Instructor: Nachiket M' Kharalkar

1
Introduction to Microcontrollers

• Instructor Nachiket M. Kharalkar
•  
• Lecture 8
• Date 06/18/2007
• E-mail knachike_at_ece.utexas.edu
•  

2
Todays Agenda

• Exam 1 June 25th
• Stack
• Assembly language development process
• Switch LED interface
• Quiz 1

3
Lab 2 demo
4
Shift operations

• The N bit is set if the result is negative.
• The Z bit is set if the result is zero.
• The V bit is set on a signed overflow, change in
  the sign bit.
• The C bit is the carry out after the shift.
• asla RegARegA2 (same as lsla)
• aslb RegBRegB2 (same as lslb)
• asld RegDRegD2 (same as lsld)
• lsla RegARegA2 (same as asla)
• lslb RegBRegB2 (same as aslb)
• lsld RegDRegD2 (same as asld)
• asra RegARegA/2 (signed)
• asrb RegBRegB/2 (signed)
• asrd RegDRegD/2 (signed)
• lsra RegARegA/2 (unsigned)
• lsrb RegBRegB/2 (unsigned)
• lsrd RegDRegD/2 (unsigned)
• rola RegArol(RegA) (C?A7??A0?C)
• rolb RegBrol(RegB) (C?B7??B0?C)
• rora RegAror(RegA) (C?A7??A0?C)

5
Friendly software

• Set bit 2 of port T
• ldaa 00000100
• oraa PTT
• staa PTT
• Clear bit 5 of port AD
• ldaa 11011111
• anda PTAD
• staa PTAD

6
Subroutines and the stack

• classical definition of the stack
• push saves data on the top of the stack,
• pull removes data from the top of the stack
• stack implements last in first out (LIFO)
  behavior
• stack pointer (SP) points to top element
• many uses of the stack
• temporary calculations
• subroutine (function) return addresses
• subroutine (function) parameters
• local variables

7
ldaa 1 psha
lds 4000
ldaa 2 psha
ldaa 3 psha
pulb
8

• The push and pull instructions
• psha push Register A on the stack
• pshb push Register B on the stack
• pshx push Register X on the stack
• pshy push Register Y on the stack
• des S S-1 (reserve space)
• pula pull from stack into A
• pulb pull from stack into B
• pulx pull from stack into X
• puly pull from stack into Y
• ins SS1 (discard top of stack)
• The following are important transfer instructions
• tsx transfer S to X
• tsy transfer S to Y
• txs transfer X to S

9
We use the term subroutine all functions or
procedures

• whether or not they return a value
• develop modular software
• called by either bsr or jsr
• subroutine returns using rts

10

• org 4000
• main lds 4000 initialize stack
• clra
• loop bsr sub branch to subroutine
• bra loop
• Purpose increment a number
• Input RegA, range 0 to 255
• Output RegAInput1
• Errors Will overflow if input is 255
• sub inca adds one to Input
• rts
• org fffe
• fdb main

11
Execution of the bsr instruction

• Opcode fetch R 0x4004 0x07 from EEPROM
• Operand fetch R 0x4005 0x02 from EEPROM
• Stack store lsbW 0x3FFF 0x06 to RAM
• Stack store msbW 0x3FFE 0x40 to RAM

12
The stack before and after execution of the bsr
instruction
13
Execution of the rts instruction

• Opcode fetch R 0x4009 0x3D from EEPROM
• Stack read msb R 0x3FFE 0x40 from RAM
• Stack read lsb R 0x3FFF 0x06 from RAM

14
The stack before and after execution of the rts
instruction
15
Assembly language development process
16
Assembly steps

• The first pass
• check for syntax errors
• size of each line of object code
• create the symbol table
• The second pass
• check for syntax errors, errors reported in
  listing file
• create object code, S19 file if needed for real
  6812
• recreate the symbol table, phasing error if
  different
• put object code, details and source into listing
  file
• add symbol table to the end of the listing file
• A symbol table is a mapping between symbolic
  names
• PTT equals 0240

17
Errors that occur during the assembly process

• 1) Label previously defined error label occurs
  multiple times
• 2) Undefined opcode error operation does not
  exist
• 3) Operand error syntax error within the operand
•      expression error
•      undefined symbol in expression
•      addressing mode error
• size of allocated storage too big, e.g., ds
  1001000
• 4) Phasing error value of symbol changes from
  pass1 to pass2
• 5) Can't program address error
• 6) Branch too far error destination address is
  too far away
• 7) Illegal string termination e.g., "Hello
  World!