Ch 8. Analog Interface

Transcription

1 EE414 Embedded Systems Ch 8. Analog Interface Part 1/2 Byung Kook Kim School of Electrical Engineering Korea Advanced Institute of Science and Technology

2 Overview 8.1 Introduction 8.2 A/D Conversion 8.3 ADC Interface 8.4 ADC Interface in AM Sensor Interface 8.6 D/A Conversion 8.7 PWM 8.8 PWM Interface in AM3359 Embedded Systems, KAIST 2

3 8.1 Introduction How to sample external voltages and convert them into digital values Sources Sensors Convert physical quantity into electrical quantity Represent light levels, temperature, vibration Analog signals Output of microphone or audio system Conversion Analog-to-digital converter How to turn digital data into an analog output voltage Destinations Speaker, motor Conversion Digital-to-analog converter Embedded Systems, KAIST 3

4 8.2 A/D Conversion Signal flow Sensor Signal conditioning amplifier ADC up Amplifier Increases a given input voltage Ex: Sensor output amplification from 5mVpp to 5Vpp Gain = V_out / V_in = 1000 Implementation Using vacuum tubes, transistors, or OP amps Inverting/Non-inverting/Differential amplifier Restrictions Frequency response Home stereo: 20 Hz 20 khz Sensor: flat (ideally) Distortion Total Harmonic Distortion (THD) in audio amp. Embedded Systems, KAIST 4

5 A/D Conversion (II) Analog-to-Digital converter (ADC) A device that converts an analog input voltage to a digital number Coder part of codec (Coder-DECoder) Used in maaaaaany areas Microphone in PC CD mastering DVD mastering Sensor processing Types Integrating ADC: Counter, DAC, Comparator. 2^(n-1) clocks average. Successive-approximation ADC: Binary search. n clocks. Flash ADC (parallel ADC): (2^n 1) comparators, encoding logic. Embedded Systems, KAIST 5

6 Analog-to-digital converters e/vmax = d/(2 n -1) V max = 7.5V 7.0V 6.5V 6.0V 5.5V 5.0V 4.5V 4.0V 3.5V 3.0V 2.5V 2.0V 1.5V 1.0V 0.5V 0V analog input (V) time t1 t2 t3 t Digital output analog output (V) t1 t2 t3 t4 time Digital input Proportionality Analog to digital Digital to analog Embedded Systems, KAIST 6

7 ADC using successive approximation Given an analog input signal whose voltage should range from 0 to 15 volts, and an 8-bit digital encoding, calculate the correct encoding for 5 volts. Then trace the successive-approximation approach to find the correct encoding. 5/15 = d/(28-1) d = 85 Encoding: Vin = 5V ½(V max V min ) = 7.5 volts V max = 7.5 volts. Successive-approximation method ½( ) = 5.16 volts V max = 5.16 volts ½( ) = 3.75 volts V min = 3.75 volts. ½( ) = 5.63 volts V max = 5.63 volts ½( ) = 4.69 volts V min = 4.69 volts ½( ) = 4.93 volts V min = 4.93 volts ½( ) = 5.05 volts = 5.05 volts. V max ½( ) = 4.99 volts Embedded Systems, KAIST 7

8 A/D Conversion Sampling Sample rate: samples per second >2x signal bandwidth to avoid aliasing: Nyquist sampling theory Faster: expensive ADC required Quantization Accuracy of each sample (resolution) Number of bits in digital data Quantization level: Full_scale_voltage/(2^n) 8-bit ADC: 1/ bit ADC: 1/ bit ADC: 1/ More expensive Ex: Temperature sensor with a range 0 to 100 deg.c, 0.5 deg. Resolution: 8- bit ADC is sufficient. Ex2: CD - 16-bit, 44.1 khz, stereo. 600 MB/h. Conversion equation Analog signal = (digital sample / max value) * reference voltage. Embedded Systems, KAIST 8

9 8.3 ADC Interface Wide range of ADCs Low-cost, low-speed ADCs: Simple voltage conversion High-speed, precise (and expensive) ADCs: Sampling video streams Built-in ADCs in microcontrollers: Simple interface Considerations Number of bits (resolution): 8/12/14/16 Conversion rate (speed) Interface: Parallel, serial, SPI, I2C Analog multiplexer, Sample-and-hold: Internal or external Package Cost Embedded Systems, KAIST 9

10 ADC Interface (II) Maxim MAX1245 A good general-purpose ADC for sensor applications 8 channels of analog input 100K samples/sec 12-bit resolution Internal track-and-hold (sample and hold) Interface: SPI, microwire, serial (TI DSP) DOUT: MISO, DI: MOSI, SCLK: SCLK Operation Start command to ADC via the SPI interface Specifies the channel and other ADC settings Internal/external clocks SPI SCLK can be used as ADC clock Wait for conversion: 12 clocks Get data via SPI Embedded Systems, KAIST 10

11 ADC Interface (III) MAX1245 Interface -> Embedded Systems, KAIST 11

12 8.4 ADC Interface in AM3359 ADC Interface [Datasheet] 12-Bit Successive Approximation Register (SAR) ADC 200K Samples per Second Input Can be Selected from any of the Eight Analog Inputs Multiplexed Through an 8:1 analog Switch Can be Configured to Operate as a 4-wire, 5-wire, or 8-wire Resistive Touch Screen (TSC) Interface. Embedded Systems, KAIST 12

13 ADC Interface in AM3359 (II) Analog Front End (AFE) [TRM] The AFE features are listed below, and some are controlled by the TSC_ADC_SS: 12-bit ADC Sampling rate can be as fast as every 15 ADC clock cycles Support for internal ADC clock divider logic Support for configuring the delay between samples also the sampling time Embedded Systems, KAIST 13

14 ADC Functional Block Diagram Functional Block Diagram Embedded Systems, KAIST 14

15 8.5 Sensor Interface A. Temperature Sensor Applications Room temperature: heating & cooling systems Temperature recorder: shipment of fruits, vegetables, frozen foods, and flowers AD22100/22103 temperature sensors by Analog Devices Easy to use 3-pin device: power, GND, and Vout -> 5V (AD22100), 3.3V (AD22103) Embedded Systems, KAIST 15

16 Sensor Interface (II) Temperature sensor (Cont d) AD22100/22103 Temperature range: -50 deg.c to 150 deg.c 22.5 mv/deg.c for AD22100 Vout = (Vs/5)*( *TA) or TA = (((Vout*5)/Vs) 1.375) / mv/deg.c for AD22103 Vout = (Vs/3.3)*( *TA) or TA = (((Vout * 3.3) / Vs-0.25) / Interfacing Embedded Systems, KAIST 16

17 Sensor Interface (III) B. Light Sensor Applications Artificial lighting systems Security detector: Checks light interruption TAOS TSL250R Texas Advanced Optical Solutions Inc. Consists of a photodiode and an integrated amplifier Simple 3-pin device: Vcc, GND, Output Spectral response Supply voltage between 2.7V to 5.5V Consumes typically only 1.1 ma Output: 0 to 4V Embedded Systems, KAIST 17

18 Sensor Interface (IV) Light sensor (Cont d) Amplifying the light sensor 4V to 5V. Gain: 1.25 AD623: A good general-purpose op amp Rail-to-rail operation, Single supply voltage Requires very little current, Easy to use Single external resistor to set gain R_G = 100 kohm / (Gain 1) 1% accurate R req d Amplifier circuit -> Embedded Systems, KAIST 18

19 Sensor Interface (V) C. Accelerometer ADXL150 (Analog Devices) Single-axis (one-dimensional) accelerometer Resolution 10 mg (1 g = 9.8 m/s 2 ) Full-scale range +-50g ADXL250 Dual-axis (two-dimensional) accelerometer Applications Measure linear acceleration of vehicles Gentle vibrations and shifts Seismometer Vibrations of ground shift in mines, in tunnels, or at building sites Monitor motion. Embedded Systems, KAIST 19

20 Sensor Interface (VI) Accelerometer (Cont d) Axis of sensitivity for ADXL150 -> Use strong glue under the chip ADXL150 circuit -> No external components except power supply Incorporates sensor, signal conditioning, and amplification Output directly interfaced to an ADC TESTPOINT: Used during manufacturing process Power supply 4 6 V (5 V exact desirable) V_out = Vs/2 (sensitivity * Vs/5 * acceleration) Sensitivity: 33.0 to 43.0 (38.0 nominal) for range +-50g Sensitivity doubling Connect output to the OFFSET_NULL pin (+-25g) SELF-TEST: Verify correct operation (artificial force) Embedded Systems, KAIST 20

21 Sensor Interface (VI) D. Pressure sensors Applications Air pressure for weather monitoring and prediction Cars: manifold pressure Washing machine: water level Biomedical: blood pressure Measure altitude (air pressure dep. on height above sea level) Ocean depth Sensing methods Deflection of a diaphragm separating two chambers Absolute, differential, gauge (wrt atmosphere) Embedded Systems, KAIST 21

22 Sensor Interface (VIII) Pressure sensors (II) Motorola MPXA6115A Absolute pressure sensor 5 V supply Output voltage 0.2V to 4.8V (15kPa to 115kPa) Bias compensation required. Integrates signal conditioning, temperature compensation Requires External power supply and decoupling capacitor only RC filter at the output Embedded Systems, KAIST 22

23 Sensor Interface (IX) Pressure sensors (III) KP100 by Infineon Absolute pressure sensor Incorporates a built-in ADC Much less susceptible to noise and interference SPI interface 5V supply and decoupling capacitor READY output: may interrupt the processor Separate CLK input: 4MHz or 8MHz Embedded Systems, KAIST 23

24 Sensor Interface (X) E. Magnetic field sensor AD22151 by Analog Devices Measure position and proximity Magnetic source as a reference point Built-in temperature compensation and amplification Sensor circuit -> R1: Temperature compensation resistor R2, R3: Gain R4: Voltage offset Embedded Systems, KAIST 24

25 8.6 D/A Conversion Digital-Analog Converter Take digital data and convert it into an analog signal Digital input Bus, SPI, or I 2 C MAX525 by Maxim 12 bit DAC with SPI interface -> 4 channels of analog output OUTA, OUTB, OUTC, OUTD Output amplifiers on-chip Feedback inputs: FBA, FBB, FBC, FBD Voltage reference inputs: REFAB, REFCD At least 1.4V or more below VCC Output voltage Vout = (Vref * code / 4096) * gain Embedded Systems, KAIST 25

26 D/A Conversion (II) MAX525 (II) Daisy chaining multiple MAX525s CLb input: All outputs to lowest value Low-power mode under software control PDLb input: power-down lockout UPO: User Programmable Output General-purpose Embedded Systems, KAIST 26

27 D/A Conversion (III) MAX525 (III) Nonunity gain amplifier -> Gain = 1 + R2 / R1 Bipolar output Use external amplifier with bipolar supplies -> Embedded Systems, KAIST 27

28 8.7 PWM Pulse Width Modulation Use one digital output to generate analog output Use a constant frequency (or period) Change the duty cycle Average value of the output is proportional to the duty cycle Low-pass filter Averaging Convert the pulse to an analog voltage Applications Drive LEDs Drive a speaker Frequency and duty -> pitch and volume Embedded Systems, KAIST 28

29 PWM (II) Motor Control: DC motor drive DAC + linear amplifier Poor low-speed operation Low power efficiency PWM + switching amplifier Better low-speed operation High power efficiency H-bridge -> Bidirectional drive with single power supply Q1, Q3 simultaneous ON: short circuit! Embedded Systems, KAIST 29

30 PWM (III) MC33186 by Motorola More functionality Easier to control V+: 5 to 28V TTL compatible inputs Switch continuous current up to 5A Built-in short-circuit and over-current protection Pins CP: Charge pump Forward: DI1=0, DI2=1, IN1=1, IN2=0 Backward: DI1=0, DI2=1, IN1=0, IN2=1 Freewheeling: DI1=0, DI2=1, IN1=IN2 Disabled: DI1=1 or DI2=0 SF output: Status Flag. Embedded Systems, KAIST 30

31 8.8 PWM in AM3359 Pulse-Width Modulation Subsystem (PWMSS) A. ehrpwm Dedicated 16 bit time-base with Period / Frequency control Can support 2 independent PWM outputs with Single edge operation Can support 2 independent PWM outputs with Dual edge symmetric operation Can support 1 independent PWM output with Dual edge asymmetric operation Supports Dead-band generation with independent Rising and Falling edge delay control Provides asynchronous over-ride control of PWM signals during fault conditions Supports trip zone allocation of both latched and un-latched fault conditions Allows events to trigger both CPU interrupts and start of ADC conversions Support PWM chopping by high frequency carrier signal, used for pulse transformer gate drives. High-resolution module with programmable delay line. Programmable on a per PWM period basis. Can be inserted either on the rising edge or falling edge of the PWM pulse or both or not at all. Embedded Systems, KAIST 31

32 PWM in AM3359 (II) B. ecap Dedicated input Capture pin 32 bit Time Base (counter) 4 x 32 bit Time-stamp Capture registers (CAP1-CAP4) 4 stage sequencer (Mod4 counter) which is synchronized to external events (ECAPx pin edges) Independent Edge polarity (Rising / Falling edge) selection for all 4 events Input Capture signal pre-scaling (from 1 to 16) One-shot compare register (2 bits) to freeze captures after 1 to 4 Time-stamp events Control for continuous Time-stamp captures using a 4 deep circular buffer (CAP1-CAP4) scheme Interrupt capabilities on any of the 4 capture events C. eqep Input Synchronization Three Stage/Six Stage Digital Noise Filter Quadrature Decoder Unit Position Counter and Control unit for position measurement Quadrature Edge Capture unit for low speed measurement Unit Time base for speed/frequency measurement Watchdog Timer for detecting stalls. Embedded Systems, KAIST 32

33 epwm Submodules Embedded Systems, KAIST 33

34 References [1] Steve Heath, Embedded Systems Design, 2 nd Ed., Newnes, [2] John Catsoulis, Designing embedded hardware, O-Reilly, [3] AM3359 Datasheet, AM335x ARM Cortex-A8 Microprocessors (MPUs) (Rev. D), [4] Technical Reference Manual - AM335x ARM Cortex-A8 Microprocessors (MPUs) Technical Reference Manual (Rev. F), Embedded Systems, KAIST 34