I have a couple of memory IC's lying around (16 of them) which I would like to use in my homebrew Z80 project. These IC's were produced by Samsung in 1988, the KM41256A 256K x 1 bit. I realise I can't hook them all up as the Z80 can't address more then 64K and I'd still like a rom and some IO.
However, I thought I'd use 4 of them to create 16K which seems enough for a test setup. I know I'd need a line decoder for the chip select pins but I'm a bit at a loss how to approach this.
My questions:
- First and for most would it even be possible to use these chips? Or
do I need chips with a word equal to the data width of my Z80
(surely this can't be true?). - How much address lines would I need to address 16K of memory?
- Which line decoder would I need 2-to-…. ?
I hope I phrased the questions in such a way that they're understandable. I'm completely new to this type of design. I've tried to read up online as much as I could but these matters left me confused.
Thanks.
Best Answer
These 256K x 1 chips are only 1 bit wide. With four of them you would only get 1 nibble at a time, and the Z80 doesn't do 4-bit data.
It is possible to use them, but not what I'd call practical, and would take far more than 8 chips, unless you want to use an FPGA to make a RAM controller. You need a controller anyways, in order to generate your dynamic RAM timing. But let's concentrate on simple read/write operations.
You need, at a minimum, a 9-channel 2x1 multiplexer for the addresses. You also need some time delays to generate RAS and CAS strobes. With a single 41256 you could produce a 32K x 8 RAM, by using page mode. Using the upper 15 address bits as the 32K address and the 3 lsbs of address to provide the 8 bits of data, you would perform a page mode read or write , then step through the 3 data addresses to sequentially read or write the 8 data bits into or from the RAM. Page mode cycle time takes 100 to 150 nsec/cycle, depending on your chip speed, so a single read or write cycle would take on the order of 1 to 1.5 usec.
Chip count would be something like: 2 ea. 74HC157 2-1 mpx (although this would restrict you to a 16k equivalent), 1 ea 74HC374 for the RAS addresses, 2 ea 74HC161 for the CAS addresses, 1 ea. 74HC161 for the data bit counter, 1 ea 74HC151 8-1 mpx for writing data, 1 ea 74HC374 as a read shift register to store the output data before sending it to the Z80, and a 74HC244/245 to drive the data onto the Z80 data bus. Plus a few more for glue logic, at a guess. That's a minimum of about 11 chips. Plus, assuming you're using an 8-MHz Z80, it will run at an equivalent of about a 2-MHz chip. All your memory reads and writes will take a loooong time.
You can, of course, speed things up by using 4 of them with a two-stage read or write cycle with almost no loss in speed.
What you really need to do is use 8. Then you have a good match between your CPU and your memory, and you should be able to get decent speed and minimal chip count.