EENG 383

Lab 11 - Direct Digital Synthesis

Lab:	11
Status	Live

InLab 11

Some self guided activities.

Lab 11 assignment

Software

The software will use an interrupt service routine associated with timer 0 to generate a DDS sine wave that updates about 30us from a 64 entry table. You may modify this update rate in order to make your math come out easier. The phaseIncrement and phaseAccumulator should be stored in uint16_t. You will have to determine their fixed point format using the information given.

Calculation
With a 64-entry sine wave LUT and a uint16_t data type phaseAccumulaor, what fixed point format should the phaseIncrement and phaseAccumulator have? I will refer to this value as I.F; 'I' for the number of integer bits and 'F' for the number of fractional bits.

Calculation
Write out a units factoring expression that describes the frequency of the output sine wave from the 64-entry LUT as a function of the phaseIncrement and the update rate. Now set the output sine wave frequency to 0.5 Hz and determine what update rate achieves this frequency with the smallest possible phaseIncrement (2^-F, where F is the number of bits of fractional bits in your fixed point format). At this update rate, 1 LSB unit change of the phaseIncrement corresponds to a 0.5 Hz change in the output frequency. Just double the requested frequency and store it as the phaseIncrement - you can do this with a shift operation.

Your program needs to include a function that allows the user, through the terminal interface, to type in a multi-digit decimal number, echoing the digits back to the terminal as they are typed, and, when the Enter key is pressed, convert the multi-digit value into a numerical value and return it as a uint16_t. Your function should ignore non-digit characters that may be typed in. Your function does not need to concern itself about overflow resulting from the user entering a value larger than 65535. One approach to this problem is to keep a running total of the number's value. Every time a new digit is entered, multiply the running total by 10 and adding the newly entered digit (minus ASCII '0') to the running total. The function prototype for this function in my program is:

uint16_t convert(void);

Your program will be sweeping the frequency from a non-zero starting frequency (in Hz) and an ending frequency (in Hz). To accomplish this the sweep should iterate through each phaseIncrement between the two frequencies using a for-loop. Hold each phaseIncrement for some duration of time that depends on the start and end frequencies as well as the sweep period. Make sure not to use TMR0 as the delay timer - it is already being used to generate interrupts every 30us or so.

Calculation:
Write out a units factoring expression for the number of 1:1 prescaled timer counts that will divide the sweep period into unit sized phaseIncrements. The number of timer counts will depend on the start and end frequencies as well as the sweep period.

At start-up your program should present a splash screen - this would be a great place for some ASCII art. The splash screen should also contain connection instruction for the developent board; for this assignment tell the user there are jumpers to install! When you press "?" at the terminal you should be greeted with the following menu.

-------------------------------------------------
--	Fixed
--		Frequency:	440Hz
--		Amplitude:	5/16
--	Sweep:
--		Starting:	100Hz
--		Ending:		1000Hz
--		Duration:	1 second
-------------------------------------------------
?: help menu
o: k
Z: Reset processor
z: Clear the terminal
S/s: Start/stop fixed frequency sine wave
f: enter Frequency
A/a: increase/decrease Amplitude select amplitude
W: sWeep sine wave 
t: enter sTarting frequency
e: enter Ending frequency
d: enter Duration
-------------------------------------------------

Print all values in decimal even through they may be represented in terms of different units inside your program such as timer counts.

• ?
  Prints out the ever useful help menu. Display the current upper and lower audio thresholds as part of the menu.
  
  
• o
  Returns "k" so that you know that the terminal is in working order.
  
  
• Z
  Reset the processor so that we can see that splash screen.
  
  
• z
  Clear the terminal using a bunch of new lines.
  
  
• S/s
  Start or stop the generation of a sine wave with a constant frequency given by the variable phaseIncrement (from "f" command), and an amplitude determined by the "A/a" commands. The ISR generating the sine wave should use a global flag to enable or disable the generation of the sine wave - there are times when we want the sine wave output disabled.
  
  
• f
  This command changes the phaseIncrement used to generate the sine wave inside the ISR using the convert function discussed above.
  
  
• A/a
  This command changes the amplitude of the sine wave generated inside the ISR. To do this, inside the ISR take the retried value from the sine wave LUT and store it in a local variable. The create 4 shifted right versions of this local variable, creating local/2, local/4, local/8 and local/16. then, depending on the bits of the amplitude variable add the fractional amplitudes into a total amplitude. Send this amplitude to the PWM -> digital to analog converter. For example:

  	// This is rough pseudo code and not intended to directly
  	// represent your final algorithmic implementation.
  
  	amplitude = 11;			// In binary  = 0b1011
  	local = sin[index];
  	localDiv2  = local >> 1;
  	localDiv4  = local >> 2;
  	localDiv8  = local >> 3;
  	localDiv16 = local >> 4;
  
  	// Since amplitude has bits set in position -1, -3, -4
  	total = localDiv2 + localDiv8 + localDiv16;
  	PWMduty4(total);
  

  Note, that this version of the amplitude does not allow a full-scale output, you will only get to with 15/16^th of the maximal output.
  
  
• W
  Generate a sweeped frequency sine with a frequency that starts at startPhaseIncrement and ends and endPhaseIncrement over a time period given by the sweep period.
  
  
• t
  This command changes the startPhaseIncrement used to generate the sweeped sine wave using the convert function discussed above.
  
  
• e
  This command changes the endPhaseIncrement used to generate the sweeped sine wave using the convert function discussed above.
  
  
• d
  This command changes the sweepPeriod (measured in milliseconds) used to generate the sweeped sine wave using the convert function discussed above. The number entered should be assumed to be in millisecond.
  
  

  Working from Home

  You can test this assignment at home by playing the DDS output through a speaker then using a cell phone application that displays a frequency spectrum of the audio. Just add the jumper across the LPF_out / AMP_in jumper, then connect your speaker to the screw terminals. I found a bunch of appropriate apps by searching for "FFT Spectrum Analyzer". You could probably also search for a guitar tuner app and set your DDS output to correspond to a particular note. The cell phone apps should display a graph showing the power at each frequency, the biggest spike being the audio output of your development board.

  Turn-in

  You may work with a single partner (or alone) to complete this lab. Submit your main.c file on Canvas using the instructions posted there. You should take note of the Rubric that will be used to evaluate your assignment. Please form a group before submitting using the instructions posted on Canvas. You will demonstrate your code at the beginning of lab.