Embedded Systems and Software

Transcription

1 Embedded Systems and Software Some Hardware Considerations Embedded Systems in Vehicles Hardware, Slide 1

2 Logic States Digital signals may be in one of three states State 1: High, or 1. Using positive logic polarity, this corresponds to a voltage level closer to the power supply than to ground. Example. For an ATmega88PA and Vcc = 5 V, the data sheet indicates that an input voltage larger than 0.6 Vcc = 3 V is considered high, or logic 1. State 2: Low, or 0. Using positive logic polarity, this corresponds to a voltage level closer to ground than to the power supply. For example, for an ATmega88PA and Vcc = 5 V, the data sheet indicates that an input voltage larger than 0.6 Vcc = 3 V is considered logic high, or 1. Example. For an ATmega88PA and Vcc = 5 V, the data sheet indicates that an input less than 0.3 Vcc = 1.5 V is treated as a logic low or 0 In negative logic polarity, voltages levels closes to the supply than to ground are 0, and voltage levels closer to ground than to the power supply are 1. This is used in some serial communication protocols. State 3: Tristate or high-z, or simply Z. In this case the digital line is in a high-impedance state or floating. External components, often a pull-up resistor can pull the bus high or low. Hardware, Slide 2

3 Driving an LED V cc = 5 V RR llllllllll AVR PB2 Use the LED s V-I characteristics from its datasheet to determine the forward voltage for a given current. Then apply Ohm s Law to determine RR llllllllll Hardware, Slide 3

4 Driving a High Power Load Solid State Relay (SSR) SIP SSR Ratings of 660 VAC SCR output for heavy industrial loads AC or DC control Zero-crossing (resistive loads) or random-fire (inductive loads) output Cost ~ $14 Load Control Hardware, Slide 4

5 Driving a High-Power External Load V cc = 5 V Depending on the SSR, one may need an external resistor here to limit current out of microcontroller port AVR PB2 R limit SSR Control ma 300 Ω Opto-isolated Solid State Relay 220 V, 5 A Load Hardware, Slide 5

6 Driving a High-Power External Load V cc = 5 V AVR PB2 R limit V on = ma Opto-isolated Solid State Relay 220 V, 5 A Load Example: The absolute max rating for I/O current is 40 ma, and the SSR has V on = ma. Determine R limit Hardware, Slide 6

7 Well-defined Reset Pin It is important that the RESET pin has a well-defined voltage in normal operation, otherwise it may randomly reset. Also, if a mechnical switch is used for reset, consider contact bounce As a minimum, pull the line high and add a capacitor to filter out any glitches that may get to the RESET line Hardware, Slide 7

8 Well-defined Reset Pin This supervisor chip makes sure the MCUs RESET line is well-behaved when the RST button is pushed and released Reset circuitry for 28-pin development board Hardware, Slide 8

9 Power Supplies Bridge Rectifier Voltage Regulator Hardware, Slide 9

10 Power Supplies Power supply section from 28-pin development board Bridge rectifier converts ac to dc. Also allows us to use dc. Hardware, Slide 10

11 Power Supplies Power supply section from 28-pin development board Smoothing Capacitor Hardware, Slide 11

12 Power Supplies Power supply section from 28-pin development board Three Terminal Regulator Hardware, Slide 12

13 Power Supplies Power supply section from 28-pin development board Sets Output Voltage Hardware, Slide 13

14 Power Supplies Power supply section from 28-pin development board Jump for 3.3V Hardware, Slide 14

15 Power Supplies Power supply section from 28-pin development board Needed for Stability Hardware, Slide 15

16 Power Supplies Reverse-Polarity Protection Bridge provides reversepolarity protection What voltage is supplied to embedded system if the input voltage is 5 V? Assume Si diodes. Diodes should be Schottky rather than regular Si diodes. Why? Hardware, Slide 16

17 Power Supplies Linear Three-Terminal Regulator Varying input voltage, say 7-14 V. Must be larger than output voltage + some headroom, typically 1-2 V Constant voltage: 3 V, 5 V, etc. Note that linear regulators cannot step up a voltage, but regulate down. Hardware, Slide 17

18 Power Supplies Linear Three-Terminal Regulator LM78XX Three-terminal regulator pinout in TO220 package Heat sink to cool down regulator Hardware, Slide 18

19 Power Supplies Linear Three-Terminal Regulator These capacitors are easily-overlooked but are crucial for proper operation. Read the data sheet for your regulator. Hardware, Slide 19

20 Power Supplies Linear Three-Terminal Regulator Quiescent current for LM78xx regulators are large, and can waste lots of energy not suitable for batteryoperated equipment. Hardware, Slide 20

21 Power Supplies MAX604 Linear Regulator Quiescent current for MAX604 a few micro-amps, thus much more suitable for battery-operated equipment. Hardware, Slide 21

22 Power Supplies MAX724 DC/DC switching Regulator Very wide input range, efficient conversion to output voltage Note that switching regulator can step up voltages Hardware, Slide 22

23 Decoupling Capacitors Consider a microntroller that runs on a 10 MHz clock. This means the clock period is 100 ns. The rise and fall time of the clock is thus on the order of 10 ns or shorter. The microcontroller s internal electronics (logic gates) create significant switching noise that appears on the power supply lines. The current is drawn in very short spikes on the clock edges, and if I/O lines are switching, the spikes will be even higher. This kind of current spike cannot be delivered over long power supply lines; the main source is (or should be) the decoupling capacitor. Decoupling capacitor ~ 0.1 μf The decoupling capacitor s (local) energy provides local storage to supply to switching circuits. Used properly, a decoupling capacitor can greatly reduces switching noise. Sometimes called a bypass capacitor Hardware, Slide 23

24 Power Supply Decoupling Wrong way to decouple power supply Decoupling capacitor is ~ 0.1 μf Note: high loop current flows through the ground and can contaminate other circuits Hardware, Slide 24

25 Power Supply Decoupling Better way to decouple power supply Inductor blocks residual current spikes from entering power supply. Typical value is 47 nh. Decoupling capacitor is very close to the controller, across its GND and Vcc pins. Large current spikes never flow through the circuit ground. Hardware, Slide 25

26 External Clock Sources For crystals with frequencies larger than 400 khz For crystals with frequencies less than 400 khz, typically kHz watch crystal. Not supported on all AVRs Hardware, Slide 26

27 External Clock Sources Crystals For simplicity, the oscillator capacitors are not always indicated, but they are crucial for proper operation, and in most cases the oscillator will not oscillate without these capacitors Capacitor values are ~ 20 pf, which are small. Hardware, Slide 27

28 External Clock Sources Ceramic Resonators Connect to ucontroller 10 MHz ceramic resonator. Cost ~ $0.25 Equivalent circuit Lower Q than crystals. This means less stable, but also faster start-up time Are available with built-in capacitors Hardware, Slide 28

29 External Clock Sources Question: What does ppm mean? Answer: Parts per million To convert from ppm to %, divide the number by 10,000. To convert from % to ppm, multiply by 10,000 Cost ~ $0.25 Cost ~ $0.75 Hardware, Slide 29

30 Board Layout Hardware, Slide 30

31 In normal operation, the 5 V linear regulator provides clean 5 V. D 1 drops V D(on). This is greater than (3 V + V D(on) ), so D 2 is reverse-biased and open. Without main power, D 2 is forward biased turn on, and powers the controller. Question: what type of diodes should D1, D2, be, and why? Answer: Schottky diodes, because they have lower turn-on voltages than Si diodes. Thus, these diodes dissipate less power. Hardware, Slide 31

32 Recall Earlier Slide Example. Turning LEDs on and off Connecting LEDs Note LEDs will light up when ports pins are pulled LOW Data Direction Register Configuring port pins for output Turn LED at PB1 OFF sbi DDRB,1 ; PB1 is now output sbi DDRB,2 ; PB2 is now output sbi PORTB,1 ; LED at PB1 off Turn LED at PB1 ON cbi PORTB,2 ; LED at PB2 on Hardware, Slide 32

33 DDRB, PORTB, PINB, I/O ports on ATmege88P: PORTB, PORTC, and PORTD Individual pins of the ports are PB1, PD4, etc. Complex circuitry sits behind each port pin. Pins can be digital input, digital output, and analog Input Three registers located in I/O space DDRB Data Direction Register for PORTB PORTB Digital Output Register for PORTB Simplified I/O equivalent schematic PINB Input Register for PORTB Hardware, Slide 33

34 DDRB, PORTB, PINB, All AVR ports have true Read-Modify-Write functionality when used as general digital I/O ports. This means that the direction of one port pin can be changed without unintentionally changing the direction of any other pin with the SBI and CBI instructions. The pin driver is strong enough to drive LED displays directly. All port pins have individually selectable pull-up resistors with a supply-voltage invariant resistance. The same applies when changing drive value (if configured as output) or enabling/disabling of pullup resistors (if configured as input). Each output buffer has symmetrical drive characteristics with both high sink and source capability. All I/O pins have protection diodes to both VCC and Ground Pull-up resistor ensures that is at expected logic levels if external devices are disconnected. Protection Diodes The idea is that is that it weakly "pulls" the pin to Vcc. The resistor is high-resistance enough that, if something else strongly pulls the wire toward 0V, the pin will go to 0V. Hardware, Slide 34

35 DDRB, PORTB, PINB, Configuring a PIN DDRB Data Direction Register for PORTB PORTB Digital Output Register for PORTB PINB Input Register for PORTB Hardware, Slide 35

36 Unconnected Pins If some pins are unused, it is recommended to ensure that these pins have a defined level. Even though most of the digital inputs are disabled in the deep sleep modes, floating inputs should be avoided to reduce current consumption in all other modes where the digital inputs are enabled (Reset, Active mode and Idle mode). The simplest method to ensure a defined level of an unused pin, is to enable the internal pullup. In this case, the pull-up will be disabled during reset. If low power consumption during reset is important, it is recommended to use an external pull-up or pull-down Connecting unused pins directly to VCC or GND is not recommended, since this may cause excessive currents if the pin is accidentally configured as an output. Hardware, Slide 36

37 Example. Consider the table below, extracted from the DC Characteristics section of the ATtiny45/V microcontroller. Assume the power supply voltage is 2 V, and that PB0 is configured as digital input. Determine the threshold voltages for logic 1 and 0 on input. Solution. PB0 is not XTAL or RESET, so V IL and V IH apply. V I0 = 0.2Vcc = = 0.4 V, and V I1 = 0.7Vcc = = 1.4 V Hardware, Slide 37

38 Pull-Ups Revisited The Pull-up Disable PUD bit in MCUCR disables the pull-up function for all pins in all ports when set. If PORTxn is written logic one when the pin is configured as an input pin, the pull-up resistor is activated. To switch the pull-up resistor off, PORTxn has to be written logic zero or the pin has to be configured as an output pin. Self-Study Give a code snippet to configure PB1 on the ATmega88PA as an input with the internal pull-up resistor activated. Comment your code. Hardware, Slide 38

39 Pull-Ups Revisited Pull-up resistors can have a significant spread between parts, and even individual pins on a port, so beware Hardware, Slide 39

40 Microcontroller Fuses AVR ATtiny45 8-Pin Microcontroller Except for GND and VCC all other pins can perform at least 4 functions PB5 can be hardware RESET or an ADC input PB3 & PB4 can be ADC inputs or where crystal for external oscillator goes How does one configure controller for the external environment? Hardware, Slide 40

41 Electrical Connections Test circuit, from the data sheet Pullup resistors Low-pass filter Low-pass filter Remember contact bounce? Hardware, Slide 41

42 Switch Bounce and Debouncing Bounce Hardware, Slide 42

43 Switch Bounce and Debouncing When the switch closes, the capacitor discharger through Rb Voltage Question: what is the fall time? Make sure you can calculate this. t Hardware, Slide 43

44 Wired-Or Application: Bus Internal Pull-up Resistor OFF Bus is normally high Internal Pull-up Resistor OFF Any microcontroller can pull the bus low Hardware, Slide 44

45 Reading a Simple (3 4) Keypad Note the weak pull-ups on PDB- PD7. These must be enabled in software Hardware, Slide 45

46 Reading a Simple (3 4) Keypad 0 Step 1: Detecting a key press a) Drive PC4, PC5, PC6 low. b) Read PORTD and look for zero on any of PD4-PD7 c) If so, a key has been pressed 0 0 Hardware, Slide 46

47 Reading a Simple (3 4) Keypad Step 2: Debounce. Wait an appropriate period for the key switch to stabilize (10 ms is probably a good waiting time) Hardware, Slide 47

48 Reading a Simple (3 4) Keypad 0 1 Step 3: Search for the pressed key a) Drive PC4 Low, PC5,PC6 High b) Read PORTD PD4 = 0 1 pressed PD5 = 0 4 pressed PD6 = 0 7 pressed PD7 = 0 * pressed c) If pressed key is not found in column 1, repeat the process as necessary for the other two columns 1 Hardware, Slide 48

49 Reading a Simple (3 4) Keypad 1 0 Step 4: Search for the pressed key a) Drive PC5 Low, PC4,PC6 High b) Read PORTD PD4 = 0 2 pressed PD5 = 0 5 pressed PD6 = 0 8 pressed PD7 = 0 0 pressed c) If pressed key is not found in column 2, repeat the process as necessary for the other two columns 1 Hardware, Slide 49

50 Reading a Simple (3 4) Keypad 1 Step 5: Search for the pressed key a) Drive PC6 Low, PC4,PC5 High b) Read PORTD PD4 = 0 3 pressed PD5 = 0 6 pressed PD6 = 0 9 pressed PD7 = 0 # pressed 1 0 Hardware, Slide 50

51 Hardware, Slide 51

52 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. Hardware, Slide 52

53 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. Hardware, Slide 53

54 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 Hardware, Slide 54

55 Driving a Seven Segment Display Common Anode Pulling a line low turns the corresponding segment on Hardware, Slide 55

56 Driving a Seven Segment Display Common Anode Hardware, Slide 56

57 Driving a Seven Segment Display Common Anode Hardware, Slide 57

58 Driving a Seven Segment Display Common Anode Turn on multiple LEDs to create the desired number Hardware, Slide 58

59 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 Hardware, Slide 59

60 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 Hardware, Slide 60

61 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 Hardware, Slide 61

62 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 Hardware, Slide 62

63 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 Hardware, Slide 63

64 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. Hardware, Slide 64

65 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 Hardware, Slide 65

66 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 Hardware, Slide 66

67 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. Hardware, Slide 67

68 The LDS-A514RI Display Pin 10 Pin 6 Pin 10 Pin1 Pin 5 Pin1 Pin 5 Hardware, Slide 68

69 The LDS-A514RI Display Mapping from pin numbers to segments Hardware, Slide 69

70 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 Hardware, Slide 70

71 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. Hardware, Slide 71

72 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 Hardware, Slide 72

73 Microcontroller Configuration/Fuses Set this to the internal RC oscillator Hardware, Slide 73

74 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) SDI CLK LE ~OE 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. Hardware, Slide 74

75 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) Hardware, Slide 75

76 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 Hardware, Slide 76

77 Switch Bounce and Debouncing Bounce ~ 5 ms Hardware, Slide 77

78 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. Hardware, Slide 78

79 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 Hardware, Slide 79

80 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. Hardware, Slide 80

81 More on TLC5916 Operation Diagram extracted from TLC5916 data sheet LEDs turn on when output pins are pulled low Hardware, Slide 81

82 More on TLC5916 Operation Hold OE (drivers are off) Hardware, Slide 82

83 More on TLC5916 Operation Hold OE (drivers are off) Latch Enable (LE) is low Hardware, Slide 83

84 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 Hardware, Slide 84

85 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 Hardware, Slide 85

86 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 Hardware, Slide 86

87 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 Hardware, Slide 87

88 More on TLC5916 Operation Diagram extracted from TLC5916 data sheet ~OE LEDs turn on when output pins are pulled low 1 Latch data Hardware, Slide 88

89 Timing diagram from data sheet With a 8-MHz clock, a clock cycle is 125 ns Hardware, Slide 89

90 tt WW CLK Clock Pulse Duration (tt ww CLK ) tt WW CLK < 1 8 MHz Hardware, Slide 90

91 tt WW CLK SDI Setup Time (tt ssss D ) tt SSSS D tt ssss D < 1 8 MHz Hardware, Slide 91

92 tt WW CLK SDI Hold Time (tt h D ) tt h D tt h D < 1 8 MHz Hardware, Slide 92

93 ~OE Pulse Width (tt WW OE ) tt WW OE tt h D tt ww OOOO > 1 8 MHz Hardware, Slide 93

94 Note that, in general, timing values depend on power supply values, and often on temperature as well Hardware, Slide 94

95 Hardware, Slide 95