It is currently Sun Jul 15, 2018 11:50 pm

All times are UTC - 7 hours





Post new topic Reply to topic  [ 8 posts ] 
Author Message
PostPosted: Thu May 03, 2018 7:25 am 
Offline
User avatar

Joined: Sat Jul 04, 2015 9:58 am
Posts: 736
Location: -29.794229 -55.795374
Hello.
Recently I've found in this page a schematic of a MegaDrive game wich uses an 8bit ROM and a circuit to "convert" it to 16 bit.
Here's the schematic:
Attachment:
File comment: 8 to 16
conversão 8bits para 16 bits.jpg
conversão 8bits para 16 bits.jpg [ 145.58 KiB | Viewed 1120 times ]

My doubt is: would this simply work out of the box or the pirate game was somehow altered to make this possible?
I know that would be simple to just use 2 8bit ROMs with the odd/even bytes, but this circuit seems interesting, specially to me who only have a single 8bit ROM to try.

Thanks in advance.


Top
 Profile  
 
PostPosted: Thu May 03, 2018 7:44 am 
Offline

Joined: Sun Sep 19, 2004 11:12 pm
Posts: 20249
Location: NE Indiana, USA (NTSC)
Latching the upper and lower bytes on each access and assembling the 16-bit words before they get to the CPU isn't conceptually that different from the N64 RAM being only 9 bits wide and then the RCP memory controller reassembling words before the R4300i sees them. But you'd need a fairly fast ROM for this, and I remember reading somewhere that some early Genesis games couldn't DMA from ROM to VRAM because they were manufactured on slow ROM. Instead, they had to PIO-copy data to VRAM, as if it were a filthy Master System, or copy it to work RAM and then DMA it from there.

But I don't see much point, particularly given that the larger 5.0 V EPROMs (27C160 and 27C322) are 16-bit anyway, to the point where people end up using multiplexer circuits to get a 27C322 working with the Super NES's 8-bit data bus. Or were 16-bit EPROMs not made in smaller capacities?


Top
 Profile  
 
PostPosted: Thu May 03, 2018 8:14 am 
Offline
User avatar

Joined: Sat Jul 04, 2015 9:58 am
Posts: 736
Location: -29.794229 -55.795374
tepples wrote:
isn't conceptually that different from the N64 RAM being only 9 bits wide

That's pretty interesting!
Also I didn't know about the DMA, wich is interesting too.
tepples wrote:
But I don't see much point...

Sure! I agree with that too.
But that's not a matter of getting new parts, but to try to (re)use the ones I have.


Top
 Profile  
 
PostPosted: Thu May 03, 2018 8:16 am 
Offline
User avatar

Joined: Tue Dec 04, 2012 3:28 pm
Posts: 345
Location: Canada
tepples wrote:
Or were 16-bit EPROMs not made in smaller capacities?


There were lots of them. The 27C400 and 27C800 were both 16-bit chips with 4Mbit and 8Mbit sizes, respectively.


Top
 Profile  
 
PostPosted: Thu May 03, 2018 10:15 am 
Offline
User avatar

Joined: Wed Feb 13, 2008 9:10 am
Posts: 640
Location: Estonia, Rapla city (50 and 60Hz compatible :P)
http://www.tmeeco.eu/BitShit/8bitROMonMD.jpg

This is what is done in Turrican cartridge (seems to be same as the schematic earlier). It works but only on real original hardware that has no overclock or TMSS bypass mod done to it. It will not work on any clones except the superclones that use 1:1 copied parts.

If you have to use 8bit ROMs use pair of them, it is guaranteed to work and allows you to save money on bulk ordering of the memory chips.

_________________
http://www.tmeeco.eu


Top
 Profile  
 
PostPosted: Thu May 03, 2018 10:54 am 
Offline

Joined: Sun Apr 13, 2008 11:12 am
Posts: 7303
Location: Seattle
The pictured schematic uses B21 / "/CAS2", which is "asserted for reads and writes within 000000-7FFFFF". I'm vaguely surprised that it's a useful amount of phase delay relative to the other signals to use as a demultiplexing clock.

getafixx wrote:
There were lots of them. The 27C400 and 27C800 were both 16-bit chips with 4Mbit and 8Mbit sizes, respectively.
Were being the key word. There are no new 5V 16-bit ROMs, other than Atmel's expensive OTP offerings.

TmEE wrote:
If you have to use 8bit ROMs use pair of them, it is guaranteed to work and allows you to save money on bulk ordering of the memory chips.
The volume discount doesn't hit 50% though.


Top
 Profile  
 
PostPosted: Thu May 03, 2018 11:33 am 
Offline
User avatar

Joined: Wed Feb 13, 2008 9:10 am
Posts: 640
Location: Estonia, Rapla city (50 and 60Hz compatible :P)
From my experience pair of 4Mbit chips are only tiny bit more expensive than single 8Mbit chip, you'll be reaching the first discount tier pretty quickly where things even out and pretty quickly after that a point where they are cheaper. Of course it only matters when you start doing tens of carts rather than one or two at a time...

The circuit works only because of the slight timing difference between various signals, there's some tens of ns between the edges.

_________________
http://www.tmeeco.eu


Top
 Profile  
 
PostPosted: Fri May 04, 2018 4:04 am 
Offline
User avatar

Joined: Sat Jul 04, 2015 9:58 am
Posts: 736
Location: -29.794229 -55.795374
That's great!
Thanks for the info!!


Top
 Profile  
 
Display posts from previous:  Sort by  
Post new topic Reply to topic  [ 8 posts ] 

All times are UTC - 7 hours


Who is online

Users browsing this forum: No registered users and 0 guests


You cannot post new topics in this forum
You cannot reply to topics in this forum
You cannot edit your posts in this forum
You cannot delete your posts in this forum
You cannot post attachments in this forum

Search for:
Jump to:  
Powered by phpBB® Forum Software © phpBB Group