ACU ROM board and 16k chip compatibility (lack of address lines)

[geebus]

Hi all, I was ordering some RAM chips to upgrade one of my PCW8256's and seen some 16k eeprom chips whilst I was there. thought I might as well get them are it was a silly price for 10 of them.
I was originally thinking i could make an ACU ROM board as per the instructions ion the scanned pages in the wiki. However, these chips (at28c17) only have address lines going from 0-10, not like the rom board connections that go from 0 - 13. The pins are there of course, but the a11-13 are NC on those chips.
My question is, Is there a way to use these chips on the ACU rom board or would I have to work out some other sort of way to use them with logic gates?
Or are these chips just not suitable for use on the CPC systems at all?

[Bryce]

The AT2817 is a 2K ROM which only needs up to A10. It's 16Kbits in total, but when you divide that by 8 (for an 8Bit system) you end up with 2Kx8 which it states on the first page (on the right under ATMEL) of the datasheet you linked to.

Bryce.

[geebus]

The AT2817 is a 2K ROM which only needs up to A10. It's 16Kbits in total, but when you divide that by 8 (for an 8Bit system) you end up with 2Kx8 which it states on the first page (on the right under ATMEL) of the datasheet you linked to.

AH! thanks very much for clearing that up. Yet another lesson in electronics haha.
So what i really need is a 128Kbit ROM that's 16x8?

[Bryce]

Yup, a 27C128 or something compatible.

Bryce.

[geebus]

Fantastic, thanks.The ROM chips I got won't go to waste. I'll use them on my Ben Eater 8 bit breadboard computer.Thanks again for explaining whats going on.

[geebus]

@Bryce If you'd be so humble as to help me understand a little better.


I've been looking at the http://www.cpcwiki.eu/index.php/MegaROM that you designed and read the following:

The MegaROM is a low-cost replacement for a standard ROMBoard, but rather than using an array of 16 kb EPROMs as traditional ROMBoards use, the MegaROM uses a single 8x128k (1Mbit) or 8x256k (2Mbit) EPROM (or Flash EPROM) divided into 16 kb blocks. This not only reduces the size of the device significantly, but due to the fact that 16 kb EPROMs are a dying breed, it is much easier and cheaper to find a single 8x256k EPROM than buy sixteen 16 kb EPROMs.

The 16kb roms and blocks you speak of, are those 16kbit roms/blocks? as later in the paragraph you state that the MegaROM uses 8x128k (1Mbit) chips... is that not a 16kByte chip?
Meaning each ROM/Block is 2Kbyte in length?


Does that mean that the roms used by the CPC series are 2kByte roms and the chips I have (16x8) will in fact work with a rom board for the CPC?


I'm getting so confused about some of this and I quite fancy taking some electronic course to be able to better understand whats going on, then hopefully be able to make something work in the future.


If anyone could recommend where I can do some basic electronics course that would help, i'd love to hear about it.

[Bryce]

Yup, I could have worded that slightly better perhaps. It's a problem that often pops up because people (me included) swap between kbit and kbyte. The chip I used is 1Mbit = 8x128kbit = 8x16kbyte. Each block is in fact 16kbytes in size. Does that make sense?

As for understanding retro electronics: start by reading these: https://www.cpcwiki.eu/forum/amstrad-cpc-hardware/understanding-retro-electronics/  and if you have any request for explanations of other things, let me know. I can recommend some books too, but these are free


Bryce.

[geebus]

So, trying to get this in my head..
1Mbit is 128kBYTE, but it's an 8x128kBITE ROM so the 128 bits is 16bytes.. meaning it's 8x16kBYTE spaces/blocks?
Much better if I write these things down to go over it with myself haha.



Edit --------
Just been looking at this again.
1Mbit is 128kBit x 8
meaning 16kBYTE (aka 128KBIT) x 8