All
the questions asked below are related to the programming of PIC16F877A.
- If any
line of the program is added with semicolon in front, how does the
assembler interpret it?
Assembler will ignored it
- Write a
short program to add the content in memory address 0x20 with 5, and save
the result in 0x22 address.
LIST
P=16F877A
#INCLUDE
“P16F877A.INC”
__CONFIG
_CP_OFF & _WDT_OFF & _XT_OSC & _PWRTE_ON
P1
EQU 0X20
P2
EQU 0X22
ORG 0X00
MAIN
NOP
MOVF P1,W
ADDLW 0X5
MOVWF P2
SLEEP
END
- In what
condition instruction of “GOTO
$” or “SLEEP”, is normally put in
the end of the program?
If a program does not run
in an infinite loop the last instruction must be SLEEP.
(or goto $)
- “END”
at the end of the program is a directive or instruction?
Directive.
- #include
is normally put at the beginning of a program, what will happen during the compilation if it is not included in the program?
In
PIC16F877A.inc file, all the parameters used in programming are well defined.
If the program did not include PIC16F877A.inc file, compiler would not be able
to translate the parameters in the program, translation to machine code will
fail.
- List
down four possible fields on the source code line
There are four possible fields on a source code line:
·
The label is the
address in the program memory where the instruction is stored. A label always starts
in the first position of the line.
·
The mnemonic of
the instruction (for example MOVF, BSF).
·
Operands which
always belong to the instruction.
·
The comment is
always preceded by a semicolon.
- Instruction
set of PIC16F877A is consisted of 36 instructions. It can be divided into
three categories, namely, byte-oriented
file register operations, bit-oriented file register operations and literal
and control operations. Refer to datasheet, show the differences among
these categories by giving an examples for each of it.
Byte-Oriented File
Register Operations
f = Register file address
(0x00 to 0x7F)
Destination select; d = 0:
store result in W,
d = 1: store result in file
register f.
Default is d = 1.
Example:
ADDWF f, d
Bit-Oriented
File Register Operations
b = 3-bit bit address
f = 7-bit file register
address
Example: BCF
f, d
Literal and Control Operations
Example: ADDLW k
k = Literal field, constant data or label
- Write a “one millisecond delay” program
If
the fosc = 4MHz, one instruction cyle = 1us
(refer
to LED.VBB)
DELAY1 ;1ms
MOVLW .250
MOVWF DelayCounter
DELAY.Loop
NOP
DECFSZ DelayCounter,F
GOTO DELAY.Loop
RETURN
- Refer
the program below explain what will happen to output at PortB.
MOVLW .255
XORWF PORTB,1
CALL DELAY
GOTO MAIN
All
the pins at port B will be toggled to “1” or “0” alternatively. Every time
after it toggles port B, it will delay for a while before another toggle.
- Write a
subprogram to set port D as output and port C as input.
;======Begin
PORT
BSF STATUS, RP0 ;PORT TRIS Registers are on page 1
;======PORT C
MOVLW .255 ;PORTC
Configuration Bits
MOVWF TRISC
;======PORT D
MOVLW .0 ;PORTd Configuration Bits
MOVWF TRISD
BCF STATUS, RP0 ;Restore Page0
- Write a
subprogram to input the signal from port C and output it to port D.
MOVF PORTC,W
MOVWF PORTD
- Write a
subprogram which enables external interrupt from pin RB0/INT.
BCF INTCON, INTF ;Clear Interrupt flag
BSF INTCON,INTE ;Enable
the INB0 Interrupt
BSF INTCON,GIE ;Enable the Interrupts
- Write a
subprogram to trigger the interrupt when falling edge occur at pin
RB0/INT.
BCF
INTCON, INTF ;Clear Interrupt flag
BSF
STATUS,RP0 ;Access bank 1
BCF
OPTION_REG,INTEDG ;Falling Edge Detect
BCF
STATUS,RP0 ;Restore bank 0
BSF
INTCON,INTE ;Enable the INB0 Interrupt
- For
keypad interfacing, which
RB4:RB7 have interrupt-on-mismatch
feature, together with software
configureable pull-ups on
these four pins, allow easy interface to a keypad and make it possible for wake-up
on key depression.
- Write a
program to enable Port B pull-ups.
BSF
STATUS,RP0 ;Access Bank1
Configuration Bits
BCF OPTION_REG,NOT_RBPU
BCF
STATUS,RP0
- When
the LCD display is not enabled, data lines are tri-state, what is meant by
tri-state?
data
lines are tri-state means they are in a state of high impendence (as though
they are disconnected) and this means they do not interfere with the operation
of the microcontroller when the display is not being addressed.
- The LCD
requires 3 "control" lines from the microcontroller, namely Enable
(E), Read/Write (R/W)
and Register select (RS). Describe
the function of each of the line.
Enable (E)
|
This line allows
access to the display through R/W and RS lines. When this line is low, the
LCD is disabled and ignores signals from R/W and RS. When (E) line is high,
the LCD checks the state of the two control lines and responds accordingly.
|
Read/Write (R/W)
|
This line determines the
direction of data between the LCD and microcontroller. When it is low, data
is written to the LCD. When it is high, data is read from the LCD.
|
Register select (RS)
|
With the help of this line,
the LCD interprets the type of data on data lines. When it is low, an
instruction is being written to the LCD. When it is high, a character is
being written to the LCD.
|
- The LCD
controller needs 40 to 120 microseconds (uS) for writing and reading.
Other operations can take up to 5 mS. During that time, the
microcontroller can not access the LCD, so a program needs to know when
the LCD is busy. Suggest two methods to overcome this problem.
One
way is to check the BUSY bit found on data line D7. This is not the best method
because LCD's can get stuck, and program will then stay forever in a loop
checking the BUSY bit. The other way is to introduce a delay in the program.
The delay has to be long enough for the LCD to finish the operation in process.
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