Ch2 microcontroller architecture

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CHAPTER 2 – MICROCONTROLLER ARCHITECTURE & ASSEMBLY LANGUAGE PROGRAMMING
PIC18 Microcontroller families
 PIC is a family of modified Harvard architecture microcontrollers made by Microchip
Technology, derived from the PIC1650 originally developed by General Instrument's
Microelectronics Division. The name PIC initially referred to "Peripheral Interface
Controller".
 1989 – Microchip Technology Corporation introduced it’s first 8-bit microcontroller
PIC18 Architecture
 PIC microcontrollers are based on advanced RISC architecture.
 RISC stands for Reduced Instruction Set Computing. In this architecture, the instruction
set of hardware gets reduced which increases the execution rate (speed) of system.
 CPUs use many register to store data temporarily.
 To program in Assembly Language, we must understand the register and architecture
 PIC microcontrollers follow Harvard architecture for internal data transfer
 PIC microcontrollers are designed using the Harvard Architecture which includes:
 Microprocessor unit (MPU)
 Program memory for instructions
 Data memory for data
 I/O ports
 Support devices such as timers

The data RAM file Register
 CPUs use many registers to store data temporarily.
 To program in assembly language, we must understand the registers and architecture of
given CPU and the role they play in processing data.
WREG register
 Registers are used to store information temporarily
 The information could be a byte of data to be processed, or and address pointing to the
data to be fetched
 The WREG is the most widely used register in PIC microcontroller

 The 8-bit WREG register is the most widely used register in the PIC micro controller.
 WREG stands for working register, as there is only one. The WREG register is the same
as the accumulator in other microprocessors.
 The WREG register is used for all arithmetic and logic instructions. To understand the
use of the WREG register, we will show it in the context of two simple instructions:
MOVLW and ADDWL.
MOVLW Instruction
Notice that in MOVLW, the letter L (literal) comes first and then the letter W (WREG),
which means "move a literal value to WREG, " the destination. The following
instruction loads the WREG register with a literal value of 25H (i.e., 25 in hex).
o MOVLW 25H ; move value 25H into WREG
o MOVLW 87H ; load 87H into WREG
ADDLW Instruction
The ADD instruction tells the CPU to add the literal value K to register WREG and put the
result back in the WREG register. Notice that in ADDLW, first comes the letter L (literal)
and then the letter W (WREG), which means "add a literal value to WREG," the
destination
MOVLW 25H ; load 25H into WREG
ADDLW 34H ; add value 34 to WREG
W = W + 34H Executing the above lines results in WREG = 59H (25H + 34H = 59H)
File register (SFRs and GPR)
 File Register (data RAM) is read/write memory used by CPU for data storage, scratch
pad and register for internal use and function
 Divided into two sections:
o SFRs – Special Function Registers
o GPR – General-Purpose Register

Special Function Register (SFRs)
 Special-Function registers are RAM memory locations, their purpose is
predetermined during manufacturing process and cannot be changed.
 Dedicated to specific functions – ALU status, timers, serial communication, I/O ports
ADC etc.
 Fixed by CPU designer at the time of design
 8 bits register

General Purpose Register (GPR)
 8-bit registers
 are a group of RAM locations in the file register that are used for data storage and
scratch pad.
 the space that is not allocated to the SFRs typically used for general-purpose registers
MOVWF Instruction
Notice that in MOVWF, the letter F stands for a location in the file register, while W
means WREG. The MOVWF instructions tells the CPU to move (in reality, copy) the
source register of WREG to a destination in the file register (F)
MOVLW 55H ; WREG = 55H
MOVWF PORTB ; Copy WREG to Port B (Port B = 55H)
MOVWF PORTC ; Copy WREG to Port C (Port C = 55H)
MOVWF PORTD ; Copy WREG to Port D (Port D = 55H)

MOVF Instruction
The MOVF mnemonic is intended to perform MOVFW. It move the contents of file
register into WREG or to itself
MOVF PORTB,W ; move from file register of Port B to WREG
MOVF PORTB,0 ; move content of file register into WREG
MOVF PORTB,1 ; move content of file register into itself
Review Question
1. State five example of SFR
2. What is GPR
3. Write an instruction to move value of 58H into WREG
4. Write an instruction to move value of 58H into PORTB
5. What is the difference between the MOVWF and MOVF instruction
6. Explain each instruction below:
a. MOVLW 55H
b. MOVF PORTB,W
Status Register
 One of the most important register in PIC. Most of PIC has this register.
 8-Bit register. Sometime referred as flag register.
 Only 5 bits of it are used by the PIC18.
o called conditional flags
 The three unused bits are unimplemented and read as 0.

 C (Carry/Borrow Flag) set when an addition generates a carry and a subtraction
generates a borrow.
 DC(Digit Carry Flag):also called Half Carry flag;set when carry generated from Bit3 to
Bit4 an arithmetic operatio.n
 Z(Zero Flag):set when result of an operation is zero.
 OV(Overflow Flag):set when result of an operation of signed numbers goes beyond
seven bits-if the results fall outside 127(0x7F)and -128(0x80).
 N(Negative Flag):set when bit B7 is one of the result of an arithmetic/logic operation.
Example
1. Show the status of the C, DC and Z flag after the addition of 38H and 2FH.
Solution:
38H 00111000
+ 2FH + 00101111
67H 01100111
C = 0 ; DC = 1; Z = 0

2. Show the status of the C, DC and Z flags after the addition of 9CH and 64H in the
following instructions:
MOVLW 9CH
ADDLW 64H
3. Write the instruction for addition operation of 88H and 93H and show the status of the
C, DC and Z flags.
Solution:
MOVLW 88H
ADDLW 93H
Review Question
1. Determine the state of status register after the instruction below:
a. MOVLW 0F5H
ADDLW 0BH
b. MOVLW 9FH
ADDLW 61H

2. Show the status of the C, DC and Z flags after the addition of 9CH and 64H in the
following instructions:
a. MOVLW 9CH
ADDLW 64H
b. MOVLW 67H
ADDLW 99H
c. MOVLW 87H
ADDLW 22H
Data format and directives
 PIC data type has only one data type. 8-bits. Size of register also 8-bits ( 00-FF, or 0-255)
 Data format representation.
o Hex,
o Binary,
o Decimal,
o ASCII
HEX number
 Use h, or H right after the number. Example MOVLW 12H
 Put 0x. Example MOVLW 0x12;
 Put nothing. Eg. MOVLW 12
 Put h in front of number. MOVLW h’12’
Binary number
 Only one way to represent binary numbers,
Example: MOVLW B’00010010’
1 2 in hex
Decimal number
 Two ways to represent. MOVLW D’12’ , (in hex 0x0C)
 Or MOVLW .12

ASCII code
To represent ASCII data in PIC assemble we use the letter A as follows
MOVLW A’2’ ; WREG = 00110010 or 32 in hex ( see ASCII chart)
MOVLW A’c’ ; WREG = 01100011 or 63 in hex (see ASCII chart)
MOVLW ‘9’ ; WREG = 39H another way for ASCII

ASSEMBLER Directive
 Directive give direction to he assembler..
 Example EQU, ORG, END, #INCLUDE, LIST, _CONFIG
EQU (equate)
 Used to define constant value or fixed address.
 Associates a constant number / address label
 Eg. COUNT EQU 0x25
MOVLW COUNT ; WREG = 25
ORG (Origin)
 Indicate the beginning of the address.
 Example ORG 0x00
END
 Important to indicate End of code, last line
LIST
 Indicates to the assembler the specific PIC chip for which the program should be
assembled. Eg. LIST P=18F458
#INLCUDE
 Tell the assembler to use the libraries associated with the specific chip
_CONFIG
 To configure bits for the targeted PIC. Read during the power-up, wrong configuration
makes PIC unusable.

Example code
Instruction set of PIC18
 PIC18F2455/2550/4455/4550 devices incorporate the standard set of 75 PIC18 core
instructions.
 The instruction set is highly orthogonal and is grouped into four basic categories:
o Byte-oriented operations
o Bit-oriented operations
o Literal operations
o Control operations

Arithmetic Instruction and Operation
a. Addition and subtraction
MOVLW 0xF5 ; WREG = F5
ADDLW 0x0B ; WREG = F5 + 0B = 00 and C = 1
MOVLW 0x23 ; WREG = 23H
SUBLW 0x3F ; WREG = 3F - WREG

b. Multiplication
The PIC supports byte-by-byte multiplication only. The byte is assumed to be unsigned
data. The syntax is as follows. After multiplication, the result is in the special function
registers PRODH and PRODL; the lower byte is in PRODL, and the upper byte is in
PRODH.
MULLW K ; W x K 16 bit is result in PRODH:PRODL

c. Addition of BCD data
 BCD stands for binary coded decimal. BCD is needed because in everyday life we use
the digits 0 to 9 for number, not binary or hex number
 Binary representation of 0 to 9 is called BCD . In computer literature, one encounters
two terms for BCD numbers: (i) unpacked BCD, and (ii) packed BCD.
 In unpacked BCD, the lower 4 bits of the number represent the BCD number, and
the rest of the bits are O. Example: "0000 100 I" and "0000 0101" are unpacked BCD
for 9 and 5, respectively. Unpacked BCD requires I byte of memory, or an S-bit
register, to contain it.
 In packed BCD, a single byte has two BCD numbers in it: one in the lower 4 bits, and
one in the upper 4 bits. For example, "0101 1001" is packed BCD for 59H. Only I byte
of memory is needed to store the packed BCD operands. One reason to use packed
BCD is that it is twice as efficient in storing data.
 There is a problem with adding BCD numbers, which must be corrected. The
problem is that after adding packed BCD numbers, the result is no longerBCD. Look
at the following.
MOVLW 0x17
ADDLW 0x28
 Adding these two numbers gives 0011 IIII B (3FH), which is not BCD. A BCD number
can only have digits from 0000 to 1001 (or 0 to 9).
 The result above should have been 17 + 28 = 45 (01000101). To correct this problem,
the programmer must add 6 (0 II 0) to the low digit: 3F + 06 = 45H.
 The same problem could have happened in the upper digit (for example, in 52H +
87H = D9H). Again, 6 must be added to the upper digit (D9H + 60H = 139H) to ensure
that the result is BCD (52 + 87 = 139).
 This problem is so pervasive that most microprocessors such as the PIC 18 have an
instruction to deal with it. In the PIC 18 instruction "DAW" is designed to correct the
BCD addition problem.
 The DAW (decimal adjust WREG) instruction in the PIC 18 is provided to correct the
aforementioned problem associated with BCD addition.
 The mnemonic "DAW" works only with an operand in the WREG register. The DAW
instruction will add 6 to the lower nibble or higher nibble if needed; otherwise, it will
leave the result alone. The following example will clarify these points


Logic Instruction and Bit Manipulation
 Apart from I/O and arithmetic instructions, logic instructions are some of most widely
used instructions.
 Boolean logic instructions such as AND, OR, Exclusive-OR (XOR), and complement.
 Logical operations are useful for looking for array elements with certain properties (e.g.,
divisible by power of 2) and manipulating I/O pin values (e.g., set certain pins to high,
clear a few pins, toggle a few signals, and so on).
 The logical instruction allow user to perform AND, OR, exclusive-OR and complementing
on 8-bit numbers.
AND
ANDLW K ; WREG = WREG AND K
Example : Show the result of the following
MOVLW 0x35h
ANDLW 0x0Fh

OR
IORLW K ; WREG = WREG OR K
MOVLW 0x04h
IORLW 0x30h
EX-OR
XORLW K ; WREG = WREG XOR K
MOVLW 0x54h
XORLW 0x78h

COMF (Compliment file register)
This instruction complements the contents of a file register. The complement action changes
the 0s to 1s and the 1s to 0s. This is also called 1’s complement
Bit Manipulation
 BCF – Bit Clear File register (set the bit: bit = 1)
 BSF – Bit Set File register (clear the bit: bit = 0)
 BTG – Bit Toggle File register (complement the bit)
 BTFSC – Bit Test File register, skip if clear (skip next instruction if bit = 0)
 BTFSS– Bit Test File register, skip if set (skip next instruction if bit = 1)
 RRNCF – Rotate right f (no carry)
 RLNCF – Rotate left f (no carry)
 RRCF – Rotate right f through carry
 RLCF – Rotate left f through carry
Example

Review Question
1. Explain each of the register below:
a. WREG Register
b. File Register
c. Status Register
2. Include directive is used to ……………
3. EQU is used to ………………………………
4. Illustrate the result after the following code is executed

Ch2 microcontroller architecture

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