Post by feedingdragon on Jun 16, 2019 20:33:33 GMT
I did a search for what I'd like to find out, but didn't find anything. I have a couple of 28-pin 64k EEPROMs that I was planning to program for use as internal & external (in my 1750 REU,) function ROMs. I know how to set them up to switch between the 2 banks (32k each.) What I'm wanting to find out is if there is a way to disable them completely with the switch. What I had in mind was to use a DPDT switch that would select the Lo bank when to the left & the Hi bank when to the right, with the middle position disabling the chip all together. I've even selected the 4 function ROMs I'd like to have installed (have the burner,) though I'm having problems finding TASS 128, the link here doesn't work any more. I know that most have a means of exiting back to normal functions, but I prefer to just disable the chip if I can. The best I can work out would be to ground out (or maybe just disconnect,) the chip enable pin (pin 20.) But would like confirmation before actually implementing it on my physical system.
Also, I cannot seem to find the specs for the resistor that is placed between pins 1 (A15) & 28 (VCC) for the 64K chips. The clearest picture I could find (on this board actually,) wasn't quite clear enough on my monitor
Get a single pole 3 position switch. Connect the common to 5v and the first two outputs to a pull down resistor to gnd. The first output goes to the address pin. So selecting this position pulls it high. The second output (middle) goes to the eprom chip enable pin which is active low, selecting this pulls it high and turns the chip off. The third position is unconnected, so the CE pin and address pin both go low, enabling the other bank.
Basically, like this? Couldn't find a DT switch schematic Please mentally add a middle position & move the arms there, thank you. Could calculate the size of the resistors based on the current requirements of the chip. Except, the datasheet doesn't want to tell me how much current CE or A15 wants to pull. Could be missing something, I'll keep reading. This design is based mainly on an image elsewhere of a ROM selector wired like this (without the Chip Enable pin connections, of course,) and the new knowledge that CE is active low instead of high.
You will see the chip enable pin normally has a line over it, this denotes it's inverted.
So the chip is enabled when this input is low, when it's High the chip has it's outputs disabled. For pullup/down resistors this is VERY common in TTL logic. You can google this easily. (Ok, I googled it. People make this WAY more complex than it needs to be.) Just stick a 10k in there unless you have problems.
I can't quite tell from your schematic, but if you stick to my description all should be good. Anybody else can chime in if they think I got this wrong!
Post by feedingdragon on Jun 18, 2019 23:58:29 GMT
I had trouble, at first, figuring out your description. We're working from 2 different types of switches. I like your's better I'm working with a toggle switch, and your design is for a selector switch. I haven't actually purchased the switches yet, so I might go to the selector switch instead, as it is a simpler design.
That is with a slide switch. Would much prefer a 3 position rotary for this, though. Then I would change the layout to
Middle (or up) would be disabled, left would be lower bank & right would be upper For the slide switch, I'd put far left as off with middle being lower & far right being upper.
This also allows for moving the resistor to the switch instead of having to mount it on the sandwich or the chip. I could even put everything on the chip, depending on heat dissipation issues.
This is also a better design in that you are never grounding out VCC (even through a resistor, it's something that should be avoided.) But with the switches I'm used to working with, that becomes problematical. the middle position has everything open, so pulling CE high won't work. Of course, I could switch the low bank & off positions on the switch.... Will update that schematic Found a DPDT switch diagram in the library after all anyways. Though, the new design would work with a SPDT switch as well.