Chroma 81 - SCART and Colour interface for the ZX81

Discussions about Sinclair ZX80 and ZX81 Hardware
Post Reply
olofsen
Posts: 162
Joined: Wed Jan 08, 2014 12:29 pm

Re: Chroma 81 - SCART and Colour interface for the ZX81

Post by olofsen » Wed Nov 19, 2014 10:01 pm

Could it be that the ULA of the ZX81 generates HSYNCs independent of INTs such as the design of the ZX97 (Wilf Rigter's pages)? And that the driver does not work on a ZX80 with a ZX81 ROM and NMI generator (if Chroma supports the ZX80)?

User avatar
Andy Rea
Posts: 1530
Joined: Fri May 09, 2008 2:48 pm
Location: notts UK

Re: Chroma 81 - SCART and Colour interface for the ZX81

Post by Andy Rea » Wed Nov 19, 2014 11:02 pm

Thats a cool video driver, i have tinkered a little with Sponzy on the 20Mhz Zeddy, indeed it runs at a very respectable speed :-)

@pokemon the hsync generator is free running, you cannot stop it on a real ZX81 ( unless you generate int ack's every 16 cycles or less, but thats another story )

Regards Andy
6 x ZX81, 1 x TS1500 , 1 x +3e, 1 x timex 2040 printer, 1 x timex 2020 cassette deck, siclair printer and some spectrum

User avatar
PokeMon
Posts: 2225
Joined: Sat Sep 17, 2011 6:48 pm

Re: Chroma 81 - SCART and Colour interface for the ZX81

Post by PokeMon » Wed Nov 19, 2014 11:30 pm

Andy Rea wrote:@pokemon the hsync generator is free running, you cannot stop it on a real ZX81 ( unless you generate int ack's every 16 cycles or less, but thats another story )
This is new for me and a significant difference to the ZX80 (with NMI generator circuit).
I think even the NMI generator from Grant Searle does not produce a steady hsync.
So this would mean, that some HRG programs may not run depending on how they are realized.
Am I right ?

And how is synchronisation done ? Just waiting for next NMI in the program ? Or any other mechanism ?

User avatar
PokeMon
Posts: 2225
Joined: Sat Sep 17, 2011 6:48 pm

Re: Chroma 81 - SCART and Colour interface for the ZX81

Post by PokeMon » Thu Nov 20, 2014 12:17 am

PokeMon wrote: I think even the NMI generator from Grant Searle does not produce a steady hsync.
Okay - it does give a steady sync, as I checked.
http://searle.hostei.com/grant/zx80/zx80nmi.html

So this is then a HRG problem of the old ZX80CORE. Will solve it on the new version. ;)

User avatar
stefano
Posts: 391
Joined: Tue Dec 11, 2012 9:24 am
Contact:

Re: Chroma 81 - SCART and Colour interface for the ZX81

Post by stefano » Thu Nov 20, 2014 10:15 am

>@pokemon the hsync generator is free running, you cannot stop it on a real ZX81 ( unless you generate int ack's every 16 cycles or less, but thats another story )

Probably it is the reason for the garbled picture I got with my crap attempt. I mention it just to be precise, since I vaguely referred to the R register before; I stop developing on it because of the lack of colours where the only benefit of such timing was the possibility to loop between many text line buffers.

User avatar
PokeMon
Posts: 2225
Joined: Sat Sep 17, 2011 6:48 pm

Re: Chroma 81 - SCART and Colour interface for the ZX81

Post by PokeMon » Thu Nov 20, 2014 2:41 pm

Andy Rea wrote: ( unless you generate int ack's every 16 cycles or less, but thats another story )
Is the INT ACK used for synchronizing the hsync generator ?

User avatar
Andy Rea
Posts: 1530
Joined: Fri May 09, 2008 2:48 pm
Location: notts UK

Re: Chroma 81 - SCART and Colour interface for the ZX81

Post by Andy Rea » Thu Nov 20, 2014 4:11 pm

Exactly, the int ack resets the hsync counter and will hold it until the int ack releases, the hsync pulse then starts 16 clock cycles later

Regards Andy
6 x ZX81, 1 x TS1500 , 1 x +3e, 1 x timex 2040 printer, 1 x timex 2020 cassette deck, siclair printer and some spectrum

User avatar
Andy Rea
Posts: 1530
Joined: Fri May 09, 2008 2:48 pm
Location: notts UK

Re: Chroma 81 - SCART and Colour interface for the ZX81

Post by Andy Rea » Fri Nov 21, 2014 8:50 am

The delay between int ack and the hsync is why aszimic & other display drivers can have scan lines more than 207 cycles long, towards the end of the scan line we are expecting an int to occur some cycles before the hsync ia due, however if the int is delayed by say 4 cycles the hsync counter will reset and the hsync will also be delayed.

Regards andy
6 x ZX81, 1 x TS1500 , 1 x +3e, 1 x timex 2040 printer, 1 x timex 2020 cassette deck, siclair printer and some spectrum

User avatar
PokeMon
Posts: 2225
Joined: Sat Sep 17, 2011 6:48 pm

Re: Chroma 81 - SCART and Colour interface for the ZX81

Post by PokeMon » Fri Nov 21, 2014 12:14 pm

Many thanks.

User avatar
stefano
Posts: 391
Joined: Tue Dec 11, 2012 9:24 am
Contact:

Re: Chroma 81 - SCART and Colour interface for the ZX81

Post by stefano » Wed Dec 03, 2014 10:13 pm

With the Olofsen contribution, z88dk is now Chroma-81 compatible ;)
All the improvements will be online soon.

zcc +zx81 -subtype=chroma -clib=chroma -create-app -Dspritesize=8 -DBANNERS -DSPECTRUM -O3 wall.c
...powerful, isn't it ?

.. would you mind to take a picture of the real hardware in action ? :roll:
Attachments
chroma-wall.p
(9.78 KiB) Downloaded 76 times

Post Reply