EEE3410 Microcontroller Applications Department of Electrical Engineering. Lecture 10. Analogue Interfacing. Vocational Training Council, Hong Kong.

Transcription

1 Department of Electrical Engineering Lecture 10 Analogue Interfacing 1

2 In this Lecture. Interface 8051 with the following Input/Output Devices Transducer/Sensors Analogue-to-Digital Conversion (ADC) Digital-to-Analogue Conversion (DAC) 2

3 Transducer/sensors Digital computer/microcontrollers use binary values, but in the physical world most things is in analog nature (continuous). Data (such as temperature, pressure, humidity, velocity, voltage) are analog data. A device called transducer is used to convert the physical quantity to electrical signals (i.e. voltage, current). Transducer are also referred to as sensors. Sensors for temperature, velocity, pressure, light, and many other natural quantities can produce an output voltage (or current) which the value is proportional to the quantity being measured. Then an analog-to-digital converter is used to translate the analog voltage to digital numbers so that microcontroller can read and process them. 3

4 Temperature sensors (Thermistor( Thermistor) Temperature can be converted to electrical signals by thermistor. Thermistor is a kind of resistor responds to temperature change by changing its resistance. But its response is not linear, as seen in the Table 10.1 below. Table 10.1 Temperature ( C) Thermistor Resistance (kω)

5 Temperature sensors (LM34 and LM35) EEE3410 Microcontroller Applications LM34 series are precision integrated-circuit temperature sensors whose output voltage is linearly proportional to Fahrenheit temperature. LM35 series are precision integrated-circuit temperature sensors whose output voltage is linearly proportional to Celsius temperature. They require no external calibration since it is internally calibrated. Their output voltage changes 10mV for each degree of temperature change. Table 10.2 Item LM34 LM34C LM35 LM35C LM35D Temperature Range -50 F to +300 F -40 F to +230 F -55 C to +150 C -40 C to +110 C 0 C to +100 C Accuracy +3.0 F +3.0 F +1.5 C +1.5 C +2.0 C Output 10 mv/f 10 mv/f 10 mv/ C 10 mv/ C 10 mv/ C 5

6 Analog-to to-digital converter (ADC ) Need to have some means to convert them into digital signal so that computers can handle Analog-to-digital converter (ADC) is a device which can convert analogue voltage to digital numbers so that microcontrollers can handle and process the data. ADC are the most widely used devices for data acquisition ADC has n-bit resolution, where n can be 8, 12, 16 or even 24 bits. The higher-resolution ADC provides a smaller step size. Step size is the smallest change that can be recognized by ADC. 6

7 Analog-to to-digital converter (ADC ) An ADC has a resolution of 8 bits, the range is divided into 2^8=256 steps (from 0 255). But there are 255 quantization levels. Step Size Vcc = 2 n 1 Vcc is the reference voltage of ADC Table 10.3 : Resolution versus Step Size for ADC (if Vcc = 5V) n-bit Number of steps 2^8 = 256 2^10 = ^12 = ^16 = Step Size (mv) 5/255 = /1023 = /4095 = /65535 =

8 ADC0804 Chip (Free Running Mode) +5V 20 10k POT 10k Vin(+) Vin( ) A GND Vref/2 CLK R CLK in 150 pf ADC Figure 10.1 CS RD D GND 0804 D0 D1 D2 D3 D4 D5 D6 D7 WR INTR To LEDs Normally open START 8

9 ADC0804 Chip CS Active low input used to activate the ADC0804 chip. RD (data enable) Active low input used to get converted data out of the ADC0804 chip. When CS = 0, if a high-to-low pulse is applied to the RD pin, the 8-bit digital output shows up at the D0-D7 data pins. WR (start conversion) Active low input used to inform the ADC0804 to start the conversion process. If CS = 0 when WR makes a low-to-high transition, the ADC0804 starts converting the analog input value of Vin to an 8-bit digital number. When the data conversion is complete, the INTR pin is forced low by the ADC

10 ADC0804 Chip CLK IN and CLK R Connect to external capacitor and resistor for self-clocking, f = 1/(1.1RC). The clock affect the conversion time and this time cannot be faster than 110µs. INTR (end of conversion) This is an active low output pin. When the conversion is finished, it goes low to signal the CPU that the converted data is ready to be picked up. After INTR goes low, we make CS = 0 and send a high-to-low pulse to the RD pin to get the data out of the ADC0804 chip. Vin (+) and Vin ( ) These are the differential analog inputs where Vin = Vin (+) Vin ( ). Often the Vin ( ) pin is connected to ground and the Vin (+) pin is used as the analog input to be converted to digital. 10

11 ADC0804 Chip V CC This is the +5V power supply. It is also used as a reference voltage when the Vref/2 (pin 9) input is open. Vref/2 Input voltage pin used for the reference voltage. If this pin is open, the analog input voltage for the the ADC is ranged from 0 to 5 volts. Table 10.4 Vref/2 (V) Vin (V) Step Size (mv) Not connected 0 to 5 5/255 = to 4 4/255 = to 3 3/255 = to /255 = to 2 2/255 = to 1 1/255 =

12 ADC0804 Chip D0 D7 D0 D7 are the digital data output pins. These are the tri-state buffered and the converted data is accessed only when CS = 0 and RD is forced low. The output voltage: D out = Vin Step size Analog Ground and Digital Ground Analog ground is connected to the ground of the analog signal while digital ground is connected to the ground of the Vcc pin. 12

13 Signal control on ADC Make CS = 0 and send a L-to-H pulse to pin WR to start the conversion. 2. Keep monitoring the INTR pin. If INTR is low, the conversion is finished and we can go to the next step. If INTR is high, keep polling until it goes low. 3. After the INTR has become low, we make CS = 0 and send a H-to-L pulse to the RD pin to get the data out of the ADC0804 IC chip. 13

14 The timing diagram of ADC0804 CS WR D0 D7 Data out INTR RD Start conversion End conversion Figure 10.2 Read it Note: CS is set to low for both RD and WR pulses 14

15 Testing the ADC ADC V P2.5 P2.6 RD WR V CC CLK R CLK IN P1.0 P1.7 D0 : : : : : D7 V in (+) V in ( ) A GND Vref/2 D GND 10k POT P2.7 INTR CS Figure

16 Example 10-1 EEE3410 Microcontroller Applications Write a program to monitor the INTR pin and bring the digital input value into register A. Then call a subroutine to display the input value on a 7-segment display. Do this continuously. ; P2.6 = WR (Start conversion needs a L-to-H pulse) ; P2.7 = INTR (When low, end-of-conversion) ; P2.5 = RD (H-to-L will read the data from ADC chip) ; P1.0 P1.7 = D0 D7 of the ADC0804 ; MOV P1, #0FFH ; make P1=input BACK: CLR P2.6 ; WR=0 SETB P2.6 ; WR=1 L-to-H to start conversion HERE: JB P2.7, HERE ; wait for end of conversion CLR P2.5 ; conversion finished, enable RD MOV A, P1 ; read the data ACALL DATA_DISPLAY ; display the data SETB P2.5 ; make RD=1 for next round SJMP BACK 16

17 Referred to Example 10.1, the digital values output from the ADC0804 correspond to the analogue voltage inputs. It is calculated by: Step Size Vcc = 2 n Step Size = = = V Analogue input voltage V in (V) Digital Value Output D out 0/ = 0 0.5/ = / = / = / =

18 Example of ADC Application Temperature detection A temperature sensor (LM34 or LM35) is interfaced to the 8051 via an ADC (ADC0804) The output voltage from the LM34/LM35 is linearly proportional to the measuring temperature The ADC0804 converts the output voltages from the LM34/LM35 into digital signals, which correspond to the measured temperature. They are then handled by the

19 Interfacing with the LM35 (Temperature Sensor) 8051 ADC V XTAL1 XTAL2 P2.5 P2.6 P1.0 RD WR D0 V CC CLK R CLK IN LM35 D Q Q D Q Q P1.7 P2.7 D7 INTR V in (+) V in ( ) A GND Vref/2 D GND CS Set to 1.28V Clock freq 4 Figure

20 Example 10.2 ADC Application EEE3410 Microcontroller Applications The ADC0804 converts the output voltages from the LM35 into digital signals, which correspond to the measured temperature. Step size of the ADC0804 = (1.28)x(2)/255 = V Clock input to the ADC0804 = clock frequency 4 If the following data of LM35 are given the temperature range of the temperature sensor LM35 is -55 C to 150 C and its output scale is 10mV/ C, and the output value of the ADC0804 is 00H when the LM35 senses -55 C then the value output from the ADC0804 for a measuring temperature 100 C is: (100 ( 55)) x.01 D out = = = 155 or 9BH

21 Digital-to to-analog (DAC) Conversion The digital-to-analog converter (DAC) is a device widely used to convert digital values to analog signals. It does the reverse operation of an ADC The resolution of DAC depends on the no. of binary bits input to it. The common ones are 8, 10 and 12 bits. An 8-input DAC provides 256 discrete voltages (or current) levels of output. (The 12-bit DAC gives 4096 discrete levels). Digital inputs D0 DAC Analog output (Voltage or current) D7 Figure

22 DAC0808 Chip (MC1408) +5V V ref (e.g +5V) P2.5 P2.6 V CC V in (+) R ref1 I ref = V ref /(R ref1 +R ref2 ) = 2mA (Typical) P1.0 D0 V in ( ) I ref R ref 2 R DAC0808 Comp. I out + To Scope V out = 0 to 10V P1.7 D7 V ee 15V GND Figure

23 DAC0808 Chip (MC1408) Operating Principle In the DAC0808, the digital inputs are converted to corresponding current (I out ). Then by connecting a resistor to the I ou t pin, the current will further convert to voltage level. The total current provided by the output I out pin is a function of the reference current (I ref ) and is follows: I out = I ref Converting I out to voltage (V out ) 2 D + 4 D + 8 D D D D Connect the output pin I out to a output standard resistor and hence convert this current to a voltage. + D 256 D Output voltage, V out = I out x R o 23

24 Example 10.3 Assume that Ro = 5kΩ,, R ref1 = 1k Ω, R ref2 = 1.5k Ω and using a standard 5V supply connected to V ref(+), calculate V out for the following binary inputs. (a) (99H) (b) (C8H) Solution: (a) 99H = I ref = = 2mA I out = ( 2mA) = mA 256 V out = ( 1.195mA)(5000kΩ) = V (b) C8H = I out = ( 2 ma ) = ma 256 V out = ( 1.562mA)(5000kΩ) = V 24

25 Example 10.4 In order to generate a star-step ramp, set up the circuit as in Figure 10.6 and connect the output to an oscilloscope. Then write a program to send data to the DAC at port 1 to generate a stair-step ramp. Solution: ORG 0H CLR A AGAIN: MOV P1, A ; send data to DAC INC A ; count from 0 to FFH ACALL DELAY ; let DAC work and recover SJMP AGAIN END 25

26 Example 10.5 Use the circuit in Figure 10.6, write a program to generate a sine wave of full-scale 10V output. If the output is connected to a CRO, draw the display on the CRO screen. Solution: Assume 10V full-scale voltage for the DAC output. V out = [5 + (5 sin θ)] V A table is drawn up to list the magnitude of the sine of angles between 0 and 360 degrees. The table values are integer numbers representing the voltage magnitude for the sine θ. Table 10.5 gives the angles, the sine values, the voltage magnitudes and the integer values representing the voltage magnitude for each angle (with 30 interval). 26

27 Table 10.5 Angle vs Voltage Magnitude for Sine Wave Angle θ (degree) Sine θ Vout 5V + (5sin θ ) Voltage Values sent to DAC

28 Program Generating a Sine Wave ORG 0H AGAIN: MOV DPTR, #TABLE MOV R2, #COUNT BACK: CLR A MOVC A,@A+DPTR MOV P1,A INC DPTR DJNZ R2, BACK SJMP AGAIN ; ORG 300 TABLE: DB 128, 192, 238, 255, 238, 192 DB 128, 64, 17, 0, 17, 64, 128 ; ; To get a better looking sine wave, regenerate the look-up table for 2-degree angles END 28

29 Display on the Generating Sine Wave on CRO Volts Degrees 29

30 Read reference The 8051 Microcontroller and Embedded Systems - Using Assembly and C, Mazidi Chapter 13 P.373 P

31 Review Questions 1. In the ADC0804, the INTR signal is an (input, output) 2. In the ADC0804, to begin conversion, send a(n) pulse to pin. 3. Which pin of the ADC0804 indicates end-of-conversion? 4. In a ADC, input is (digital, analog) and output is (digital, analog). 5. Find the step size for an ADC0804 chip if V ref /2 =1.9V. 6. With V ref /2 =0.64V, find the Vin for the D7 D0 = output. 7. In a DAC, input is (digital, analog) and output is (digital, analog). 8. DAC0808 is a9n) -bit D-to-A converter. 9. The output of DAC0808 is in (current, voltage). 10. Find the I out for the digital input to DAC0808. Assume I ref = 2mA. 11. To get a smaller step, we need a DAC with (more, fewer) digital inputs. 12. The LM35 provides mv for each degree of (Fahrenheit, Celsius). 31

32 Department of Electrical Engineering END of Lecture 10 Analogue Interfacing 32