Developing a multi-thread product

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Developing a multi-thread product
Introduction(ENCM491 real time operating
systems in 1 hr)

• M. Smith
• Electrical Engineering, University of Calgary
• Smithmr _at_ ucalgary.ca

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References

1. Understanding GPS Principles and Applications,
   1996, Elliott D. Kaplan
2. Digital Signal Processing A Practical Approach,
   1993, Emmanuel C. Ifeachor, Barrie W. Jervis
3. ADSP-TS101 TigerSHARC and Blackfin Processor
   Programming References, Analog Devices
4. Articles submitted to Circuit Cellar magazine by
   M. Smith, March 2004

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Introduction

• GPS traditionally done with ASIC/Processor
  combination
• Looking at FPGA/DSP combination for low end GPS
  receivers
• Technological interest in software radio
• Cheaper, quicker development cycle.
• Customizations for special applications
• From a talk by Darrell Anklovitch for ENEL619.23

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What is GPS?
Global Positioning System
24 satellite (SV) constellation
Orbiting 20,000 km from the surface of the Earth
in 12 hour cycles
Orbits are set-up to give global coverage 24
hours a day
Need at least 4 satellites in view to calculate a
position
(1)
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GPS Positioning Concepts
(1)

• For now make 2 assumptions
• We know the distance to each satellite
• We know where each satellite is
• Require 3 satellites for a 3-D position in this
  ideal scenario
• Requires 4 satellites to account for local
  receiver clock drift.

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GPS Signal Structure

• Each satellite transmits 2 carrier frequencies
  referred to as L1 (1575 MHz) and L2 (1227 MHz)
• Each carrier frequency is BPSK modulated with a
  unique PRN (pseudo random number) code
• The PRN code on L1 is called CA code (coarse
  acquisition), The PRN code on L2 is called P code
  (precise)
• CA code takes 1 ms for full PRN transmission at
  1MHz chip (bit) rate. P code takes 1.5 s for full
  PRN transmission at 10MHz chip rate
• Also modulated on each carrier is 50 Hz data that
  includes the current position of the satellite

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Determining Time
(1)

• Use the PRN code to determine time
• Use time to determine distance to the satellite
  distance speed of light time

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Algorithms to Find PRN Phase

• Time-domain Cross correlation 1/N ? x1 (n)
  x2(n)
• Coding equivalent to FIR filter, but need to
  filter N sets of data, each shifted by one data
  point looks like a final exam question to me.
• Correlation of perfectly matching signals gives a
  maximum value
• Correlation of 2 random data sequences tends to 0
• PRN code from different satellites are designed
  to correlate to 0.
• Frequency domain correlation 1/N F-1X1(k)X2(k)
  where F-1 is the inverse Discrete Fourier
  Transform and the Xs are the Discrete Fourier
  Transforms of two sequences

D
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Timing

• Frequency Domain 1/N F-1X1(k)X2(k)
• 1024 point FFT (2 NLOG2N)
• 1024 MULTS (N)
• 1024 point INV FFT (NLOG2N)
• Time Domain 1/N ? x1 (n) x2(n)
• 
  
  n 0
• 1024 MACs (N)
• 1024 Phases (N)

30,000 Complex operations
N-1
1,048,576 operations
(N2)
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TigerSHARC -- TS101 and TS201
TS101
TS201
Can do COMPLEX arithmetic
Low-cost version 45 / chip Evaluation boards
950 eacheducational price
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Implementing a multi-thread systemworking on
batch data audio example
Collect N pts _at_44 kHz ? array1 Collect N pts _at_44 kHz ? array2 Collect N pts _at_44 kHz ? array3 Collect N pts _at_44 kHz ? array1 Collect N pts _at_44 kHz ? array2 Process array1 Process array2 Process array3 Process array4 Transmit N pts _at_44 kHz ? array1 Transmit N pts _at_44 kHz ? array2 Transmit N pts _at_44 kHz ? array3
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Implementing a multi-thread system-- audio
example
Collect N pts _at_44 kHz ? array1 Collect N pts _at_44 kHz ? array2 Collect N pts _at_44 kHz ? array3 Collect N pts _at_44 kHz ? array1 Collect N pts _at_44 kHz ? array2 Move array1 ? array4SimulateComplex Move array2 ? array5SimulateComplex Move array3 ? array6SimulateComplex Move array1 ? array4SimulateComplex Transmit N pts _at_44 kHz ? array4 Transmit N pts _at_44 kHz ? array5 Transmit N pts _at_44 kHz ? array6
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Essentially

• Take an audio Talk-through program
• for loop
• Read_a_sample Perform operation
  Write_a_sample
• Turn into 5-threads running under interrupts
• Idle thread
• Initialization thread sets up system, when
  ready launches the other threads then
  activates the first thread
• ReadValueThread,
• ProcessValueThread with simulated Complex
  Algorithm
• WriteValueThread

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Initialization Thread
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Main Thread example
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Need to investigate and understand system
behaviour and limitations
Concept of task priority
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Using real-time audio-threads -- Write
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VDK Status History
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Adding the Initialization thread
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Making the InitializationThread a Boot Thread
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Add the thread programming control
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Avoid the free-running code
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Then add semaphores to control flow
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Essential, if not exact, concept of
multi-threading code

• Do all the initial preparation of the board
• Set up stack
• Set up C/C environment
• Set up processor timer
• Default on Blackfin ADSP-BF533 board every 0.05
  ms (called a TIC) an interrupt occurrs
• Start with an IDLE Thread
• When first TIC occurs the interrupt handler
  will cause the Scheduler ISR to run

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Scheduler ISR

• Save all the registers (etc) of the IDLE thread
  to the IDLE thread context buffer
• Recover all the registers for the scheduler ISR
  context buffer (saved somewhere during the
  initialization procedure)
• There had better be a boot thread otherwise
  system hangs
• VDK tool will not let you build a system without
  at least one boot thread
• Decide which boot thread has the highest
  priority?
• Save all the registers from the Scheduler ISR
  back into the context buffer
• Recover all the registers for the boot thread
  from its context buffer
• Return from ISR
• We have now performed a context switch between
  the IDLE thread and the BOOT thread.

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Boot thread

• The boot thread now executes until the first TIC
  occurs (next ISR call)
• We now switch back into Scheduler
• Save all the registers (etc) of the FIRST BOOT
  THREAD thread to the thread context buffer
• Recover all the registers for the scheduler ISR
  context buffer
• Other threads need launching?
• If there are other Boot threads then launch them
  depending on their priority and the ROUND ROBIN
  scheduling behaviour set by the programmer for
  tasks of equal priority
• If a boot thread has requested that other threads
  need launching then launch those. Unclear when
  the VDKCreateThread operation occurs

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The launching of threads
Looks like threads get launched during a TIC
meaning that anothercontext switch occurs for
each VDKCreateThread ( )
Does that apply to VDKPostSemaphores(
) too?
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Back in scheduler

• Other threads need launching?
• If there are other Boot threads then launch them
  depending on their priority and the ROUND ROBIN
  scheduling behaviour set by the programmer for
  tasks of equal priority
• If a boot thread has requested that other threads
  need launching then launch those.
• Have threads posted semaphores?
• Store them in a posted semaphore table.
• Threads can also post messages but I have not
  worked that out yet
• Are threads pending semaphores?
• Depending on which task is running now, and its
  relative priority to tasks that are pending
  semaphores then either perform context switching
  or not
• How do you handle conflicts? I think that is my
  problem with my final version of Lab. 5 part 3

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Original audio-talk through program
ISR routineChannel to Channel Copy
Multi-tasking versionof ISR routine
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Step 1 Add Talk-through program
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Step 2 Investigate Thread Behaviour
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Step 3 Fix Thread Behaviour
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Step 4 Start migrating code to the various
threads -- Fix ISR behaviour
ORIGINAL
NEW VERSION
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Fix Thread Behaviour

• Initialization thread
• Creates other threads andthen waits for ever
• ReadThread
• Moves my_In Value ? Process Value
• ProcessThread
• Moves Process Value ? ProcessDone Value
• Calls a non-optimizable to nothing routine
  SimulateMoreComplexProcess(cycles_to_waste)
• WriteThread
• Moves ProcessDone Value ? my_Out Value

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Final ReadThread
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Final ProcessThread
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Final WriteThread
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Thread Behaviour depends on Task priorities
ALL TASKS HAVE EQUAL PRIORITY
WRITE TASK HAS HIGHER
PRIORITY THAN PROCESS TASK1) Read Task sends
semaphore to Process Task2) Process Task sends
semaphore to Write Task and starts to waste
cycles3) Scheduler determines that Write Task
can start, send semaphore to Read Task, and
finish and then4) Scheduler lets Process Task
finish (? Why not let Read Task restart?)
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Thread Behaviour

• Useless as system is free running and the
  signals input and output have no relationship to
  samples generated by ISR
• Some samples repeated many times, others are not
• Number of repeats depends on the time that
  ProcessThread takes to execute

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Need to add an ISR semaphore
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Read Thread starts on ISR semaphore
STANDARD APPROACHVDKPostSemaphore( ) DOES
NOT WORK
Blackfin Assembly codelooks like 68K With LINK,
UNLINK, RTS instructions
MACRO
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Many issues still need handling

• How much time is available before losing sound
  quality?
• What are the best priorities for the tasks, and
  does that priority depend on how much time is
  spent in ProcessTask?
• What is the best setting for the task scheduler
  TIC time (based on processor internal timer)?
• Too fast too much time saving / recovering
  registers during task switching
• Too slow problems with interrupts being missed
  or values being over-writtem

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Scheduling based on TIC time
DEFAULT TIC 0.05 ms
TIC 0.005 ms Dont forget TICs are shortened
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Which is the Slower / Faster TIC time?
Question how does the thread status history
reflect sound quality?