Instructor: Nachiket M' Kharalkar

1
Introduction to Microcontrollers

• Instructor Nachiket M. Kharalkar
•  
• Lecture 2
• Date 06/04/2007
• E-mail knachike_at_ece.utexas.edu
•  

2
Todays Agenda

• Things to buy
• Recap
• Arithmetic operations
• Promotion, Ceiling Floor   
• Overflow, drop-out, truncation roundoff
• 9S12C32 architecture
• Few basic assembly language instructions   

3
Things to buy

• Simple digital multi-meter
• Wire strippers

4
Recap
Store 03E8 in big and little/small endian format
Freescale Store MSB first
Intel Store LSB first
Store 12345678
5
Number formats in TExaS

• h 8-bit unsigned hexadecimal
• h 8-bit signed hexadecimal (h and -h are the
  same format)
• d 8-bit unsigned decimal
• d 8-bit signed decimal (d and -d are the same
  format)
• b 8-bit unsigned binary
• b 8-bit signed binary (b and -b are the same
  format)
• 2d two unsigned decimal numbers
• H 16-bit unsigned hexadecimal
• H 16-bit signed hexadecimal (H and -H are the
  same format)
• D 16-bit unsigned decimal
• D 16-bit signed decimal (D and -D are the same
  format)
• B 16-bit unsigned binary
• B 16-bit signed binary (B and -B are the same
  format)

6
MC9S12C32 16-Bit Microcontroller

• 16-bit HCS12 CPU
• Multiplexed External Bus
• 32K byte Flash EEPROM
• 2.0K byte RAM
• 8-channel Analog-to-Digital Converter, 10-bit
  resolution
• 1M bit per second, CAN 2.0 A, B software
  compatible module
• 8 channel 16-bit timer module
• 6-channel 8-bit Pulse Width Modulator

7
9S12C32 registers
8
Condition code register (CC or CCR)
C set after an unsigned add if the answer is
wrong V set a signed add if the answer is wrong
Why do we care about these bits?
9
Unsigned number wheel
22464
9664
C bit Set
C bit Cleared
The carry bit, C, is set after an unsigned
addition or subtraction when the result is
incorrect.
10
Trick
Whenever you directly add 2 unsigned 8-bit
numbers and the answer is more than 255, C bit is
SET. The final answer is obtained by subtracting
256 from the direct addition. For example 255
5 260, C 1 and the actual answer is 260-256
4
11
Unsigned number wheel
32-64
160-64
C bit Cleared
C bit Set
The carry bit, C, is normally set when we cross
over from 255 to 0 while adding or cross over
from 0 to 255 while subtracting.
12
Trick
Whenever you directly subtract 2 unsigned 8-bit
numbers and the answer is negative, C bit is SET.
The final answer is obtained by adding 256 to the
direct subtraction. For example 5 - 255
-250, C 1 and the actual answer is -250256 6
13
Signed number wheel
9664
-3264
V bit Cleared
V bit Set
The overflow bit, V, is set after a signed
addition or subtraction when the result is
incorrect.
14
Signed number wheel
-96-64
32-64
V bit Cleared
V bit Set
The overflow bit, V, is normally set when we
cross over from 127 to -128 while adding or cross
over from -128 to 127 while subtracting.
15
Addition summary

• Let the result R be the result of the addition
  AB.
• N bit is set
• if unsigned result is above 127 or
• if signed result is negative.
• N R7
• Z bit is set if result is zero
• V bit is set after a signed addition if result is
  incorrect
• V
• C bit is set after an unsigned addition if result
  is incorrect
• C

16
Subtraction summary

• Let the result R be the result of the addition
  A-B.
• N bit is set
• if unsigned result is above 127 or
• if signed result is negative.
• N R7
• Z bit is set if result is zero
• V bit is set after a signed subtraction if result
  is incorrect
• V
• C bit is set after an unsigned subtraction if
  result is incorrect
• C

17
P1
When you perform (32 129) in an 8-bit system
what is the status of the NZVC bits? Answer
159 NZVC 1011
18
Unsigned promotion

• Promotion involves increasing the precision of
  the input numbers, and performing the operation
  at that higher precision
• decimal 8-bit 16-bit
• 224 1110,0000 0000,0000,1110,0000
• 64 0100,0000 0000,0000,0100,0000
• 288 0010,0000 0000,0001,0010,0000

19
Unsigned ceiling floor
20
Signed promotion
To promote a signed number, we duplicate the sign
bit decimal 8-bit 16-bit
-96 1010,0000 1111,1111,1010,0000 -
64 -0100,0000 -0000,0000,0100,0000 -160
0110,0000 1111,1111,0110,0000
21
Signed ceiling floor
22
Common errors

• Overflow result exceeds the range of number
  system
• promotion and ceiling/floor.
• Drop-out occurs after an integer right shift or a
  divide
• intermediate result looses the information
• divide last when performing multiple calculations
• 100(N/51) ? (100N)/51

23
Common errors

• Roundoff is the error that occurs as a result of
  an arithmetic operation. discarding the least
  significant bits of the product
• multiplication of two 64-bit mantissas yields a
  128-bit product.
• Roundoff during addition and subtraction
• two n-bit numbers are added the result is n1
  bits
• Truncation is the error that when a number is
  converted from one format to another.
• For example when a real number is converted to a
  short real format

24

• Architecture defines how the various computer
  components are connected.
• The 6812 has a von Neumann architecture (I/O
  devices are connected to the processor in a
  manner similar to memory).

25
The term von Neumann architecture (memory mapped
I/O) refers to a computer design model that uses
a single storage structure to hold both
instructions and data.
26
In an Isolated I/O system, the control bus
signals that activate the I/O are separate from
those that activate the memory devices
27
9S12C32
28
http//www.technologicalarts.com/myfiles/jpeg/9s12
c32block.jpg
29
4 basic components of a 6812 processor
30
Bus interface unit (BIU)

• The bus interface unit (BIU)
• reads data from the bus during a read cycle,
• writes data onto the bus during a write cycle.
• always drives the address bus and the control
  signals
• effective address register (EAR) contains the
  data address

31
Control unit (CU)

• The control unit (CU)
• orchestrates the sequence of operations
• issues commands to ALU, BIU
• instruction register (IR) contains the op code

32
Registers

• The registers
• high-speed storage devices located in the
  processor
• do not have addresses like regular memory
• specific functions explicitly defined by the
  instruction
• Accumulators contain data (A, B, D)
• Index registers contain addresses (X, Y)
• Program counter (PC) points to instruction to
  execute next
• Stack pointer (SP) points to the top element on
  the stack
• context switch when calling and returning from a
  function
• pass parameters
• save temporary information
• implement local variables
• Condition code register (CCR) the status of the
  previous operation

33
6812 has 6 registers
34
Condition code register (CCR)
35
Arithmetic logic unit (ALU)

• The arithmetic logic unit (ALU)
• Arithmetic operations
• Addition
• Subtraction
• Multiplication
• Division
• Logic operations
• And
• Or
• Exclusive or
• Shift

36
Memory map of 9S12C32
37
Basic 8-bit load instructions

• The 8-bit load instructions
• ldaa w RegAw
• ldaa U RegAU
• ldab w RegBw
• ldab U RegBU
• Condition code bits are set
• N result is negative NR7
• Z result is zero
• V signed overflow V0

38
Basic 8-bit store instructions

• The 8-bit store instructions
• staa U URegA
• stab U URegB
• Condition code bits are set
• N result is negative NR7
• Z result is zero
• V signed overflow V0

39
8-bit addition

• These instructions work for both signed and
  unsigned data.
• adda w RegARegAw
• adda U RegARegAU
• addb w RegBRegBw
• addb U RegBRegBU
• Condition code bits are set after RXM,
• N result is negative NR7
• Z result is zero
• V signed overflow
• VX7M7not(R7)not(X7)not(M7)R7
• C unsigned overflow
• CX7M7M7not(R7)not(R7)X7

40
8-bit subtraction

• These instructions work for both signed and
  unsigned data.
• cmpa w RegA-w
• cmpa U RegA-U
• cmpb w RegB-w
• cmpb U RegB-U
• suba w RegARegA-w
• suba U RegARegA-U
• subb w RegBRegB-w
• subb U RegBRegB-U
• tsta RegA-0
• tstb RegB-0
• Condition code bits are set after RX-M
• N result is negative NR7
• Z result is zero
• V signed overflow
• VX7not(M7)not(R7)not(X7)M7R7
• C unsigned overflow