You may have done your own timer code by now, I didn't check.
Here is the code for my colour change timers, I have moved it from SysVia to UsrVia as I hadn't checked if you still had the OS running.
The important bit to making it steady is starting T1 at vsync and then using the free running latches so as not to introduce any more errors. You could just set T1 free running and just reset the count at vsync for an even steadier result, but you would have to run non-interlaced.
To move the who lot up, subtract from VSyncToTop, or add to move down. ScanLineUs (64) is one virtical pixel, so you can roughly judge moving sideways.
You can move the transition area off the side of the screen, but as you have a blank pixel between rows, I would do it there so that you have the maximum variation without seeing anything.
I would use Pal4Col0 to get the timing right, and then swap to the brick colour.
I wouldn't recommend using it like (don't know what you might have using T1) this, but as an example in an emulator it should be OK, possibly disable HW if it doesn't look correct.
As I said, I use SysViaT1 and no OS, so this is just an example. RichTW is far better on configuring the timers than I am.
20 REM Sample colour timer code
30 *K.10O.|MREN.|ML.07|ML.|M
40 MODE 1
You could use mode 5, or for breakout, even mode 4 would be OK, but the palette code is slightly different.
timer related stuff
50 Reload = 2
60 ScanLineUs = 64
70 RowPeriod = 8 * ScanLineUs - Reload
80 VSyncToTop = 4 * 8 * ScanLineUs - Reload
various useful addresses and values
90 IRQV1 = &204
100 UsrViaT1CL = &FE64
110 UsrViaT1CH = &FE65
120 UsrViaT1LL = &FE66
130 UsrViaT1LH = &FE67
140 SysIntVSync = &02
150 UsrIntT1 = &40
160 SysViaIFR = &FE4D
170 UsrViaACR = &FE6B
180 UsrViaPCR = &FE6C
190 UsrViaIFR = &FE6D
200 UsrViaIER = &FE6E
210 VideoULAPalette = &FE21
220 Pal4Col0 = &00
230 Pal4Col1 = &20
240 Pal4Col2 = &80
250 Pal4Col3 = &A0
260 PaletteWhite = &00
270 PaletteCyan = &01
280 PaletteMagenta = &02
290 PaletteBlue = &03
300 PaletteYellow = &04
310 PaletteGreen = &05
320 PaletteRed = &06
330 PaletteBlack = &07
340 :
somewhere to put the code
350 DIM code 500
360 FOR pass = 0 TO 2 STEP 2
370 P%=code
380 [OPT pass
redirect IRQ1V to our handler and then chain the original vector to exit
390 sei
400 lda IRQV1 : sta prev_irq1
410 lda IRQV1+1 : sta prev_irq1+1
420 lda #handler MOD 256 : sta IRQV1
430 lda #handler DIV 256 : sta IRQV1+1
clear any pending T1 interrupt (not really needed)
440 lda #&00 OR UsrIntT1
450 sta UsrViaIER : sta UsrViaIFR
configure T1 in freerunning mode (best check with RichTW)
460 lda #4 : sta UsrViaPCR
470 lda #64 : sta UsrViaACR
480 cli
490 rts
500 :
our inserted interrupt handler
510 .handler
520 lda SysViaIFR
530 and #SysIntVSync
540 beq not_vsync
it was a vsync, set up first timer to bottom of first line and then latch a value for every 8 pixels down (this will keep firing every char row).
550 lda #VSyncToTop MOD 256 : STA UsrViaT1CL
560 lda #VSyncToTop DIV 256 : STA UsrViaT1CH
570 lda #RowPeriod MOD 256 : STA UsrViaT1LL
580 lda #RowPeriod DIV 256 : STA UsrViaT1LH
590 lda #0 : sta counter
enable T1 interrupts
600 lda #&80 OR UsrIntT1
610 sta UsrViaIER
620 .done
630 jmp (prev_irq1)
640 :
650 .not_vsync
660 lda UsrViaIFR
670 and #UsrIntT1
680 beq done
690 :
clear the T1 interrupt
700 sta UsrViaIFR
change palette to next colour and if not last, done
710 tya : pha
720 ldy counter
730 lda colours,Y
740 jsr set_palette_colour
750 iny : sty counter
760 cpy #colours_end-colours
770 pla : tay
780 bcc done
790 :
last colour change, disable T1 until vsync (but I think stuff uses T1 if OS enabled)
800 lda #&00 OR UsrIntT1
810 sta UsrViaIER
820 jmp (prev_irq1)
830 :
change this routine if you aren't going to use a four colour mode.
840 .set_palette_colour
850 sta VideoULAPalette : eor #&10
860 sta VideoULAPalette : eor #&40
870 sta VideoULAPalette : eor #&10
880 sta VideoULAPalette
890 rts
900 :
few bits of storage
910 .prev_irq1
920 EQUW 0
930 .counter
940 EQUB 0
950 .colours
960 EQUB Pal4Col0 OR PaletteWhite
970 EQUB Pal4Col0 OR PaletteCyan
980 EQUB Pal4Col0 OR PaletteMagenta
990 EQUB Pal4Col0 OR PaletteBlue
1000 EQUB Pal4Col0 OR PaletteYellow
1010 EQUB Pal4Col0 OR PaletteGreen
1020 EQUB Pal4Col0 OR PaletteRed
1030 EQUB Pal4Col0 OR PaletteBlack
1040 .colours_end
1050 .y_save
1060 EQUB 0
1070 ]
1080 NEXT pass
1090 PRINT~code
1100 PRINT P%-code
1110 CALL code
after starting the routine, it will just carry on, changing the background.
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