Shop Learn
VGA_Text_Demo for 6.25 MHz crystal — Parallax Forums

VGA_Text_Demo for 6.25 MHz crystal

RaymanRayman Posts: 12,371
edited 2022-01-10 21:27 in Propeller 1

This is a pretty obvious thing to do, but not 100% sure I've seen it and can't find it anywhere...

Anyway, if you replace the standard 5 MHz crystal with a 6.25 MHz crystal, you can get real 640x480 resolution with the normal 25 MHz dot clock.

This increases rows from 32 to 40, visible horizontal pixels from 512 to 640, pixel clock from 20 to 25 MHz.

This is set by "text.start(8)" for a basepin of 8. You may need to change this to 16 or something to match your hardware setup.

Unfortunately, I can't find a 6.25 MHz crystal to test this out with. But, I'm testing on a P1V in fpga and pretty sure it's good.

If someone can see it work with a real P1, please let me know.

Comments

  • evanhevanh Posts: 12,207
    edited 2022-01-09 09:59

    There is spare blanking time. It could be reduced and still be fine with a CRT. 6.00 MHz crystals are far more common than 6.25 MHz. The below should be suitable for 6.00 MHz (24 MHz dot clock):

                            ...
                            long    640             'hd  'setting timings for 24 MHz dot clock and 640x480 visible pixels
                            long    48              'hf
                            long    64              'hs
                            long    48              'hb
                            long    480             'vd
                            long    1               'vf
                            long    2               'vs
                            long    21              'vb
                            long    0               'rate
    

    If the vertical is too tight there then trading a little h-blanking would make up the diff.

  • evanhevanh Posts: 12,207
    edited 2022-01-09 13:40

    That's always been there, but shipping from USA is very pricey. Also, smaller surface mount parts are plentiful at 6.00 MHz.

  • ColeyColey Posts: 1,101

    I think it was Cluso99 that made this popular in the 2010s, pretty sure his RAM Blade used this crystal.

  • Cluso99Cluso99 Posts: 18,037

    Sapieha ran my TriBlade P1s at 115MHz. The DIP P1 can run slightly faster than the QFP44 P1.

    My P1s usually run at 104MHz (6.5MHz xtal) or 96MHz (12MHz smt 3225 xtal). The drop to 96MHz was to be able to do USB but I never found the time to get it working properly.

    Bill used 6.25MHz and persuaded Parallax to stock them.

    But, to run at these higher speeds requires special decoupling designs.

  • ColeyColey Posts: 1,101

    Ray even commented on that thread ;)

  • RaymanRayman Posts: 12,371

    I'm having trouble finding old threads... search here doesn't seem to work well and google with site:www.parallax.com is better, but not perfect...

    Anyway, thanks for that link, was looking for it. I'll see if it mentions VGA at 640x480.

  • RaymanRayman Posts: 12,371

    Ok, looks like I just lived with 75 Hz frame rate and didn't adjust any VGA settings.
    Don't know why I didn't think of doing the real 640x480 back then...

  • @Rayman said:
    This is a pretty obvious thing to do, but not 100% sure I've seen it and can't find it anywhere...

    Anyway, if you replace the standard 5 MHz crystal with a 6.25 MHz crystal, you can get real 640x480 resolution with the normal 25 MHz dot clock.

    This increases rows from 32 to 40, visible pixels from 512 to 640, pixel clock from 20 to 25 MHz.

    This is set by "text.start(8)" for a basepin of 8. You may need to change this to 16 or something to match your hardware setup.

    Unfortunately, I can't find a 6.25 MHz crystal to test this out with. But, I'm testing on a P1V in fpga and pretty sure it's good.

    If someone can see it work with a real P1, please let me know.

    @Rayman, This is SUPER its just what I was looking for. Thank you very much.

    @Cluso99 said:
    Sapieha ran my TriBlade P1s at 115MHz. The DIP P1 can run slightly faster than the QFP44 P1.

    My P1s usually run at 104MHz (6.5MHz xtal) or 96MHz (12MHz smt 3225 xtal). The drop to 96MHz was to be able to do USB but I never found the time to get it working properly.

    Bill used 6.25MHz and persuaded Parallax to stock them.

    But, to run at these higher speeds requires special decoupling designs.

    @Cluso99 , Please could you elaborate about the special decoupling design? Thank You.

  • RaymanRayman Posts: 12,371

    @JaanDoh Thanks, let me know if you get it to work on a real P1 with 6.25 MHz crystal.

  • google with site:www.parallax.com is better

    I use site:forums.parallax.com with google. It works pretty well.

    -Phil

  • If you use Vivaldi, you can even set such site searches up as custom search prefixes. So I only have to type "gp vga driver" or smth to that into the bar and it just does it.

  • @Rayman said:
    @JaanDoh Thanks, let me know if you get it to work on a real P1 with 6.25 MHz crystal.

    I will do Rayman, its likely to be a while yet since I still have to solder parts onto my pcb, but i'll still let you know here.

Sign In or Register to comment.