Embedded Systems and Software

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1 Embedded Systems and Software Notes on Lab 2 Embedded Systems in Vehicles Lecture 2-4, Slide 1

Lab 02 In this lab students implement an interval timer

two seven-segment displays When the user presses the

ATtiny45 Pushbutton Switch LED Driver If there is an

2 Lab 02 In this lab students implement an interval timer using a pushbutton switch, ATtiny45, an LED driver, and two seven-segment displays When the user presses the button, the microcontroller start counting from 0 to 9.9 in 100 ms intervals. When the user presses the button again, the counter stops. ATtiny45 Pushbutton Switch LED Driver If there is an overflow (user wait longer than 9.9 s), the display should show 9.9 and flash once per second. Lecture 2-4, Slide 2

3 Seven Segment Displays Each of the seven segments and the decimal point is a LED By turning a subset of the LEDs on, one can display the numbers 0 9 and some letters as well Segments Decimal Point These displays are sold in single- or multiple character units. They come in all shapes and sizes Other variations include multiple decimal points, colons, + and signs etc. Lecture 2-4, Slide 3

4 Common Anode, Common Cathode Each segments and the decimal point is an LED Most of these displays connect either all the cathodes or all the anodes together. They are called Common Anode or Common Cathode displays Lecture 2-4, Slide 4

5 Driving a Seven Segment Display Common Anode Pulling a line low turns the corresponding segment on Lecture 2-4, Slide 5

6 Driving a Seven Segment Display Common Anode Lecture 2-4, Slide 6

7 Driving a Seven Segment Display Common Anode Lecture 2-4, Slide 7

8 Driving a Seven Segment Display Common Anode Turn on multiple LEDs to create the desired number Lecture 2-4, Slide 8

This will quickly deplete the number I/O pins on a microcontroller

9 Issues #1 Too manly Lines Common Anode To drive the display directly from a microcontroller means we need 8 lines per seven segment module This will quickly deplete the number I/O pins on a microcontroller Turn on multiple LEDs to create the desired number Lecture 2-4, Slide 9

10 Issues #2 LED Temperature and Aging Effects Common Anode An LED s forward bias voltage is a function of ambient temperature With resistors, this means the current and therefore the brightness depends on temperature Lecture 2-4, Slide 10

LED Driver ICs To address the issues with the simplistic driving of

ensure LED current does not change with temperature SDI CLK OE Control

11 LED Driver ICs To address the issues with the simplistic driving of displays and LEDs in general, several companies market special LED drivers ICs. A bit pattern is shifted in on the DATA line Current sources/sinks ensure LED current does not change with temperature SDI CLK OE Control lines latch the data and turn on the current sources The data is also shifted out so that one can put several IC in series Lecture 2-4, Slide 11

12 Adding More Displays SDI SDI CLK OE1 CLK OE2 To add another display, we need just one more line In operation, shift the desired 16-bit pattern in, and turn on the drivers. We control individual modules with their Output Enable (OE) lines Lecture 2-4, Slide 12

13 Adding More Displays - Alternative Method SDI SDI CLK OE1 CLK OE2 All displays use the same control lines In operation, shift the desired 16-bit pattern in, and turn on the drivers. We control individual modules with by shifting in the proper bit pattern Lecture 2-4, Slide 13

14 The TLC5916IN LED Driver Overview To use the IC successfully, you have to read the data sheet. In the next few slides we give an overview. Lecture 2-4, Slide 14

The TLC5916IN LED Driver Overview VV CCCC (5 V) II OOOOOO = 15 1.

15 The TLC5916IN LED Driver Overview VV CCCC (5 V) II OOOOOO = RR EEEEEE Serial Data In Clock Latch Enable VV CCCC TLC5916IN Serial Data Out ~OE This notation is the same as OE Lecture 2-4, Slide 15

Notation A bar above a pin means that the pin is active low For an output pin, then means the pin will be pulled low when it is activated For an input pin, it means it must be pulled

16 Notation A bar above a pin means that the pin is active low For an output pin, then means the pin will be pulled low when it is activated For an input pin, it means it must be pulled to perform its function VV CCCC Output drivers are turned on when OE is pulled low Sometimes this is written as ~OE LEDs turn on when output pins are pulled low Lecture 2-4, Slide 16

17 Cascading TLC5916IN LED Design RR for 10 ma For this lab, cascade two drivers is this fashion. This means you will need 4 output lines and 1 input line. Lecture 2-4, Slide 17

18 The LDS-A514RI Display Pin 10 Pin 6 Pin 10 Pin1 Pin 5 Pin1 Pin 5 Lecture 2-4, Slide 18

19 The LDS-A514RI Display Mapping from pin numbers to segments Lecture 2-4, Slide 19

20 The LDS-A514RI Display Be sure not to exceed maximum ratings. For the lab, design the LED driver so that each segment gets 10 ma Lecture 2-4, Slide 20

21 Assigning ATtiny45 Pins We need 4 control lines and one input line. However, here is the status of the development board: Reset Line VV CCCC (5 V) These are used by the ISP programmer when it downloads code Question - what is ISP? Seems like we don t have enough pins. This is a typical constraint in embedded systems The game plan is to disable disconnect the crystal and that frees up two pins. If we are careful, we can also use the ISP lines. Lecture 2-4, Slide 21

Thus, we must first configure the Attiny45 to use the internal oscillator and then remove these

22 Use ATtiny45 s Internal RC Oscillator These jumpers connect the crystal to the microcontroller We want to use the internal RC oscillator so that can use PB3 and PB4 for controlling the LED drivers Thus, we must first configure the Attiny45 to use the internal oscillator and then remove these jumpers. If you remove the jumpers first then ATtiny45 has no clock and will not run Lecture 2-4, Slide 22

23 Microcontroller Configuration/Fuses Set this to the internal RC oscillator Lecture 2-4, Slide 23

7K resistors to avoid your hardware conflicts with programming.

24 Using the ISP Lines Reset Line VV CCCC (5 V) These are used by the ISP programmer when it downloads code Reset Line VV CCCC (5 V) Insert 4.7K resistors to avoid your hardware conflicts with programming. Note that this may not be needed if the pins are uses as output only. Lecture 2-4, Slide 24

switch down => continue rjmp chk ; Yes, PB3 is high, wait some more.

25 Testing Status of an I/O PIN chk: down:... cbi DDRB,3 ; Make PB3 input sbis PINB,3 ; See if PB3 is set (high) rjmp down ; No, PB3 is low => switch down => continue rjmp chk ; Yes, PB3 is high, wait some more... This instruction tests a single bit in an I/O register and skips the next instruction if the bit is set. Pullup resistor. Value not critical, 4.7K 100K VV CCCC (5 V) Lecture 2-4, Slide 25

.. Also, there is a companion instruction SBIC (Skip next instruction if bit in I/O

26 Testing Status of an I/O PIN VV CCCC (5 V)... cbi DDRB,3... sbis PINB,3... For input use PINB, not PORTB... cbi DDRB,3... sbic PINB,3... Also, there is a companion instruction SBIC (Skip next instruction if bit in I/O is Clear) You could use either SBIS or SBIC in a loop to test if the pushbutton switch has been pressed Lecture 2-4, Slide 26

27 Switch Bounce and Debouncing Bounce ~ 5 ms Lecture 2-4, Slide 27

28 Switch Bounce and Debouncing When the switch closes, the capacitor discharges through Rb Voltage t Question: what is the fall time? Make sure you can calculate this. Lecture 2-4, Slide 28

29 Software Debounce One idea. Sample n times at regular intervals, say 10 ms apart. Count how many times the switch is zero. In this is larger than the number of times the switch is high, consider the switch pressed. char ispressed(void) { char ones=0, zeroes=0, i; } for(i=0;i<=10-1;i++){ if(pina&0x01){ // read pin == 1 ones++; } else { // read pin == 0 zeroes++; } _delay_ms(10); } return (ones > zeroes); We will use software debounce later in the course. For now, use hardware debounce Lecture 2-4, Slide 29

30 Which PIN to Use? There are 3 available PINs for use as switch input: PINB0, PINB1, PINB2. Which one should you use? These are used by the programmer when it downloads code, so adding a debounce capacitor may interfere. You may have to experiment which PIN works best as an input pin. Consider using PB3 for the pushbutton switch. Lecture 2-4, Slide 30

31 More on TLC5916 Operation Diagram extracted from TLC5916 data sheet LEDs turn on when output pins are pulled low Lecture 2-4, Slide 31

32 More on TLC5916 Operation Hold OE (drivers are off) Lecture 2-4, Slide 32

33 More on TLC5916 Operation Hold OE (drivers are off) Latch Enable (LE) is low Lecture 2-4, Slide 33

34 More on TLC5916 Operation Hold OE (drivers are off) Latch Enable (LE) is low Place bit on SDI line Make CLK go low high to clock data in Lecture 2-4, Slide 34

35 More on TLC5916 Operation Hold OE (drivers are off) Latch Enable (LE) is low Place bit on SDI line Make CLK go low high to clock data in Once the number of bits are clocked in, pulse LE Latch data Lecture 2-4, Slide 35

36 More on TLC5916 Operation ~OE Hold OE (drivers are off) Latch Enable (LE) is low Place bit on SDI line Make CLK go low high to clock data in Once the number of bits are clocked in, pulse LE Finally, pull OE low. While OE is low output drivers are on and LEDs light up Lecture 2-4, Slide 36

37 More on TLC5916 Operation Hold OE (drivers are off) Latch Enable (LE) is low Place bit on SDI line Make CLK go low high to clock data in Once the number of bits are clocked in, pulse LE Finally, pull OE low. While OE is low output drivers are on and LEDs light up Lecture 2-4, Slide 37

38 More on TLC5916 Operation Diagram extracted from TLC5916 data sheet ~OE LEDs turn on when output pins are pulled low 1 Latch data Lecture 2-4, Slide 38

39 Timing diagram from data sheet With a 8-MHz clock, a clock cycle is 125 ns Lecture 2-4, Slide 39

40 tt WW CLK Clock Pulse Duration (tt ww CLK ) tt WW CLK < 1 8 MHz Lecture 2-4, Slide 40

41 tt WW CLK SDI Setup Time (tt ssss D ) tt SSSS D tt ssss D < 1 8 MHz Lecture 2-4, Slide 41

42 tt WW CLK SDI Hold Time (tt h D ) tt h D tt h D < 1 8 MHz Lecture 2-4, Slide 42

43 ~OE Pulse Width (tt WW OE ) tt WW OE tt h D tt ww OOOO > 1 8 MHz Lecture 2-4, Slide 43

44 Note that, in general, timing values depend on power supply values, and often on temperature as well Lecture 2-4, Slide 44

45 Lecture 2-4, Slide 45

Embedded Systems and Software. Rotary Pulse Generators

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