EE 319K Introduction to Embedded Systems

1
EE 319KIntroduction to Embedded Systems

• Lecture 8 Periodic Timer Interrupts,
  Digital-to-Analog Conversion, Sound, Lab 6

2
Agenda

• Recap
• PLL
• Data structures
• FSMs, linked structure
• Interrupts
• Agenda
• Periodic Interrupts
• Digital to Analog Conversion
• Nyquist Theorem
• Sound generation

SysTick ISR
Output one value to DAC
3
Lab 6 Lab 8 Lab 9
77 total
Lab 10
INTERRUPT VECTORS
4
Nested Vectored Interrupt Controller (NVIC)

• Hardware unit that coordinates among interrupts
  from multiple sources
• Define priority level of each interrupt source
  (NVIC_PRIx_R registers)
• Separate enable flag for each interrupt source
  (NVIC_EN0_R and NVIC_EN1_R)
• Interrupt does not set I bit
• Higher priority interrupts can interrupt lower
  priority ones

5
NVIC Registers

• High order three bits of each byte define
  priority

6
NVIC Interrupt Enable Registers

• Two enable registers NVIC_EN0_R and NVIC_EN1_R
  
• Each 32-bit register has a single enable bit for
  a particular device
• NVIC_EN0_R control the IRQ numbers 0 to 31
  (interrupt numbers 16 47)
• NVIC_EN1_R control the IRQ numbers 32 to 47
  (interrupt numbers 48 63)

7
Interrupt Rituals

• Things you must do in every ritual
• Initialize data structures (counters, pointers)
• Arm (specify a flag may interrupt)
• Configure NVIC
• Enable interrupt (NVIC_EN0_R)
• Set priority (e.g., NVIC_PRI1_R)
• Enable Interrupts
• Assembly code CPSIE I
• C code EnableInterrupts()

8
Interrupt Service Routine (ISR)

• Things you must do in every interrupt service
  routine
• Acknowledge
• clear flag that requested the interrupt
• SysTick is exception automatic acknowledge
• Maintain contents of R4-R11 (AAPCS)
• Communicate via shared global variables

9
Interrupt Events

• Respond to infrequent but important events
• Alarm conditions like low battery power
• Error conditions
• I/O synchronization
• Trigger interrupt when signal on a port changes
• Periodic interrupts
• Generated by the timer at a regular rate
• Systick timer can generate interrupt when it hits
  zero
• Reload value frequency determine interrupt rate
  

10
Synchronization
Use global variable to communicate

• Semaphore
• One thread sets the flag
• The other thread waits for, and clears
• Mailbox to be presented for Lab 8
• FIFO queue to be presented for Lab 9

11
Periodic Interrupts

• Data acquisition samples ADC
• Lab 8 will sample at a fixed rate
• Signal generation output to DAC
• Audio player (we use the Systick interrupt to
  write samples out periodically in Lab 6)
• Communications
• Digital controller
• FSM
• Linear control system (EE362K)

Demo PeriodicSystickInts starter C code
12
Digital Representation of Analog Signals

• Digitization Amplitude and time quantization

13
Conversion from Digital to Analog

• Range
• 0 to 3.3V
• Resolution
• 3.3V/15 0.22V
• Precision
• 4 bits
• 16 alternative
• Speed
• Monotonic

http//users.ece.utexas.edu/valvano/Volume1/E-Boo
k/C13_Interactives.htm
14
Digital ? Analog Conversion
Sampled at a fixed time, Dt
15
Digital ? Analog Conversion
Digital in voltage and in time
fs 1/Dt Signal has frequencies 0 to ½ fs
16
Digital-to-Analog Converter (DAC)

• Binary Weighted DAC
• One resistor for each bit of output
• Resistor values in powers of 2

17
3 bit DAC
R2 10 k?
R1 20 k?
R0 40 k?
n PB2 PB1 PB0 kohm equation Vout (V)
0 0 0 0 0.000
1 0 0 3.3 R2R1 6.67 3.3(R1R2)/(R0R1R2) 0.471
2 0 3.3 0 R2R0 8.00 3.3(R2R0)/(R1R2R0) 0.943
3 0 3.3 3.3 R1R0 13.33 3.3R2/(R2R1R0) 1.414
4 3.3 0 0 R1R0 13.33 3.3(R1R0)/(R2R1R0) 1.886
5 3.3 0 3.3 R2R0 8.00 3.3R1/(R1R2R0) 2.357
6 3.3 3.3 0 R2R1 6.67 3.3R0/(R0R2R1) 2.829
7 3.3 3.3 3.3 3.300
18
Other Types of DACs

• R-2R Ladder DAC
• Binary weighted cascading ladder
• Improved precision owing to ability to select
  resistors of equal value

19
DAC Performance

• Resolution, range, precision
• Maximum sampling frequency
• Monotonicity
• Input increase causes output increase (always)

20
Resistor Network for 4-bit DAC
R3
R2
R1
R0
21
Dynamic testing
22
Sound

• Loudness and pitch
• Controlled by amplitude and frequency
• Humans can hear from about 25 to 20,000 Hz.
• Middle A is 440 Hz
• Other notes on a keyboard are determined
• 440 2N/12, where N is no. of notes from middle
  A.
• Middle C is 261.6 Hz.
• Music contains multiple harmonics

23
Tempo
Tempo defines note duration Quarter note 1
beat 120 beats/min gt ½ s duration
24
Chord

• Two notes at the same time
• Superimposed waveforms
• 262 Hz (low C) and a 392 Hz (G)

25
Instrument Characteristics
Waveform shape of a trumpet sound
Plucked string signal with envelope
26
Synthesizing Digital Music

• Nyquists Sampling Theorem
• We can reproduce any bandlimited signal from its
  samples if we sample correctly and at a
  frequency, fs, that is at least twice the highest
  frequency component of the signal, fmax.
• Where do we get the samples?
• We could sample a series of musical tones
• We can compute the samples

27
Synthesizing Digital Music (cont.)

• What is a musical tone?
• A sinusoid of a particular frequency
• Notes vary by twelfth root of 2 1.059
• What would the samples be?
• Fixed point numbers
• How do we generate a sinusoid?
• Output appropriate digital values via a resistor
  network that effectively produces an
  pseudo-analog signal
• What about frequency?
• Employ a programmable timer to tell us when to
  output the next value

28
Synthesizing Digital Music (cont.)

• 440 Hz sine wave generated by 6-bit DAC

Frequency spectrum
29
Music Generation Lab 6

• Objectives
• Employ LM4F/TM4C to generate appropriately scaled
  digital outputs at a specified frequency
• Three frequencies are required
• Frequencies are to be determined by switch
  settings
• Four digital outputs are inputs to a resistor
  network that serves as a digital-to-analog
  converter (DAC)
• Four output bits gt 16 levels

30
Music Generation (cont.)

• DAC hardware
• Employ least significant four bits of a GPIO port
• Arrange resistor network in 1, 2, 4, 8 sequence
• Each port bit can assume digital levels of 0 and
  3.3 V
• Ports are current limited max 8 mA

R3
R2
R1
R0
31
Music Generation (cont.)

• DAC software
• Interactions via device drivers
• Two device driver functions required
• void DAC_Init(void) // initializes the
  device
• void DAC_Out(unsigned char data) //
  transfers data to device
• (Device driver provides the functions associated
  with the device but hides the detailed actions
  necessary to implement the functions.)

32
Music Generation (cont.)

• Interpretation of data
• Note has three parameters
• Amplitude (loudness)
• Frequency (pitch)
• Duration
• Amplitude is a digitally approximated sinusoid
• Sinusoid varies between 0 and 3.3 volts
• Frequency is selected by switches
• Four states stop, note_1, note_2, and note_3
• Duration is period switch(es) activated

33
4-bit Sinusoid Table
SinTab 8,9,11,12,13,14,14,15,15,15,14
14,13,12,11,9,8,7,5,4,3,2
2,1,1,1,2,2,3,4,5,7
32 value sinusoid
34
Musical Notes
35
Tone Generation

• unsigned long I
• // 4-bit 32-element sine wave
• const uint8_t wave32
• 8,9,11,12,13,14,14,15,15,15,14
• 14,13,12,11,9,8,7,5,4,3,2
• 2,1,1,1,2,2,3,4,5,7
• For a 440Hz tone
• Assume a bus clock frequency of 50 MHz
• SysTick count every 20ns
• Each cycle of the 440 Hz sinusoid requires
• (50106 counts/s)/440 Hz 113636.36 SysTick
  counts
• Each cycle consists of 32 values each of
  duration
• 113636.36 interrupt counts/32 values 3551
  SysTick counts/value
• DAC values change every 71.02 us

SysTick ISR
Output one value to DAC
36
Lab 6 ISR

• Each Systick interrupt
• Output one value from the array to DAC
• Increment index to array (wrap back to zero)
• In main program
• If a switch is pressed set SysTick period (arm)
• If no switches are pressed then disarm

SysTick ISR
Output one value to DAC
37
Other Instruments
// 6-bit 64-element bassoon wave const uint8_t
Bassoon64 33,37,37,36,35,34,34,33,31,30,2
9, 30,33,43,58,63,52,31,13,4,5,10,16,
23,32,40,46,48,44,38,30,23,17,12,11,
15,23,32,40,42,39,32,26,23,23,24,25,
25,26,29,30,31,32,34,37,39,37,35,34,
34,34,33,31,30 // 6-bit 64-element guitar
wave const uint8_t Guitar64
20,20,20,19,16,12,8,4,3,5,10,17,
26,33,38,41,42,40,36,29,21,13,9,
9,14,23,34,45,52,54,51,45,38,31,
26,23,21,20,20,20,22,25,27,29,
30,29,27,22,18,13,11,10,11,13,13,
13,13,13,14,16,18,20,20,20