Lecture 7: Analog Signals and Conversion

Transcription

1 ECE342 Introduction to Embedded Systems Lecture 7: Analog Signals and Conversion Ying Tang Electrical and Computer Engineering Rowan University 1

2 Analog Signals Everywhere Everything is an analogy in the physical world Sound, light, and force, etc. We need convert them into electrical signals. Transducer: convert one type of energy to another 2

3 How to Get Digital Signals from Analog Convert physical phenomena to voltages. Physical phenomena Sensor Voltage A typical temperature sensor transfer function shown on the right Discretize continuous voltages. Voltage ADC ADC output Software Engineering Units 3

4 How to Convert Analog to Digital Comparator: compares the analog voltages on its two terminals, V+ and V. Its output is high if V+ >V and low if V+ <V.. Simple and cheap module Act like a one-bit ADC (not precise enough) 4

5 How to Get Digital Signals from Analog Discretize continuous voltages for successive approximation V(t) V(n) where v(n) is a sequence of binary values w/ a fixed number of bits V(t) f sampled (x) f (x) t T S 5

6 Represent the signal as fraction within the range of values 6

7 Accuracy vs. Precision Accuracy: How close a measurement is to its true value, which would be produced by an ideal system. This is easy to define but hard to measure Precision/Resolution: The number of distinct output values that a measurement can provide. Alternatively, it can be specified as the change in input that corresponds to the minimum change in output, 1 bit 7

8 Precision It is a function of the range and the number of bits (N) For a 10-bit ADC for 0-5 range: Q=5/2 10 = 4.88 mv For a 12-bit ADC for 0-5 range: Q=5/2 12 = 1.22 mv Note that: MSP430f5529 has a 12-bit ADC Quantization Error = V FS /2 N 8

9 How Successive Approximation Works? Example: an input voltage of V in = 0.4 V FS to a 4- bit ADC 9

10 How Successive Approximation Works? Example: an input voltage of V in = 0.4 V FS to a 4- bit ADC The input voltage V in is compared with the midpoint ½ V FS of the full range In this case V in < ½ V FS so the most significant bit (msb)=0 We now know that the input lies between 0 and ½ V FS. The input is next compared with the midpoint of this range, ¼ VFS We find Vin > ¼ VFS so the next bit is 1 10

11 How Successive Approximation Works? Example: an input voltage of V in = 0.4 V FS to a 4- bit ADC (cont.) The process illustrated in the previous slide continues until each binary bits are assigned a value Can you finish the process and find the binary representation of the analog signal V in = 0.4 V FS? 11

12 How To Implement ADC? Set control registers Specify where the input is coming from (which pin) Specify the reference range Specify the characteristics of the input signal Enable interrupt and set bits to start the conversion When the interrupt occurs, read sample from the data register Wait for a sample period and Repeat the process above 12

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14 Input 12 external analog signals ADC12_A are multiplexed w/ digital port pins 4 internal analog signals 14

15 Input One of 16 analog signal Output 12-bit digital value of input relative to voltage references 15

16 Internal External Vref+ 1.5V, 2.5V, Vcc VeRef+ Vref- AVss VeRef- Voltage reference Lower reference by default is analog ground, but it can be external Upper reference by default is analog power supply, but can use internal/external reference 16

17 Clock Conversion clock Sampling clock ADC12OSC can be used for both that is a 5MHz Oscillator Timers A and B can be chosen for sampling clock sources 17

18 Sampling Timing Consideration All port inputs default to high impedance When sample starts (SAMPCON=1), inputs are enabled and modeled as a low-pass filter The capacitor C I voltage V C must be charged to within onehalf LSB of the source voltage V S for an accurate n-bit conversion, where n is the bits of resolution required. 18

19 Conversion Memory Registers Results from each channel are stored in the low 12-bit of a 16-bit conversation memory register (ADC12MEMx) Each memory register has a corresponding memory control register (ADC12MCTLx) to control: End-of-sequence (EOS): is the channel the end on a sequence of channels that are to converted; it is used for multichannel conversion. Reference voltage selection (ADC12SREFx) Analog input channel selection (ADC12INCHx) 19

20 How To Implement ADC in Code? Set control registers Two control register: ADC12CTL0 and ADC12CTL1 Clock source (ADC12SSELx) and divider (ADC12DIVx) Sample and hold behavior -- trigger source selection (ADC12SHSx) -- sample and hold time (ADC12SHT0x/ADC12SHT1x) -- ADC 12 pulse mode (ADC12SHP) Reference voltage -- need to clear REFMSTR=0 (see user guide Table 26-4) -- turn on the internal reference voltage (ADC12REFON=1) -- select either 2.5v or 1.5 v for V ref (ADC12REF2_5V=1or 0) Select conversion mode (ADC12ONSEQx): single vs. multiple 20

21 Sampling Time Extended sample mode (ADC12SHP=0) SHI signal directly controls SAMPCON and defines the length of the sample period 21

22 Sampling Time Plus sample mode (ADC12SHP=1) SHI signal is used to trigger the sampling timer. The ADC12SHT0x and ADC12SHT1x bits in ADC12CTL0 control the interval of the sampling timer that defines the SAMPCON sample period 22

23 How To Implement ADC in Code? Set control registers Select input channel(s): ADC12INCHx bits in ADC12MCTLx ADC12 has 12 (external) input signals (A0-A7, A12-A15) that are multiplexed with Ports 6 and 7 ADC12 has 4 (internal) analog signals, among which input channels 8, 9 & 11are connected to different reference voltages, and channel 10 to an internal Temperature sensor 23

24 Let s Look at an Example A digital current meter might measure the current by measuring the voltage across a sensing resistor. Can we use the ADC12 to measure the voltage across with the accuracy of 1V, 1mV or 0.1mV? Why and how? 24

25 Let s Look at an Example Set control registers Reset REFMSTR to hand over control of internal reference voltages to ADC12_A control registers REFCTL0 &= ~REFMSTR; Initialize the control register ADC12CTL0 ADC12CTL0 = ADC12SHT0_4+ADC12REFON+ADC12REFON2_5V+ADC12ON; Initialize the control register ADC12CTL1 ADC12CTL1=ADC12SHP; 25

26 Let s Look at an Example Set control registers Set conversion memory control register ADC12MCTL0 = ADC12SREF_1+ADC12INCH0; Configure the port to be used as ADC input P6SEL = 0x01; Enable interrupt and set bits to start the conversion, Or use polling without interrupts (we use the interrupt in our example) ADC12IE = 0x01; //enable interrupt ADC12CTL0 = ADC12ENC; //enable conversion 26

27 Let s Look at an Example When the interrupt occurs, read sample from the data register; Wait for a sample period and Repeat the process above Start sampling while (1) { ADC12CTL0 = ADC12SC; // Start conversion bis_sr_register(lpm0_bits + GIE); no_operation(); // SET BREAKPOINT HERE } 27

28 When the interrupt occurs, read sample from the data register; Wait for a sample period and Repeat the process above Write ADC ISR #pragma vector = ADC12_VECTOR interrupt void ADC12_ISR(void) { adc_value = ADC12MEM0; } 28