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Re: 6502 with the RA8875
Posted: Sat Dec 06, 2025 10:00 pm
by PaulaM
so I can't draw a single pixel (nothing shows up) but I can draw a very very short line
The colour in real life is much more vibrant than the image
Re: 6502 with the RA8875
Posted: Sun Dec 28, 2025 8:07 am
by GlennSmith
Hi Paula,
If you're able, could you post a timing diagram (even written on a piece of tablecloth in good engineering style) of the EN signal and data-bus of your working parallel interface. I'm still having problems with my 6522-driven interface (no level translators as the whole system is running at 3v3 : Pi Pico2 'oblige'). At 1MHz I can seemingly get through the initialization part (the loop waiting for the graphic fill to complete works correctly) - but the screen remains (infuriatingly) blank. At higher speeds it hangs at random places.
Thanks, in advance.
Re: 6502 with the RA8875
Posted: Mon Dec 29, 2025 2:55 pm
by PaulaM
Hi Paula,
If you're able, could you post a timing diagram (even written on a piece of tablecloth in good engineering style) of the EN signal and data-bus of your working parallel interface. I'm still having problems with my 6522-driven interface (no level translators as the whole system is running at 3v3 : Pi Pico2 'oblige'). At 1MHz I can seemingly get through the initialization part (the loop waiting for the graphic fill to complete works correctly) - but the screen remains (infuriatingly) blank. At higher speeds it hangs at random places.
Thanks, in advance.
I'll grab it again in a bit, but the main thing was ensuring the E line goes high AFTER the address, data and RnW lines change.. it's a bit of a night mare to get right.
Paula
Re: 6502 with the RA8875
Posted: Mon Dec 29, 2025 3:45 pm
by PaulaM
Hi Paula,
If you're able, could you post a timing diagram (even written on a piece of tablecloth in good engineering style) of the EN signal and data-bus of your working parallel interface. I'm still having problems with my 6522-driven interface (no level translators as the whole system is running at 3v3 : Pi Pico2 'oblige'). At 1MHz I can seemingly get through the initialization part (the loop waiting for the graphic fill to complete works correctly) - but the screen remains (infuriatingly) blank. At higher speeds it hangs at random places.
Thanks, in advance.
Link to my BASIC code (if it helps) -
https://github.com/TechPaula/PC6502/blo ... T_TEST.BAS
Good luck, let us know how you get on
Re: 6502 with the RA8875
Posted: Mon Dec 29, 2025 3:52 pm
by PaulaM
Larger picture of the Mandelbrot
And a Sierpinski triangle/gasket
Re: 6502 with the RA8875
Posted: Mon Dec 29, 2025 6:50 pm
by GlennSmith
Link to my BASIC code (if it helps)
Good luck, let us know how you get on
Thanks ! The basic code was useful in order to have a known good init sequence. The timing for EN is in your CPLD logic, however.
Armed with your diagrams I'll compare with what my setup is doing.
More news soon, but I've currently been banned from the dining-room table - so progress will be whenever it's light enough and warm enough for me to be in my 'den'.
Re: 6502 with the RA8875
Posted: Mon Dec 29, 2025 9:17 pm
by BigDumbDinosaur
More news soon, but I've currently been banned from the dining-room table - so progress will be whenever it's light enough and warm enough for me to be in my 'den'. 
Re: 6502 with the RA8875
Posted: Mon Dec 29, 2025 9:24 pm
by barnacle
Heh. My (attic) workshop has no fewer than five desks, plus a temporary workbench, and they're _all_ full of stuff...
Neil
Re: 6502 with the RA8875
Posted: Tue Dec 30, 2025 12:40 pm
by PaulaM
Link to my BASIC code (if it helps)
Good luck, let us know how you get on
Thanks ! The basic code was useful in order to have a known good init sequence. The timing for EN is in your CPLD logic, however.
Armed with your diagrams I'll compare with what my setup is doing.
More news soon, but I've currently been banned from the dining-room table - so progress will be whenever it's light enough and warm enough for me to be in my 'den'.
hehe, I'm currently working on the sofa
Paula
Re: 6502 with the RA8875
Posted: Sat Jan 03, 2026 11:24 am
by GlennSmith
Hi everyone,
Well I have negotiated my corner of the table for a few days, some progress has been made. The screen is now filled with black.
Here are some traces of the dialogue (sorry I haven't been able to get my analyzer clips to stay on the data connections) System clock is currently 1MHz.
a) a global view of a 'hello world' dialogue. You can see the individual writes of each letter at the end of the trace.
- Hello world dialogue
b) a close-up of the first two "instructions" : two writes to the reset register.
- Close-up : two instructions
The protocol seems to be correct, and the WAIT signal is pulled by the RA8875 from time-to-time which seems to imply that it is doing things in response to the instructions. But no text
And to complete the package, the top-level
PLASMA code (blatently copied from PaulaM's BASIC program (thanks Paula))
Code: Select all
// --------------------------------- Main program --------------------------------
via_init()
puts("SOFT RESET"); putln
CmdWrite($01)
DataWrite($01)
Wait(1)
CmdWrite($01)
DataWrite($00)
Wait(1)
puts("Pll Init"); putln
CmdWrite($88)
DataWrite($0A)
Wait(1)
CmdWrite($89)
DataWrite($02)
Wait(1)
puts("COLOUR DEPTH & MCU INTERFACE"); putln
CmdWrite($10)
DataWrite($00)
Wait(1)
puts("PCLK INVERS"); putln
CmdWrite($04)
DataWrite($82)
Wait(1)
puts("HORIZONTAL SET"); putln
CmdWrite($14)
DataWrite($3b)
Wait(1)
CmdWrite($15)
DataWrite($00)
Wait(1)
CmdWrite($16)
DataWrite($01)
Wait(1)
CmdWrite($17)
DataWrite($00)
Wait(1)
CmdWrite($18)
DataWrite($05)
Wait(1)
puts("VERTICAL SET"); putln
CmdWrite($19)
DataWrite($0F)
Wait(1)
CmdWrite($1A)
DataWrite($01)
Wait(1)
CmdWrite($1B)
DataWrite($02)
Wait(1)
CmdWrite($1C)
DataWrite($00)
Wait(1)
CmdWrite($1D)
DataWrite($07)
Wait(1)
CmdWrite($1E)
DataWrite($00)
Wait(1)
CmdWrite($1F)
DataWrite($09)
Wait(1)
puts("ACTIVE WINDOW X"); putln
CmdWrite($30)
DataWrite($00)
Wait(1)
CmdWrite($31)
DataWrite($00)
Wait(1)
CmdWrite($34)
DataWrite($DF)
Wait(1)
CmdWrite($35)
DataWrite($01)
Wait(1)
puts("ACTIVE WINDOW Y"); putln
CmdWrite($32)
DataWrite($00)
Wait(1)
CmdWrite($33)
DataWrite($00)
Wait(1)
CmdWrite($36)
DataWrite($0F)
Wait(1)
CmdWrite($37)
DataWrite($01)
Wait(1)
puts("DISPLAY ON"); putln
CmdWrite($01)
DataWrite($80)
Wait(1)
puts("FILL SCREEN WITH BLACK"); putln
CmdWrite($91)
DataWrite($00)
Wait(1)
CmdWrite($92)
DataWrite($00)
Wait(1)
CmdWrite($93)
DataWrite($00)
Wait(1)
CmdWrite($94)
DataWrite($00)
Wait(1)
CmdWrite($95)
DataWrite($DF)
Wait(1)
CmdWrite($96)
DataWrite($01)
Wait(1)
CmdWrite($97)
DataWrite($0F)
Wait(1)
CmdWrite($98)
DataWrite($01)
Wait(1)
CmdWrite($63) // BG RED BITS 0..2
DataWrite($00)
Wait(1)
CmdWrite($64) // BG GREEN BITS 0..2
DataWrite($00)
Wait(1)
CmdWrite($65) // BG BLUE BITS 0..1
DataWrite($00)
Wait(1)
CmdWrite($90)
DataWrite($B0)
Wait(10)
puts("WAIT...")
CmdWrite(90)
repeat
temp = DataRead() & $80
until temp <> 0
puts(" OK!"); putln
puts("TEXT MODE"); putln
CmdWrite($40)
DataWrite($E0) // TEXT MODE, BLINK CURSOR
Wait(1)
CmdWrite($44) // BLINK RATE REG
DataWrite($20)
Wait(1)
puts("SET CURSOR POSITION"); putln
CmdWrite($2A) // X LSB POSITION
DataWrite($0A)
Wait(1)
CmdWrite($2B) // X MSB POSITION
DataWrite($00)
Wait(1)
CmdWrite($2C) // Y LSB POSITION
DataWrite($0A)
Wait(1)
CmdWrite($2D) // Y MSB POSITION
DataWrite($00)
Wait(1)
puts("SET TEXT COLOUR"); putln
CmdWrite($63) // FG RED BITS 0..2
DataWrite($07)
Wait(1)
CmdWrite($64) // FG GREEN BITS 0..2
DataWrite($04)
Wait(1)
CmdWrite($65) // FG BLUE BITS 0..1
DataWrite($00)
Wait(1)
CmdWrite($02) // Data write register
DataWrite('H')
DataWrite('e')
DataWrite('l')
DataWrite('l')
DataWrite('o')
DataWrite(' ')
DataWrite('W')
DataWrite('o')
DataWrite('r')
DataWrite('l')
DataWrite('d')
DataWrite('!')
puts("DONE!"); putln
done
I think I'll go and read the manual yet again !!
Re: 6502 with the RA8875
Posted: Sun Jan 04, 2026 2:29 pm
by GlennSmith
Heh, I've found some better probe hooks and I think I can see what's wrong...
- Oops, no data !
Something is terribly wrong - especially as the RA8875 inputs are pulled-up with 10k resistors...
Re: 6502 with the RA8875
Posted: Sun Jan 04, 2026 7:26 pm
by GlennSmith
Back again !
Well the terribly wrong was an error in addressing the non-handshake port A of the 6522 (ORG+$0F) : I was writing my data into unused address space
With that fixed, I was then seeing very strange behavior between bits D3 and D4 of the data lines - they were following each other - yes, you've deduced the cause : a short circuit. Caused by a fragment of a pin when it was clipped after soldering. Had to get my 'old guy' reading glasses out to find that...
So the result still isn't what I was expecting
- At last something to read.
But it's
MUCH better! Now I just need to get my head around all of the display format commands !
Thanks again, PaulaM, for your helpful inputs.
Re: 6502 with the RA8875
Posted: Tue Jan 06, 2026 1:42 pm
by PaulaM
Back again !
Well the terribly wrong was an error in addressing the non-handshake port A of the 6522 (ORG+$0F) : I was writing my data into unused address space
With that fixed, I was then seeing very strange behavior between bits D3 and D4 of the data lines - they were following each other - yes, you've deduced the cause : a short circuit. Caused by a fragment of a pin when it was clipped after soldering. Had to get my 'old guy' reading glasses out to find that...
So the result still isn't what I was expecting
At_last.jpg
But it's
MUCH better! Now I just need to get my head around all of the display format commands !
Thanks again, PaulaM, for your helpful inputs.
AWESOME!!!!!
Can I ask, do you have level shifters between your 6502 and the RA8875?
One thing I found with mine is that I had to disable the level shifter when I wasn't accessing the display (the TXS0108 has an OE pin) as I was getting interference with the main databus.
(my 6502 is running at 5V and the display signals are 3.3V)
Second question, DO you use the /WAIT signal from the display? I currently don't as I found a few things online saying it kinda worked, but not always.
My next challenge is the LT7683 driver chip, eeeep.
Re: 6502 with the RA8875
Posted: Tue Jan 06, 2026 3:55 pm
by GlennSmith
Thankyou, but I still haven't managed to get the full display to clear, and I've lost the "Hello World!" text... I'll get there one day. I'll have to be quick, 'cos my bail on the dinig table expires Friday night (family arriving)
Can I ask, do you have level shifters between your 6502 and the RA8875?
No level shifting so far, I'm using a Pi Pico2 to drive my contraption (it drives PHI2 and provides an API for file IO and other functions, as well as UART) I'm running the whole show on 3v3 for the moment. Once I've got everything running (well, walking) properly I have plans to use the level translators to isolate the 3v stuff and run the 6502 (or the 65816 proto that I'm also working on) at 5v and crank the speed up to the limit. I'll keep in mind the info about dis-activating, though.
Second question, DO you use the /WAIT signal from the display? I currently don't as I found a few things online saying it kinda worked, but not always.
Yes, I'm using it and it seems to work as advertised. For example, when you launch a graphics draw operation, !Wait is pulled low during the operation. This is how I've hooked the display up using a 6522 (my original idea was to use the '22s handshaking, but the polarity is all wrong so I'm 'bit-banging' the !CS and EN handshaking).
- Bit-bang RA8875 interface
My next challenge is the LT7683 driver chip, eeeep.
Yes, quite a complex beastie. I have a FT801Q-R still in it's sealed bag : that's a
really complex beastie! BTW exactly what is the reference of your display ? Mine is TFTM070A3-5. I can't understand why your init values don't work on my device.
Thanks again!
Re: 6502 with the RA8875
Posted: Thu Jan 08, 2026 8:31 pm
by PaulaM
but I still haven't managed to get the full display to clear, and I've lost the "Hello World!" text... I'll get there one day. I'll have to be quick, 'cos my bail on the dinig table expires Friday night (family arriving)
I had a thought, my code is set up for a 480x272 display (TFT043A4-3), as yours is a higher resolution you'll need to change the PLL timings and also window sizes.
This probably explains the double image and also random bits on the right hand side (they'll be outside the window that was cleared)
Paula