example uPD7220 GDC *character mode only* board

[Andrew Lynch]

Hi

Does anyone have any application notes, examples, schematics, etc. of a uPD7220 GDC board for *character mode only*?

The reason I am asking is because I've designed uPD7220 GDC *graphic mode* boards in the past with some success (ECB 7220, Z180GDC) however trying to use them as console is extremely difficult because the uPD7220 is dreadfully slow drawing characters and updating the screen.

I've considered modifying the uPD7220 GDC graphics design to include character mode but never figured out how to do it.  There are tiny bits of technical details on how to do this but insufficient for me to figure it out.

Now I am thinking it may be better to just design a uPD7220 GDC *character mode only* board and see if it can be made to work.  Maybe down the road merge the graphics and character mode designs?

This may be a project that is too hard to do together but can be broken up into more manageable pieces on separate boards.

Any help greatly appreciated, Thanks, Andrew Lynch

[Alan Cox]

On Tue, 9 May 2023 at 14:45, 'Andrew Lynch' via retro-comp
<retro...@googlegroups.com> wrote:
>
> Hi
> Does anyone have any application notes, examples, schematics, etc. of a uPD7220 GDC board for *character mode only*?

Yes - I found one by accident that was used in East Germany

https://hc-ddr.hucki.net/wiki/lib/exe/fetch.php/z1013/module/gdc_cc_jena_s.png?cache=

[Andrew Lynch]

Thank you Alan!  Very helpful.

I think the next step is to adapt this circuit to KiCAD and build a prototype.

Thanks, Andrew Lynch

[Andrew Lynch]

> -----Original Message-----
> From: retro...@googlegroups.com [mailto:retro-
> co...@googlegroups.com] On Behalf Of Alan Cox
> Sent: Tuesday, May 9, 2023 11:23 AM
> To: Andrew Lynch <lyn...@yahoo.com>
> Cc: Retro-comp <retro...@googlegroups.com>
> Subject: Re: [retro-comp] example uPD7220 GDC *character mode only*
> board
>

> On Tue, 9 May 2023 at 14:45, 'Andrew Lynch' via retro-comp <retro-
> co...@googlegroups.com> wrote:
> >
> > Hi
> > Does anyone have any application notes, examples, schematics, etc. of a
> uPD7220 GDC board for *character mode only*?
>
> Yes - I found one by accident that was used in East Germany
>
> https://hc-
> ddr.hucki.net/wiki/lib/exe/fetch.php/z1013/module/gdc_cc_jena_s.png?cac
> he=
>

Hi, if anyone would like more details on z1013 GDC module, you can find it here:

https://hc-ddr.hucki.net/wiki/doku.php/z1013/module/gdc

The schematic looks straight forward. IO decoding looks a little different but nothing too exotic.

Great example! Thanks! Andrew Lynch

[Andrew Lynch]

Hi

I've got the z1013 GDC module circuit PDF and am converting to KiCAD.  Also found the descriptive web site for the z1013 which I can partially translate from German to English with Google Translate.  I am studying the circuit to learn about it.

I am primarily interested in this design out of curiosity but also would like to build a character mode board to compliment the GDC 16 color graphics only board previously designed as part of the Z180GDC project.  During build, test, and debug of the Z180GDC the team discovered that while a text console mode was possible using the graphics only circuit, it was dreadfully slow.  

I think what the design needs is a dedicated character mode. However, modification of the graphics only circuit is very difficult, so I am considering a parallel design with one GDC dedicated to graphics only and another for characters only and switching between the two as needed.  You would then have a speedy console when needed and can still display full screen graphics.

Regarding the z1013 circuit, based on what limited information I can find on the internet with Google and reviewing the circuit, I think this appears to be an 80x25 character matrix display using 8x8 pixel characters.  That's good and from that we can derive a lot of information.

Total screen display is 640x200 although each pixel is not individually accessible.  Pixel clock is 16 MHz which suggests PAL or SECAM video timing (East Germany).  Could also be NTSC if programmed with appropriate parameters.

Video SRAM is 2Kx8 (6116 type) which supports 80x25 character matrix of 2000 8-bit characters with 48 bytes to spare.

In my conversion to KiCAD, I've studied the chip select logic and converted it to be slightly more conventional.  Instead of the pair of intel 8205s for IO port decoding (appear to be 3 to 8 decoders, aka 74LS138) I am replacing with 74LS688 plus two OR gates for qualifying GDC reads and writes.

I think the circuit would work just fine as it is, however, it would require a scan doubler to use a VGA monitor.  That's certainly possible but I think would be better if the circuit could drive a VGA monitor directly.  That requires boosting the pixel clock to slightly more than 25 MHz to achieve 640x400 pixels which is minimum VGA resolution.

A 640x400 total screen display using an 8x8 pixel characters results in an 80x50 text display.  Unfortunately, there is only 2K of video SRAM but since the GDC's AD10 and AD11 seem to be unconnected, they could be used to drive an 8Kx8 video SRAM (6264 type) which would give plenty of room.

Bumping the pixel clock to about 25 MHz also means some components would have to be upgraded from 74LS to 74F or 74ALS for timing.  Also, the pixel clock drives the GDC clock which appears to be a "divide by four" approach.  4 MHz GDCs are available commonly however, going to VGA pixel clock would require a 6 MHz GDC.  There are GDCs which can go up to 8 MHz available.  I think the Zilog Z7220 is 8 MHz capable.  Also, the pin compatible uPD72020 is CMOS and 8 MHz capable.

Thanks, Andrew Lynch