Question about VGA_Demo

JaanDoh

I've been looking at the VGA_Demo program which can be found in the OBEX

obex.parallax.com/object/645

I'm a little confused because of the following:

''***************************************
''* VGA Terminal 40x15 v1.0 *
''* Author: Chip Gracey *
''* Copyright (c) 2006 Parallax, Inc. *
''* See end of file for terms of use. *
''***************************************

CON

_clkmode = xtal1+pll16x
_clkfreq = 80_000_000

vga_params = 23
cols = 32
rows = 16
screensize = cols * rows

Where the confusion comes from is simple...
I see it uses the internal font found in the Propeller 1 Microcontroller.

And each font character is 16px wide by 32px high

In the very top of the source code it says..

''* VGA Terminal 40x15 v1.0 *

I understand that to be 40 columns and 15 rows

And yet in the source itself it defines 32 columns and 16 rows...
So which one is the right one?


Thank You

Jaan

jmg

JaanDoh wrote: »

And yet in the source itself it defines 32 columns and 16 rows...
So which one is the right one?

Usually, real source trumps comments, which can be out of date...

JaanDoh

Hiya JMG,
Thank You for the response,
it's much appreciated.

Although I have not tried the code,

That means the resolution of the screen is 512 x 480?
(based on character size)

Yet further in the code it also says...

vgaparams long 0 'status
long 1 'enable
long %010_111 'pins
long %011 'mode
long 0 'videobase
long 0 'colorbase
long cols 'hc
long rows 'vc
long 1 'hx
long 1 'vx
long 0 'ho
long 0 'vo
long 512 'hd
long 16 'hf
long 96 'hs
long 48 'hb
long 380 'vd
long 11 'vf
long 2 'vs
long 31 'vb
long 20_000_000 'rate

(Sorry for the spacing - it hasn't retained the spacing as in the code)

Which doesn't match?

ke4pjw

Without looking at the code, I can tell you a 16x32 pixel font that has a 40x15 character screen, is a 640x480 pixel screen.

Peter Jakacki

The font is 16 wide by 32 high, so 32 high by 15 rows = 480 which is the value that I have in VGA driver in Tachyon, not 380.

If it helps, this is the initialization values that I use in Tachyon. Only the PASM part of the driver is used and stored in upper EEPROM as a "ROM" to load into a cog at runtime on demand. Should be easy enough to follow.

TABLE vgapars PRIVATE
    0 ,                --- 0:status              ' 0/1/2 = off/visible/invisible - read-only
    1 ,                --- 1:enable              ' 0=off (disabled completely)
    16 7 + ,           --- 2:pins                ' vga pin group 0,8, or 16.
    %1000 ,            --- 3:mode                ' %tihv = tile,interlace,hpol,vpol   write-only
    vgabuf , colors ,  --- 4:screen 5:colors     ' pointers to screen (words) and colors (longs)
    32 , 15 ,          --- 6:cols 7:rows         ' horz and vert tiles (characters)
    1 , 1 ,            --- 8:hx 9:vx             ' horz and vert tile expansion
    0 , 0 ,            --- 10:ho 11:vo           ' horz and vert offset
    512 , 10 ,         --- 12:hd 13:hf           ' horz display ticks, front porch
    75 , 43 ,          --- 14:hs 15:hb           ' horz synch ticks, back porch
    480 , 11 ,         --- 16:vd 17:vf           ' vert display lines, front porch
    2 , 31 ,           --- 18:vs 19:vb           ' vert synch lines, back porch
    CLKFREQ 2 >> ,     --- 20:rate		 ' tick rate (Hz)

JaanDoh

Thank You both Ke4pjw and Peter Jakacki,

You are both right in what you say.
And together with what Jmg wrote, (importantly - source trumps comments)
Your'e all right.

So in fact the comments are wrong and the code has a slight minor typo.
Well that's cleared up everything nicely.


Moving on in the VGA_Demo code found in the link in my first thread in this post...

In the Public method begin
the first statement is ...

start(%010111)

This line passes the pins config to the method called start
and the comments towards the end of the DAT section explain the following...

'' ________
'' vga_pins
''
'' bits 5..3 select pin group:
'' %000: pins 7..0
'' %001: pins 15..8
'' %010: pins 23..16
'' %011: pins 31..24
'' %100: pins 39..32
'' %101: pins 47..40
'' %110: pins 55..48
'' %111: pins 63..56
''
'' bits 2..0 select top pin within group
'' for example: %01111 (15) will use pins %01000-%01111 (8-15)

In the example above there are only 5 bits not 6
and bits 5 to 3 are x01 where x is missing
because %001111 is the same as %01111
so this is pin group 15 to 8

but bits 2 to 0 are 111 so how do I arrive at 8 (for pin 8)?



Thank You for your input and patience.


Jaan

DigitalBob

I found some good working vga demo's on the Obex. Kwabena W Agyeman wrote a couple of good examples that work straight of the Box

Peter Jakacki

start(%010111) has %010 for bits 3 to 5 so that is pin group 16..23. The bottom two bits look like they are shortform for the VPins field (00001111 or 11110000 or 11111111) so %11 would be all 8 pins which is correct. (btw, I just guessed for the Vpins as I see there are 3 bits there, so I will go check and correct).



EDIT: That's right, the basepin is offset by +7 which is correct. Forget the Vpins guess.

btw, if you ran this in Tachyon you would be able to interactively play with all these values

JaanDoh

Thank You

That's answered my question.

I gotta get time to try this out now.


Jaan