Microcontroller Programming in Assembly Language PART 3
Microcontroller Programming in Assembly Language PART 3
In the previous two articles we discuss about how to create a MPLAB project and understood some basic features and instruction available in a PIC16F84A microcontroller. In this article I basically focus on how to write ‘if-then’ logic in assembly. Below a sample program code is provided which uses simple ‘if-then’ logic. This will give you some idea about how to use it in microcontroller Programming.
#include<p16f84a.inc> ; This is the header file
org 00h ; Reset vector
main
bsf STATUS,RP0 ; configure status bits to move into bank 1
movlw b’00000001′ ; move a binary value into the working register
movwf TRISB ; move the content of the working register into trisb
; above two instructions set RB0 as an input other PORTB pins output
bcf STATUS,RP0 ; moves into bank 0
clrf PORTB ; clears PORTB initially
loop_off ; a lable
btfss PORTB,0 ; check the status of RB0 and wait untill it is set
goto loop_off
bsf PORTB,1 ; make RB1 high when RB0 goes high
loop_on
btfsc PORTB,0 ; check the status of RB0 and wait untill it is cleared
goto loop_on
bcf PORTB,1 ; make RB1 low when RB0 goes low
goto loop_off
end ; end of the program
The main function of this program is to test the input condition on RB0 and if RB0 is high it makes RB1 high otherwise if RB0 is low it makes RB1 low.
Most of the codes include in this program I have already discussed in my previous article. So therefore I will mainly concentrate on new commands come across in this program we will start from the very beginning one by one.
#include<p16f84a.inc> - refer my previous article
org 00h - refer my previous article
main - refer my previous article
bsf STATUS,RP0 - refer my previous article
movlw b’00000001′ - this command is somewhat new but it is very easy to understand. This means that, it assigns a binary value 00000001 to working register. This working register is temporary register and if we want to assign some value to a SFR, firstly the value is assigned to the working register and then that value is moved in to the particular SFR using ‘movwf’ command.
movwf TRISB - now you know what is going to be happened in this command. The value in the working register will be moved in to the TRISB register. After that RB0 pin of the PORTB will be act as an input.
bcf STATUS,RP0 - if you refer my previous article you know what is the purpose of this command. By using this command we jump in to Bank0 of the PIC.
clrf PORTB - clears what ever in the PORTB initially.
loop_off - It is just a loop. refer my previous article
btfss PORTB,0 - this is something new. This checks the value of the pin mentioned in the particular register and if the value is 0 then the next instruction is executed and if the value is 1 then the next instruction is discarded instead instruction after that will be executed. This might be confusing for you. Therefore we will take a look at the program and try to understand what happens in it.
In here it checks the status of the 0th pin. If the pin is in low stage ‘goto loop_off’ command will be executed and if not instead of executing ‘goto loop_off’ command it will be discarded and ‘bsf PORTB,1′ will be executed.
goto loop_off - refer my previous article
bsf PORTB,1 - refer my previous article
loop_on - it is another label.
btfsc PORTB,0 - this is the opposite of ‘btfss’ command and it will operate in the same way which ‘btfss’ command does if the particular pin is in the reset stage(0).
Rest is pretty obvious for anyone who went through these two articles.
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Comments
5 Comments on Microcontroller Programming in Assembly Language PART 3
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Jessicasaw on
Mon, 11th May 2009 12:34 am
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