Help with 6845 CRTC needed

[Matthew]

I am building a video circuit to use with a z80-based project.

I am looking to use a 6845 to give a screen resolution of 512*256 with
8 colours.
I have a problem understanding the addressing of the 6845 - it seems
to be only for use with character based video.
I want to use one byte of screen memory to give me 2 pixels (I will
have 2 bits left over for other uses - flashing etc). This means I
have to address 64k memory and have 16 address lines. I also want that
memory to be able to be accessed contiguously from 0 to 64k.

Can someone explain how this could be done?

Many thanks,

Matthew

 

[R. Steve Walz]

Matthew wrote:

I am building a video circuit to use with a z80-based project.

I am looking to use a 6845 to give a screen resolution of 512*256 with
8 colours.
I have a problem understanding the addressing of the 6845 - it seems
to be only for use with character based video.
I want to use one byte of screen memory to give me 2 pixels (I will
have 2 bits left over for other uses - flashing etc). This means I
have to address 64k memory and have 16 address lines. I also want that
memory to be able to be accessed contiguously from 0 to 64k.

Can someone explain how this could be done?

Many thanks,

Matthew
---------------

Why don't you just buy an old MGP or Hercules card? That's exactly
what it is!

-Steve
--
-Steve Walz rstevew@armory.com ftp://ftp.armory.com/pub/user/rstevew
Electronics Site!! 1000's of Files and Dirs!! With Schematics Galore!!
http://www.armory.com/~rstevew or http://www.armory.com/~rstevew/Public

 

[Matthew]

"R. Steve Walz" <rstevew@armory.com> wrote in message news:<413FA501.6C35@armory.com>...

Why don't you just buy an old MGP or Hercules card? That's exactly
what it is!

Because I want to make it myself.

 

[petrus bitbyter]

"Matthew" <google@cleantalk.co.uk> schreef in bericht
news:1426aa98.0409080838.5d41cc6a@posting.google.com...

I am building a video circuit to use with a z80-based project.

I am looking to use a 6845 to give a screen resolution of 512*256 with
8 colours.
I have a problem understanding the addressing of the 6845 - it seems
to be only for use with character based video.
I want to use one byte of screen memory to give me 2 pixels (I will
have 2 bits left over for other uses - flashing etc). This means I
have to address 64k memory and have 16 address lines. I also want that
memory to be able to be accessed contiguously from 0 to 64k.

Can someone explain how this could be done?

Many thanks,

Matthew

Matthew,

The 6845 *is* a character based CRT controller but a very versatile one. You
can define the dimensions of a character and you can go graphical by
defining a hight of one pixel.Can't remember whether the width also could by
set to one pixel but the smaller the better. You need one address in the
video memory (RAM) for every character on the screen. Translation to video
is done during the scanning by a character generator, usually a bunch of
(EP)ROM. No need to say you absolutely need a datasheet of the 6845.

petrus bitbyter


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[Jean-Yves]

In article <_T10d.73293$C7.44478@amsnews05.chello.com>,
"petrus bitbyter" <p.kralt@reducespamforchello.nl> wrote:

No need to say you absolutely need a datasheet of the 6845

there is one here :
http://www.funet.fi/pub/cbm/documents/chipdata/68b45.zip

--
Jean-Yves.

 

[Chris]

In message <1426aa98.0409090231.1caaa11e@posting.google.com>, Matthew
<google@cleantalk.co.uk> writes

"R. Steve Walz" <rstevew@armory.com> wrote in message
news:<413FA501.6C35@armory.com>...
Why don't you just buy an old MGP or Hercules card? That's exactly
what it is!


Because I want to make it myself.

http://andercheran.aiind.upv.es/~amstrad/docs/hd6845s/hd6845sp.htm

http://andercheran.aiind.upv.es/~amstrad/docs/mc6845/mc6845.htm

http://andercheran.aiind.upv.es/~amstrad/docs/um6845r/um6845r.htm

http://andercheran.aiind.upv.es/~amstrad/docs/crtcnew.html

Hope these are of use!
--
Chris