Real Time Embedded Systems. Lecture 1 January 17, 2012

Transcription

1 Analog

2 Real Time Embedded Systems Lecture 1 January 17, 2012 Topic

3 Section Topic Where in the books Catsoulis chapter/page Simon chapter/page Zilog UM197 (ZNEO Z16F Series Flash Microcontroller Contest Kit User Manual) Zilog UM171 (ZiLOG Developer Studio II ZNEO User Manual) Zilog PS220 (ZNEO Z16F Series Product Specification) Zilog UM188 (ZNEO CPU Core User Manual) Assorted datasheets

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5 Signals Analog Digital Reference voltage

6 Digital-Analog Connecting a sensor to digital circuitry is simple if the sensor provides digital output. We have seen that switches, and logic encoders are easily interfaced with gate circuits due to the nature of their signals. When analog sensors are involved the interfacing becomes more complex. We need a way to electronically translate analog signals into digital quantities, and visa-versa.

7 Analog Signals How do we input and output analog signals?

8 Signal Converters Analog to Digital signal converter on the inputs Digital to Analog signal converter on the outputs

9 Digital -> Analog A Digital to Analog converter is an electronic circuit which accepts a digital number at its input and produces a corresponding analog signal (usually a voltage) at the output

10 Digital -> Analog DAC Several Types PWM output + low pass filter Binary Weighted DAC R2R Ladder Kelvin Divider

11 Kelvin Divider DAC

12 DAC R-2R ladder network is a simple Digital-Analog converter. Only two resistance values are used anywhere in the entire circuit. Modern DACs use the R-2R ladder

13 But... We don't have a Digital to Analog converter on the ZNEO (or any of the Zilog microcontrollers).

14 Fortunately we know how to add that capability if we need it Add a DAC chip I2C, SPI, 1-wire, Or parallel digital input 1000's to choose from (really!)

15 Like this one!

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17 Analog -> Digital An Analog to Digital converter is an electronic circuit which accepts an analog input signal (usually a voltage) and produces a corresponding digital number at the output

18 ADC Analog-to-Digital Conversion The workhorse of the test and measurement world. Providing a means for a microcontroller to measure analog signals.

19 ADC Several Types (ways to convert analog to digital) Digital Ramp Successive Approximation Subranging Tracking Slope Integration Flash Sigma Delta

20 Flash ADC Start with a voltage comparator Determines of a signal is greater than a reference signal (a 1-bit analog to digital converter). Vref Vin + Vout Vref Vin Vout

21 Flash ADC n-bit resolution requires 2 n -1 comparators 8-bit means 255 comparators 10-bit means 1023 comparators

22 A binary counter counts up from 0. The output of the counter to feed into a digital to analog converter. The generated analog signal is compared to the input signal. When the generated signal is greater than the input signal the counter stops and the binary value is the digital value for this signal Digital Ramp

23 Sigma Delta ADC High resolution and Low cost The Z16 uses Sigma-Delta ADC One of the more advanced ADC technologies

24 Why so many different ways?

25 Resolution vs Speed

26 ADC Signal Sampling Sampling can cause errors (aliasing) Nyquist rate = 2x highest frequency of interest Practically: always sample at least 5x, or higher

27 What kinds of things can you measure with an ADC?

28 Sensors Sensors with analog output Temperature Pressure Light intensity Position Velocity, RPM...

29 Analog Input Don't like the ADC on your microcontroller? Add an external one. Many to choose from Parallel data out I2C, SPI, 1-wire,...

30 MAX Million samples per second 16 bit

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32 MAX Million samples per second 10-bit data 1 micro amp in shutdown

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34 Why so many different ADC?

35 Why Speed (samples/second) Low Power for battery/portable applications Resolution (8,10,12,14,18 bits) Number of channels Interface Range of analog input (3v, 12v, +/- 12v,...)

36 Z16 ADC One ADC 12 Switched inputs 10-bit resolution Internal voltage reference generator Ability to supply external reference voltage.

37 Z16 ADC

38 The Pins ANA0..11 AVDD, AVSS VREF

39 ADC 10 bit conversion (needs 2 bytes for result) 12 analog input sources are multiplexed with general-purpose I/O ports Interrupt upon conversion complete Internal voltage reference generator

40 ADC Each ADC measurement consists of three phases: 1. Input sampling (programmable, minimum of 1.0 μs). 2. Sample-and-hold amplifier settling (programmable, minimum of 0.5 μs). 3. Conversion is 12 ADCLK cycles.

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42 Z16 ADC Registers ADC0CTL Control Register ADC0DH, ADC0DL ADC data ADCSST Sample Settling Time ADCST Sample Time ADCCP Clock Prescaler ADC0MAX Max Chanel Register ADCTCAP_H,L Timer0 Capture Register

43 Z16 ADC Registers Don't forget GPIO alternate function and interrupt control registers

44 ADC0CTL

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46 ADC0D_H,L

47 Interrupt? Why do I want to generate an interrupt on conversion?

48 Single Shot ADC Single ADC 1. Enable the alternate function of the GPIO pins that you want to use for analog inputs (PORT B & H) 2. Write to ADC0CTL to configure which input source (one source can be active at a time) 3. Set ADCCTL START0 to enable conversion. 4. Watch STAT0 bit until conversion is complete. 5. Read value from ADC0D_H, ADC0D_L

49 Example Example Analog_Photodiode

50 Photodiode Like a diode (allows current to flow in one direction and not the reverse) BUT, light changes that and allows current to flow in the reverse direction also. Dark = little current flow More light = more current flow

51 5v _ Photo diode LED < > /\/\/\----+ v GND +--> ANA0 LED for visual indicator

52 Photodiode is blocked, LED not on

53 Photodiode is illuminated, LED is on

54 Why would we want an External Reference Voltage?

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57 External Reference This circuit gives us access to supply our own reference voltage (on J7) or Use a MAX6160 voltage reference (and potentiometer) to provide a variable VREF

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63 VREF Internal VREF=3.3v 3FF 10 bits 3.3v 0FF 8 bits 0.82v 00F 4 bits 0.05v bit 0.003v With external VREF=1v 3FF 10 bits 1.0v 0FF 8 bits 0.25v 00F 4 bits v bit 0.001v