EECS 461代写、代做C/C++设计编程

日期：2023-09-26 11:07

EECS 461 Fall 2023

Lab 3: Analog-To-Digital Conversion

1 Overview

In this lab you will learn how to use the analog to digital converters on the NXP S32K144 microcontroller.

You will then sample a sine wave produced by the signal generator and display the result on a virtual

oscilloscope to study the phenomenon of aliasing. See Chapter 42 in the S32K144 Reference Manual:

“Analog-to-Digital Converter (ADC).” A block diagram of the ADC is found in Figure 1.

Figure 1: Block diagram of the ADC module.

EECS 461: Embedded Control Systems 1 Fall 2023

Lab 3 Analog-To-Digital Conversion ANin

GND

GND

5V

5V

6 4 2 0

7 5 3 1

Potentiometer

(Turning Knob)

PotJump (Connector)

Figure 2: Pin connection matrix between external signals and S32K144 microcontroller.

1.1 Analog-To-Digital Converter Module

The S32K144 contains two successive approximation register (SAR) A-D converter modules, referred to

as ADC0 and ADC1, each of which has 16 input channels. In Lab 3, we shall only use channel 0 of ADC0.

The ADC may be operated using either software or hardware to trigger conversions; we shall only use

software triggered conversions in Lab 3. The ADC also has single and continuous conversion modes; we

shall only use single mode conversions.

The clock for the S32K144 ADC uses an 8 MHz clock instead of the 80 MHz system clock.

2 Design Specifications

2.1 Hardware

The interface board has connections to eight of the input pins from the ADC module. The connections

are made to pins ANin0 through ANin7. Each input is connected to a four-pin header consisting of the

input signal, ground, a +5 volt reference and one unused pin.

A potentiometer is mounted permanently on the interface board to provide a variable input signal for

A/D conversion. Note the jumper that must be connected to the ANin0 signal pin for this to work,

as shown in Figure 2. In this lab, you will use this potentiometer as well as a sine wave from a signal

generator as input to the board.

2.2 Software

adc.h and adc.c

The first file, adc.h, contains function prototypes for three functions. You will be responsible for writing the definitions for these functions in the other file, adc.c (rename

adc template.c). As with Lab 2, place the adc.h file in the include directory and your

completed adc.c file in the lib directory.

lab3o.c

When using an A/D converter to sample an analog signal for an embedded control application,

it is almost always important that the samples be taken at a specified fixed interval. Variations

EECS 461: Embedded Control Systems 2 Fall 2023

Lab 3 Analog-To-Digital Conversion

in sample time are referred to as timing jitter, and are usually undesirable. To achieve fixed

interval sampling, the program lab3o.c allows you to sample the output of a signal generator

at a 50kHz sampling rate. The sample values are then transferred over the serial port to the

lab station desktop so that they may be displayed on a Matlab based virtual oscilloscope.

To achieve the desired sampling frequency, the A/D conversions are performed in a periodic

interrupt routine that executes every 0.02 msec. We have not yet learned how to use the

interrupt timers on the S32K144, and thus this file is provided to you directly and you don’t

need to make any modifications. Place lab3o.c in the lab3o directory.

virtual oscilloscope.slx

This file is a virtual oscilloscope program built in Simulink. As described in the discussion

of lab3o.c, the data used by the oscilloscope is collected by the ADC at a 50kHz sampling

rate. When executed, the virtual oscilloscope sampled date plotted in the time domain. The

other plot is a frequency domain FFT of the data, which is useful for observing the frequency

content of the data sequence from the serial port. We shall use the virtual oscilloscope to plot

the data collected at a 50kHz sampling rate using lab3o.c for sine wave outputs of various

frequencies. You will observe the e↵ects of aliasing and imperfect reconstruction in both the

frequency and time domains. See example plots in Figure 3.

Figure 3: Output of Virtual Oscilloscope Simulink Model: time samples and FFT of a 1 kHz sine wave.

EECS 461: Embedded Control Systems 3 Fall 2023

Lab 3 Analog-To-Digital Conversion

3 Pre-Lab Assignment

Pre-Lab questions must be done individually and a .pdf copy must be uploaded before the due date and

time specified on Canvas. PLEASE INDICATE YOUR ANSWERS CLEARLY. Note that you will need

refer to your Pre-Lab solutions while you are working in the lab. You must also, with your partner,

design an initial version of your software before you come to your lab section. Your software does not

need to be completely debugged, but obviously more debugging before coming to lab means less time you

will spend in the lab.

Within the file adc.c, there are three functions to be completed: init ADC0 single, ADC0 complete,

and read ADC0 single. The prototypes of these functions can be found in the adc.h file and you should

modify the code provided in adc template.c.

1. void init ADC0 single(void): There are several registers that need to be set to initialize the

ADC0 module. These are the Peripheral Clock Controller (PCC) register, the Status and Control

Registers SC1[0], SC2, and SC3, the the configurations registers CFG1 and CFG2.

• SC1[0] (Section 42.4.2): According to Table 41-2, the ADCH bitfield should be set to 0x1F

to disable the ADC0 module for configuration (the disable setting in Section 42.4.2 is not

applicable to the S32K144 processor). To enable a conversion, set ADCH to the desired channel.

The AIEN bitfield enables an interrupt when a conversion is completed; we do not use this

feature.

• CFG1 (Section 42.4.3): The MODE bitfield is set to specify the resolution of the ADC – use the

full 12-bit resolution. The ADICLK bitfield selects the ADC clock – use Alternate clock 1.

• CFG2 (Section 42.4.4): The SMPLTS bitfield selects the sample time, or time taken to acquire

an analog voltage for conversion. Use the default value of 13 ADC clock cycles.

• SC2 (Section 42.4.7): Set the ADTRG bitfield to specify software triggered conversions, and the

REFSEL bitfield to select the default reference voltages, VREFH and VREFL.

• SC3 (Section 42.4.8): Set the ADCO bitfield for single conversion mode. Set the other bitfields

to disable calibration and hardware averaging.

2. uint8 t ADC0 complete(void): This function returns the value of the COCO bit from the SC1[0]

register. The COCO bit equals 0 until the conversion is complete, when it is set to 1. Reading the

result of the ADC conversion clears the COCO bit.

3. uint32 t read ADC0 single(uint16 t inputChannel): This function does four things:

• Initiate conversion by writing the ADC channel number to the ADCH bitfield of the SC1[0]

register.

• Wait for conversion to complete.

• Read Data Result Register R[0] (Section 42.4.5).

• Convert the result to millivolts.

4. Suppose that we sample a sine wave with frequency f0 Hz in a software loop with

approximate execution time Ts seconds (and thus with sample frequency fs = 1/Ts

Hz). Assume that the frequency of the sine wave f0 is approximately (but not exactly)

equal to the sample frequency. State an expression for the apparent frequency of the

sampled sine wave. What is the approximate value of this frequency?

5. During the In-Lab, you will be asked to sample sine waves output from the signal generator in

a timed 50kHz software loop and display the results on a software oscilloscope. As described in

the handout “Sampling, Beats, and the Software Oscilloscope”, you may see a variety of di↵erent

phenomena, depending on the frequency of the sine wave with respect to the sampling frequency.

EECS 461: Embedded Control Systems 4 Fall 2023

Lab 3 Analog-To-Digital Conversion

For each of the sine wave frequencies listed below, which of the four plots in the

handout (i.e., which of Figures 1-4) corresponds to the result you should see? Note:

there is a 1-1 correspondence between Figures 1-4 and sine waves (a)-(d).

(a) f0 = 12.4 kHz;

(b) f0 = 55 kHz;

(c) f0 = 10 kHz;

(d) f0 = 24 kHz.

4 In-Lab Assignment

4.1 Single Mode Conversion Testing

1. Write a simple C program called lab3.c that uses the init ADC0 single and read ADC0 single

functions from adc.c to retrieve the value of an analog input from the potentiometer and store it

to a variable called iAnalog.

2. Run the lab3.c. Adjust the potentiometer to change the analog signal sent to the pin between 0

and 5 volts. Verify that the value of iAnalog has changed and that you can correctly read the full

voltage range.

4.2 Timing

In the Post-Lab Assignment, you will be asked to consult the S32K144 reference manual and compute

Tconv, the amount of time required for the hardware to perform one A/D conversion. We now measure

two other times of interest. Modify your lab3.c file so that, before the call to the read ADC0 single

function, one of the LEDs is set to high and is set back to low after the function returns. Connect an

oscilloscope to the GPO output pin.

1) Measure TRead, the amount of time required for one execution of the function read ADC0 single.

2) Measure TLab3, the period with which read ADC0 single is called.

Record these times for later analysis.

4.3 From Sine Wave to Square Wave

In this section of the lab, we read a sine wave output from a signal generator and display the result on

the oscilloscope. Because we can only output a digital signal, we cannot display the sine wave directly

in this way; instead, we convert the sine wave into a square wave. To do so, change your lab3.c file so

that when the ‘ANin0’ input measured in read ADC0 single is greater than approximately half of the

maximum value for the A/D conversion a digital output is set to output high. Otherwise, the digital

output will output low. Connect a function generator to the ‘ANin0’ signal pin and generate a sine wave

with parameters 0-5 V, 2.5 V o↵set, 5 Vpp, and 3 kHz. Use an oscilloscope to observe both the input

signal and the output signal. Use the stop and run features on the oscilloscope to freeze the waveform if

necessary.

As in Section 4.2, measure the amount of time Ts between successive calls to read ADC0 single, and

thus determine the frequency fs at which the sine wave is being sampled. Record these values for later

analysis. First try an input sine wave with frequency f0 much smaller (say by a factor of ten) than fs,

and record the approximate frequency of the resulting square wave. Next, try an input sine wave with

frequency f0 that is about equal to the sample frequency (say 95%), and again record the frequency of

the resulting square wave. Repeat a few times with di↵erent values until you are comfortable with the

relation between sine wave and square wave frequencies.

EECS 461: Embedded Control Systems 5 Fall 2023

Lab 3 Analog-To-Digital Conversion

4.4 The Virtual Oscilloscope

In this section of the lab, we again read sine wave output from a signal generator, and we output the

measured data over the serial port to a virtual oscilloscope program written in Simulink. To do so, use the

file lab3o.c and the Simulink model virtual oscilloscope.slx provided on the course website to plot

the data. Attach the signal generator to your interface board as shown by your instructor. Demonstrate

the working virtual oscilloscope to your instructor using a 3 KHz sinusoid. (Remember to put the signal

generator in high-Z mode). Recall that the data plotted by the virtual oscilloscope is collected by the

program lab3o.c at a 50 kHz sampling rate.

Increase the input signal frequency slowly until you see the “beat” phenomenon illustrated in Figure 3

of [1]. Record the frequency of the input signal and the resulting beat period for later analysis. Further

increase the input signal frequency until you see aliasing, as illustrated in Figure 2 of [1]. Record the

signal frequency and the resulting alias frequency for further analysis. Demonstrate to your GSI that you

can produce all four cases discussed in [1].

Save a copy of your code to hand in with the Post-Lab Assignment.

5 Post-Lab Assignment

1. According to Section 42.5.4.5 of the S32K144 Reference Manual, the time Tconversion required to

perform one A-D conversion consists of (i) the time Tsample required to sample the analog signal;

(ii) the time Tcompare required to set all 12 of the converted bits; and (iii) some additional overhead

time. Given that the ADC clock TADC is set to 0.125 µsec (fADC = 8 MHz) and the bus clock is

set to TBUS = 0.025µsec (fBUS =40 MHz), what are the values of (i) Tsample, (ii) Tcompare, and

Tconversion? Express your answer in µseconds.

2. In Section 4.2, you measured TRead, the amount of time required for one execution of read ADC0 single,

and TLab3, the period with which read ADC0 single is called. What values did you get for

TRead and TLab3? Recall that Tconversion is the amount of time required to perform one

conversion. Why is TRead greater than Tconversion? Why is TLab3 greater than TRead?

Assume e↵ects due to hardware inaccuracies are negligible. Limit your complete response to less

than 5 sentences.

3. For In-Lab part 4.3, what values did you record for the loop sample time and frequency?

For the low frequency input case, what value of input sine wave frequency did you use,

and what was the frequency of the resulting square wave? For the case fs ⇡ f0, what

values of input sine wave frequency and output square wave frequency did you record?

Verify that these values are consistent with your answer to question 4 of the Pre-Lab.

4. When you used the virtual oscilloscope in Section 4.4, you recorded signal frequencies that resulted

in aliasing, and in beats, as depicted in Figures 2 and 3 of [1].

(a) What was the frequency f0 of the sine wave from the signal generator that resulted

in aliasing? What was the frequency fA of the resulting alias? Verify that these

signals di↵er by an integer multiple of the sampling frequency by finding that

value of k for which f0  kfs = ±fA.

(b) What was the frequency f0 of the sine wave for which you observed periodic beats

on the virtual oscilloscope? What were the period and frequency fbeat of the

resulting beats? Verify that fbeat = 2(fN  f0).

Be sure to upload a copy of your code with your solutions to the Post-Lab.

If you have comments that you feel would help improve the clarity or direction of this assignment, please

include them following your postlab solutions.

EECS 461: Embedded Control Systems 6 Fall 2023

Lab 3 Analog-To-Digital Conversion

References

[1] J. S. Freudenberg, “Sampling, Beats, and the Software Oscilloscope” available from the EECS 461

Canvas website.

EECS 461: Embedded Control Systems 7 Fall 2023