Slide 1: An additional Lecture on C Programming

Creating a Signal Generator

Uli Raich

Course on Embedded Systems
UCC semester end 2017

Slide 2: C is too abstract?

I was told that you find the course on C too abstract!

• What is is good for?
• What are pointers good for?
• Why use command line arguments?
• What has all this to do with embedded systems?
• Can I use it for my Physics experiments?

Slide 3: A pulse generator

A pulse generator is a device you use to test your electronics

(e.g. electronics to read out an experiment

which generates electronic signals

On a professional pulse generator you can:

• change the wave form
  sine, rectangular, triangular, sawtooth,
  arbitrary user defined wave form
• change the frequency
• change the pulse height
• change rise and fall times
• and many parameters more

Slide 4: The oscilloscope

Before using the signal from the pulse generator

you observe the signal on an oscilloscope

How does an oscilloscope work?

What is

• vertical gain
• time base
• auto versus normal, external, single trigger
• trigger level and trigger position

Slide 5: A standard oscilloscope

A standard digital storage oscilloscope uses

• a very fast (GHz) analogue to digital converter (ADC)
• plenty of knobs and buttons to select
  gain, timebase, trigger parameters …
• a screen to display the input signal
• Price: ~ 5kUS$ - 50kUS$

I have a 60 US$ oscilloscope

(not useful for professional applications

but ok for the slow signals we produce

with our electronics) featuring:

• 2 input channels
• 40 MHz sampling rate (20 MHz band width)
• No screen
• USB connection to a computer

Slide 6: Oscilloscope Example

Slide 7: Our pulse generator

Let us create our own pulse generator!
It should provide

• sine wave
• rectangular wave

What do we need?

• A table of numerical values that describes the wave form
• A digital to analogue converter (DAC) to convert the numbers into a signal level.

Slide 8: The DAC

We have a 12 bit DAC, which creates signal

levels from 0V to Vcc

What is the max. number this DAC can take in

decimal and in hex?

What is the signal resolution in ‰

This is Physics!

Slide 9: DAC access

The DAC has 3 registers with the following bit layout:

register 0 takes the lowest 4 bits of the DAC value

register 1 takes the middle 4 bits

register 2 takes the highest 4 bits

RS selects the register

The strobe line (STR) must pulse (go high and low again)

to read/ write the data (R/W line) from/to the DAC

What is the bit combination and sequence to write the

middle 4 bits with the data 0xa

Slide 10: Breaking down the problem

Even though this will be still a very small and simple

program, let’s break it down into smaller pieces:

• Get the wave form from the user through command line arguments
• Create the wave form within a table of
  100 short (16 bit) values. The upper 4 bits will always be zero
• Function to write all 12 data bits to the DAC

Slide 11: Check the number of cmd line args

Write a piece of code that checks that the user

has entered 1 arguments

The program prints a “Usage” message and exits if the

no of arguments given by the user is not exactly 1

Slide 12: Solution: arg count

Slide 13: Argument Check

If the no of args is correct we have to check

if the argument given is either

• “sine”
• or “rect”

To do this we need a string compare function:

int strcmp(const char *s1, const char *s2)

This function returns zero if the 2 strings match

You must include <string.h> to use this function

Slide 14: Check the argument

Improve your program to include a check

if the argument given is “sine” or “rect”.

Print an error message it it is not

and a success message if it is.

Slide 15: Solution: argument check

Slide 16: Creating the wave form

As we said, the DAC takes 12 bit values which

can be stored in an array of short (16 bits)

The upper 4 bits of each element will be zero

The generation of the waveform goes into a

separate source file and its associated include file

I give you the example for the sine wave and

you write the code for the rectangular wave

Slide 17: Function test

First we create the framework

We will need:

• the function itself (genWave.c)
• an include file describing it (genWave.h) and containing
  definitions needed by it and useful to the calling program
• a main routine (createWaveForm.c) to call it

Slide 18: Write the Test function

Write a test function (genWaveTest.c) taking one argument

(the wave type) and printing which wave type has been selected.

Which definitions should go into the include file?

How do you have to modify the main program

to call the genWave function?

Slide 19: Dummy function and test (main)

Slide 20: The include file

Slide 21: The test function

Slide 22: Implementing the function

Slide 23: Was it correct?

Slide 24: The rectangular wave form

Write the code for the generation of the rectangular wave form.

The signal should be zero for the first 50 values of the wave,

4095 for the following 50 values

Slide 25: Solution: Rectangular wave

Slide 26: The rectangular wave form

Slide 27: Bit handling functions

We need to learn a few bit handling operators before

being able to prepare the data for the DAC:

| : bitwise or

& : bitwise and

data += 5 <=> data = data + 5

data |= 5 <=> data = data | 5

~data: invert all bits in data

data >> 4 all bits in data are shifted right by 4

data << 4 all bits in data are shifted left by 4

example:

data = 16  00010000
data >> 4  00000001

Slide 28: DAC preparation

Write a function which prepares a byte for the DAC

The functions takes 3 arguments:

• The data nibble (4 bits of data)
• The register selection (0,1,2)
• The read/write bit
• Leave the strobe bit zero!

What will be the result if you want to write 5

to register to the middle nibble?

Slide 29: Solution: DAC preparation

Slide 30: Now the test program

Slide 31: Strobe

Now create a strobe function which takes DAC data

and generates a strobe signal on the

STR line without touching the other bits

Slide 32: Solution: Strobe

Slide 33: Send one short value to the DAC

Now that we know how to strobe, it is easy to write

a 12 bit value to the DAC

We must do it in 3 steps:

• "and" out all the bits in the data word
  except the last 4 (first data nibble)
• “or” in the rw and register bits (must be reg 0! )
  and send the dataByte to strobe
  
  
• Shift the data word by for bits and do the same thing
  (now the register bits must be 1 of course)
  
  
• Shift again by 4 bits and repeat

Slide 34: Solution: send one data word

4

Slide 35: ...and the test program for it

Slide 36: … with the result

Slide 37: Assembling the whole thing

Now we have all the bits and pieces and

finalizing the project becomes easy.

In the file accessing the hardware we add:

and we call this in main.

Slide 38: And this is the final result

-- Uli Raich - 2017-10-09

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