ELN5622 Embedded Systems Class 3 Spring, 2003

1
ELN5622Embedded SystemsClass 3Spring, 2003
Kent Orthnerkorthner_at_hotmail.com
2
Assembly Language Programming
3
Programming Conventions

• Conventions
• Rules that a designer follows to make his/her
  program easier to understand, communicate to
  other, debug, and less prone to mistakes.
• Includes
• Comment conventions
• Naming conventions
• Drawing Conventions

4
Assembly Language Style

• Program Header
• What the program does
• Authors Name
• File Name
• Date
• Version
• History
• Function Header
• What the function does/How it works
• Authors Name
• Date
• History

5
Assembly Language Style

• Equates vs. In-line constants
• Use constants wherever possible
• Easier to understand,
• Easier to fix
• Easier to maintain
• Location of Equate Statements
• All together
• Point-of-use

6
Assembly Language Style

• Types of Equates
• System Equates
• System Functions
• Location of I/O Registers
• Port Addresses
• Constant Equates
• CR/LF, End of String, NIL
• Memory Map Equates
• Program Location
• Flash Memory Location
• Stack Pointer Location

7
Assembly Language Style

• Constant Data Definitions
• Tables, Strings, Etc.
• Located in ROM
• Often best to have at the end of the program to
  lessen the chance of them executed
• Variable Data Definitions
• System-wide Variables
• Located in RAM
• Non-volatile ROM Data
• Database for user settings
• Maintained when power is shut off

8
Assembly Language Style

• Indenting
• Not commonly used for assembly programs
• Can ease understanding by making program flow
  more obvious.
• Up to the individual designer.
• Use Boilerplate files
• All files use the same flow
• Saves Typing
• Less potential Mistakes

9
Assembly Language Style

• Commenting Style
• Headers for functional blocks
• Headers per line
• Goal To make it so the person maintaining the
  code can understand the program.

10
Assembly Language Style

• Naming Conventions
• Very important to prevent errors and make code
  easy to understand.
• Examples
• LDX SP
• LDB X,p_operand Stack variable
• LDA g_timer Global variable
• JSR s_timercheck Subroutine
• LDB X,Operand Stack variable
• LDA G_TimerVar Global variable
• JSR TIMERCHECK Subroutine

11
Assembly Language Style

• Pseudocode
• Self-commenting, less prone to error
• Makes it natural to
• design first with pseudo code
• Implement second with assembly language
• Get Temp
• If Temp gt MaxAllowed
• Turn valve off
• Else
• turn the valve on
• End if

12
Assembly Language Style

• Pseudocode
• Get Temp
• LDAA TEMP_PORT
• If Temp gt MaxAllowed
• IF_TEMPMAX CMPA MaxAllowed A-MaxAllowed
• BLE EL_TEMPMAX Branch if lt 0
• Turn valve off
• LDAA ValveOff
• STAA CONTROL_PORT
• BRA EI_TEMPMAX
• Else turn the valve on
• EL_TEMPMAX LDAA ValveOn
• STAA CONTROL_PORT
• End if
• EI_TEMPMAX ltNext Instgt

13
Assembly Language Style

• While-Do Loop
• While Temperatue gt MaxAllowed
• WH_MAXALL CMPA MaxAllowed A-MaxAllowed
• BLS EW_MAXALL Branch if lt 0
• Do
• End While
• BRA WH_MAXALL
• EW_MAXALL ltNext Instgt

14
Assembly Language Style

• Repeat-Until Loop
• Repeat
• RP_SWSTATE
• Until SwitchState END_STATE
• LDAA SwitchState
• CMPA END_STATE
• BNE RP_SWSTATE
• ltNext Instgt

15
Assembly Language Style

• For Loop
• For (ICOUNT, I--, I0)
• LDAA COUNT
• Begin
• FOR_LOOP PUSH A
• . . .
• Next
• PULA
• DECA
• BNE FOR_LOOP
• ltNext Instgt

16
Anatomy of an Embedded Program
17
Embedded System Characteristics

• A computing system embedded within a device.
• A system intended for a single purpose, which
  includes a general purpose processor.
• Often used for
• providing user control over a product
• to observe or control something in the real
  world (i.e. analog)

18
Embedded System Startup

• What is the first thing the user expects when an
  embedded system starts up?
• Begin application execution.
• When should the software in an embedded system
  finish?
• It shouldn't. It should (normally) run until the
  power is turned off.

19
Embedded Program Flow

• Environment setup
• System Equates
• Constant Equates
• Memory Map Equates
• Initialization
• Set stack pointer
• Special Register Setup
• Initialize Tasks
• Enable Interrupts
• Main Loop
• Execute Applications

20
Embedded Program Flow
Initialize Stack Pointer
Special Register Setup
Initialize Task 1
Initialize Task 2
Initialize Task 3
Enable Interrupts. ( Go! )
Execute Task 1
Execute Task 2
Execute Task 3
21
Environment Setup

• Types of Equates
• System Equates
• System Functions
• Location of I/O Registers
• Port Addresses
• Constant Equates
• CR/LF, End of String, NIL
• Memory Map Equates
• Program Location
• Flash Memory Location
• Stack Pointer Location

22
Execution Start

• 68HC11 jumps to the 'Reset Vector' located at
  0xFFFE, 0xFFFF
• PC lt- M(0xFFFE, 0xFFFF)

23
Initialization

• Initialize Stack Pointer
• Interrupts
• Functions
• Temporary Storage
• Special Register Setup
• Memory Mapping registers
• Determine the location of RAM the register
  block within the memory map.
• Can only be written at Startup

24
Initialization

• Special Register Setup (Continued)
• System Configuration Registers
• A/D Powerup
• Clock Select
• IRQE Edge-Sensitive Select
• Clock Monitor Enable
• COP Timer Rate
• I/O Control Registers
• Timer Registers
• Interrupt Mask Registers
• SCI SPI Registers
• ADC/DAC Control Registers

25
Initialization

• Task Initializatoin
• Set initial state for state machines
• Set initial values for task variables
• Pre-compute tables where necessary
• Clear / pre-set buffers

26
Main Loop Architecture

• 3 buttons need to be checked at least 10 times a
  second
• repeat
• for (i2 i-- i0)
• Checkbutton(i)
• end for
• until ( false )

27
Main Loop Architecture

• 3 buttons need to be checked at least 10 times a
  second, and 3 pins must be set 5 times a second.
• repeat
• for (i2 i-- i0)
• CheckButton(i)
• end for
• if (FifthSecondIsUp)
• for (i2 ilt3 i--)
• SetPin(i)
• end for
• end if
• until ( false )

28
Main Loop Architecture

• 3 buttons need to be checked at least 10 times a
  second, and 3 pins must be set 5 times a second.
• repeat
• for (i2 i-- i0)
• CheckButton(i)
• end for
• if (FifthSecondIsUp)
• for (i2 ilt3 i--)
• SetPin(i)
• end for
• end if
• until ( false )

29
Main Loop Architecture

• And if there are a couple more things to be done,
  all at different times
• repeat
• CheckButtons()
• SetPins()
• ControlMotors()
• SetDisplay()
• until ( false )

30
Cyclical Executive

• Architecture
• Repeat
• Task1 ()
• Task2 ()
• Task3 ()
• Until ( false )

31
Cyclical Executive

• Architecture
• Repeat
• MAINLOOP
• Task1 ()
• JSR TASK1
• Task2 ()
• JSR TASK2
• Task3 ()
• JSR TASK3
• Until ( false )
• JMP MAINLOOP

32
Cyclical Executive

• Adequate for simple applications.
• Other Names
• Round Robin Executive
• Round Robin Kernel
• Super Loop
• Rules
• Tasks may not employ busy waiting
• Tasks must do their work quickly and return to
  the main loop so that other tasks can run
• Tasks must save their place by using a state
  variable

33
Cyclical Executive

• Advantages
• Small
• Compact
• Easy to use
• Easy to understand
• Disadvantages
• No priorities
• Polls for events
• Timing responsibility is put on the programmer

34
68HC11 On-Chip PeripheralsParallel I/O
35
Parallel I/O

• Three Functions per pins
• Output (1 or 0)
• Wired-or output (1 or 0)
• Input
• Up to 40 I/O pins.
• Each pin can be used as I/O or another function.
• Some pins are fixed-direction, some are
  bidirectional.

36
Parallel I/O

• Port A
• Shared with Timer Pulse Accumulator
• PA7 Bidir
• PA6-PA3 Output Only
• PA2-PA0 Input Only
• Port B
• Shared as Expanded bus Address pins
• Output Only
• Port C
• Shared as Expanded bus Data / Multiplexed Address
  pin.
• Bidirectional

37
Parallel I/O Registers

• PORTn
• 8-bit register for each port
• Reads return the level of the pin itself for
  input/bidirectional pins, or the state of the
  logic inside the output buffer at the pin output.
• Writes cause the data to be latched, so it can be
  used for output operation. (not input pins.)
• PORTCL
• Special Port C register for handshake writes.

38
Parallel I/O Registers

• DDRn
• Register for each bidirectional pin.
• Controls the direction of data flow.
• 0 Input
• 1 Output
• Subsystem function overrides this pin function
• Ie. SCI Tx/Rx
• Control Registers
• Determine if the pin is going to be used as
  General Purpose I/O, or as a subsystem pin.
• Determines if an output will be a wired-or (open
  collector) output.

39
Handshaking

• Ports BC, with STRA input STRB output
• 3 modes
• Simple Strobe (default)
• Port B Simple Strobe Output
• Port C Simple Strobe Input
• Full-input Handshake
• Full-output Handshake
• Configured with PIOC register

40
Handshaking - Simple Mode

• Port B Output with STAB
• Port C Input with STAA

41
Handshaking - Full-Input Handshake

• Port C Input with both STAA STAB

42
Handshaking - Full-Output Handshake

• Port C Output with both STAA STAB