888888”. –All LEDs are on in all displays. –The total current required just to activate the displays is ~700 mA. This would reduce the useful battery lifetime to a few hours at best. –The solution is not to activate all displays at once.

7. Embedded Systems Design 7 Multiplexed Displays The figure below show a 4 character multiplexed display. –The LED inputs are common to all 4 but each character has a separate “enable” input. When a particular display is to be activated, its enable input is turned on, the display now uses the values on the LED pins to determine which LEDs to activate. –Total number of pins: 8 + # of characters. a b c d e f g

8. Embedded Systems Design 8 Interfacing to a Multiplexed Display To use a multiplexed display, each character will be flashed for a short period of time, the first character is activated for a brief amount of time, then it is turned off. Then the next character and so on through the four characters. Back to the first character. –Only one character is activated at any point in time. Worst case current requirement: ~70 mA. –If the multiplexing is done fast enough, the human eye will filter out the flickering and it would look like all the displays are one constantly. Most humans cannot distinguish flashes at more than 20 per second. They would consider that to be steady light. The scanning frequency should be at least 30 Hz. –Each character is on for 1/30 of a second.

9. Embedded Systems Design 9 The Program MOVB, #11H; Display sequence LOOP:MOVR2, #04; Number of characters MOVR0, #20H; Location of message NEXT:MOVP0, #00H; Turn off the displays MOVA, @R0; Get the next character LCALLCONVRT; Binary to 7-segment MOVP1, A; Display the first character INCR0; Next character in the message MOVP0, B; Turn on the appropriate display MOVA, B; Go on to next display RLA MOVB, A LCALLDELAY; On-time (1/30 seconds) DJNZR2, NEXT; Next character JMPLOOP; Start next scan

10. Embedded Systems Design 10 Interfacing a Keypad A 16-key keypad is built as shown in the figure below. –16 keys arranged as a 4X4 matrix. –Must “activate” each row by placing a 0 on its R output. Then the column output is read. If there is a 0 on one of the column bits, then the button at the column/row intersection has been pressed. Otherwise, try next row. –Repeat constantly.

11. Embedded Systems Design 11 Interfacing a Keypad Algorithm: –Drive a “0” on a row –Read all the columns –If any key had been pressed, its column will be “0”, else 1 –Keep repeating in a loop for each successive row Example: –Switch 4 is pressed R1  0, C1:C4 = 1111 R2  0, C1:C4 = 0111 –Switch 2 is pressed R1  0, C1:C4 = 1101

12. Embedded Systems Design 12 Bouncing Contacts Push-button switches, toggle switches, and electromechanical relays all have one thing in common: contacts. These metal contacts make and break the circuit and carry the current in switches and relays. Because they are metal, contacts have mass. Since at least one of the contacts is on a movable strip of metal, it has springiness. Since contacts are designed to open and close quickly, there is little resistance (damping) to their movement.

13. Embedded Systems Design 13 Bouncing Because the moving contacts have mass and springiness with low damping they will be "bouncy" as they make and break. That is, when a normally open (N.O.) pair of contacts is closed, the contacts will come together and bounce off each other several times before finally coming to rest in a closed position. The effect is called "contact bounce" or, in a switch, "switch bounce”.

14. Embedded Systems Design 14 Why is it a problem? If such a switch is used as a source to an edge- triggered input such as T0 or INT0, then the MCS-51 will think that there were several “events” and respond several times. The bouncing of the switch may last for several milliseconds. –Given that the MCS-51 operates at microsecond speed, a short ISR may execute several times in response to the above described bounciness.

15. Embedded Systems Design 15 Hardware Solution The simplest hardware solution uses an RC time constant to suppress the bounce. The time constant has to be larger than the switch bounce and is typically 0.1 seconds. The buffer after the switch produces a sharp high-to-low transition. –If the button is pushed again before the expiration of the time constant, the second press will not be recognized.

16. Embedded Systems Design 16 Software Solution It is also possible to counter the bouncing problem using software. This would require one of two approaches: –Delays: When the first edge is noticed, an “appropriate” delay is executed, then the value of the line is checked again to make sure the line has stopped bouncing. Actually, it is also necessary to execute a similar delay on the button’s release to counter the release bouncing. –Interrupts and Timers: When the first edge is noticed, a timer is started. When following edges appear, the value of the counter is checked. If the edges appear “too quickly”, they will be rejected.

17. Embedded Systems Design 17 LCD Interfacing LCDs have become a cheap and easy way to get text display for an embedded system –Various configurations (1 line by 20 characters upto 8 lines by 80 characters), starting from around $5. –Graphics LCDs are also available Intelligent LCDs have internal ASCII decoders, Character Generators and LCD control circuitry Some also have custom character generation capacity –User defined character RAM –Program this RAM with the character pattern –Then use it like ordinary ASCII characters Usually MSB decides between std ASCII and custom characters

18. Embedded Systems Design 18 Alphanumeric LCD Interfacing Pinout (statistical average) –8 data pins D7:D0 –RS: Data or Command Register Select –R/W: Read or Write –E: Enable (Latch data) –V EE : contrast control RS – Register Select –RS = 0  Command Register –RS = 1  Data Register R/W = 0  Write, R/W = 1  Read E – Enable –Used to latch the data present on the data pins. –A high-to-low edge is needed to latch the data. D0 – D7 –Bi-directional data/command pins. –Alphanumeric characters are sent in ASCII format. E R/W RS DB7–DB0 LCD controller communications bus Microcontroller 8 LCD Module

19. Embedded Systems Design 19 LCD Commands The LCD’s internal controller can accept several commands and modify the display accordingly. These commands would be things like: –Clear screen –Return home –Decrement/Increment cursor –Shift display right/left –Check the data sheet from the manufacturer. After writing to the LCD, it takes some time for it to complete its internal operations. During this time, it will not accept any new commands or data. –We can check if the LCD is busy or not by reading the busy flag. Set R/W = 1 and RS = 0, then read from the LCD –If D7 = 1, then the LCD is still busy, wait.

20. Embedded Systems Design 20 Example LCD Interface Program ORG 100H MOVA, #38H; Initialize, 2-lines, 5X7 matrix. ACALLCommand MOVA, #0EH; LCD on, cursor on ACALLCommand MOVA, #01H; Clear LCD Screen ACALLCommand MOVA, #06H; Shift cursor right ACALLCommand MOVA, #86H; Cursor, line 1 position 6 ACALLCommand MOVA, #’N’; Character N ACALLData MOVA, #’O’; Character O ACALLData Wait:SJMP Wait Command:ACALLReady MOVP1, A CLRP2.0; RS=0 CLRP2.1; R/W=0 (Write) SETBP2.2; E = 1 CLRP2.2; E = 0 – complete the 1-to-0 pulse RET Data:ACALLReady MOVP1, A SETBP2.0; RS = 1 CLRP2.1; R/W = 0 SETBP2.2; Enable pulse CLRP2.2 RET Ready:SETBP1.7; bit 7 of port 1 set to input CLRP2.0; RS = 0 – Command Reg. SETBP2.1; RW = 1 – Read Back:SETBP2.2; Enable pulse CLRP2.2 JBP1.7, Back; Wait till not busy RET

21. Embedded Systems Design 21 The Cursor Location Whenever a new data character is sent to the LCD, the cursor position will be automatically updated one position to the right. It is also possible to place the cursor at a particular location using the “set address” command. –Set RS = 0 and R/W = 0 –Set D7 = 1 – (set address command) –Set D6 = 0 for line 1 and D6 = 1 for line 2 –Set D5 – D0 for the position on the line (in binary). 1 st position (extreme left) is 0.

22. Embedded Systems Design 22 Interfacing an 8031 to an LCD Source: Intel Application Brief AB-39