Hank Fletcher
New Member
Hi all! I'm still on my quest to write simultaneously (or either) my first PIC assembly program, and a PIC program that produces an NTSC video signal. I found this post elsewhere on the internet: http://www.rohitab.com/discuss/lofiversion/index.php/t15740.html
It seems to me that it's a little more straight forward than the Rickard's website explanation (which I can't seem to find online anymore, anyway), at least in terms of being able to understand the code. I think I'm almost to the point of being able to understand it, but I still have a few questions.
1. How would I go about modifying the header to use a 16F88 and internal oscillator? I think I can get 4MHz as an internal oscillator option on a 16F88, can't I?
2. Generally, what does "org" do? Specifically, what do these lines in the program do:
?
3. Is the "Main" section of the code outdated/redundant/unnecessarily wordy? It seems to me I was called out for something along this line before, because some part of this is now articulated in the include file? At the end of the day, I believe this part of the code is just switching banks, setting PORTA as an input, and PORTB as an output, and then switching back banks again, correct? What's the best way to do that if this isn't it?
4. I don't really understand what's happening with the goto statements. What's the syntax of "goto" as it relates to this code? I think I remember reading up on what happens, but I'm either confused or have forgotten, and I'm unclear on the use of the $ symbol in this code.
5. I also don't understand what's happening in the "Delay" subroutine, and the use of the _D variable.
I just wanted to add that I think I understand what needs to happen electronically to produce an NTSC signal at this point. I'm just having trouble following the code, and getting to the point where I understand enough of it to comfortably code in assembly. I probably should clarify that the poster of this code did so with the intention of it being used to create a PAL signal, but as it doesn't use any vertical sync (just a continuous stream of horizontal patterns), and with the leeway that, as far as I know, modern NTSC televisions will permit, this shouldn't be a problem. I hope.
It seems to me that it's a little more straight forward than the Rickard's website explanation (which I can't seem to find online anymore, anyway), at least in terms of being able to understand the code. I think I'm almost to the point of being able to understand it, but I still have a few questions.
1. How would I go about modifying the header to use a 16F88 and internal oscillator? I think I can get 4MHz as an internal oscillator option on a 16F88, can't I?
2. Generally, what does "org" do? Specifically, what do these lines in the program do:
Code:
org 0x00
goto Main3. Is the "Main" section of the code outdated/redundant/unnecessarily wordy? It seems to me I was called out for something along this line before, because some part of this is now articulated in the include file? At the end of the day, I believe this part of the code is just switching banks, setting PORTA as an input, and PORTB as an output, and then switching back banks again, correct? What's the best way to do that if this isn't it?
4. I don't really understand what's happening with the goto statements. What's the syntax of "goto" as it relates to this code? I think I remember reading up on what happens, but I'm either confused or have forgotten, and I'm unclear on the use of the $ symbol in this code.
5. I also don't understand what's happening in the "Delay" subroutine, and the use of the _D variable.
I just wanted to add that I think I understand what needs to happen electronically to produce an NTSC signal at this point. I'm just having trouble following the code, and getting to the point where I understand enough of it to comfortably code in assembly. I probably should clarify that the poster of this code did so with the intention of it being used to create a PAL signal, but as it doesn't use any vertical sync (just a continuous stream of horizontal patterns), and with the leeway that, as far as I know, modern NTSC televisions will permit, this shouldn't be a problem. I hope.
Code:
list p=16f84A
#include "P16f84.inc"
__config _CP_OFF & _WDT_OFF & _PWRTE_ON & _XT_OSC
w equ 0
f equ 1
SYNC equ h'00'
BLACK equ b'00010000'
GREY equ b'00100000'
WHITE equ b'00110000'
_D equ h'12'
org 0x00
goto Main
org 0x04
goto Main
_2NOP macro
nop
nop
endm
_6NOP macro
nop
nop
nop
nop
nop
nop
endm
Main
bsf STATUS,5
movlw h'00'
movwf TRISB
movlw h'ff'
movwf TRISA
bcf STATUS,5
Line
movlw SYNC /G16
movwf PORTB ;2µs unused /G17
_2NOP ;Start 4µs SYNC S2
movlw BLACK S3
movwf PORTB S4
_6NOP ;Start 8µs BlackLevel B6
movlw WHITE B7
movwf PORTB B8
btfsc PORTA,0 ;Is the button pushed? W1/2
goto $+3 ;yes jump 3 lines forward W3
movlw h'07' ;no W3
call Delay ;25µs white line W28
movlw BLACK W4/29
movwf PORTB W5/30
btfss PORTA,0 ;is the button pushed? B1
goto $+3 ;no jump 3 lines forward B3
movlw h'07' ;yes B3
call Delay B28
movlw GREY B4/29
movwf PORTB B5/30
_6NOP G6
_6NOP G12
Nop G13
goto Line G15
Delay
movwf _D
decfsz _D
goto $-1
return
end
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