4MB SRAM Module

[BigEd]

Might have been this article: http://lwn.net/Articles/525459/

It's worth understanding the idea of mixing in entropy, and optionally accounting for it. It means your RNG can accept good or mediocre sources of randomness without losing any more randomness than necessary. So using uninitialised memory, or a MAC address, need not be a sin.

I'm pretty sure I've read somewhere, in a forensic context, that RAM has a tendency to power up in the state it's been in if it was in that state for long enough. That is, if you save a static password in the same location for years in your router, and then sell it, someone has a better than evens chance of guessing each bit.

Finally, I think it's pretty likely that a RAM would have a back bias generator, which is a good reason for it to need some warm up time. I can't quite see why it should be necessary to wait for five times as long as the spec though: what's a spec for?

Good story Jeff!

Cheers
Ed

[Dajgoro]

I bought 4 modules, but with no ram on it.
I ordered from ebay only 4 ram chips, just to test my 68020 sbc while building it, and I plan to add the ram chips when I am sure that the system is working.

[GARTHWILSON]

I updated the datasheet to fix links that had gone dead. The old one is still there at the old URL, and the new one is at http://wilsonminesco.com/WM-1_4Mx8SRAMm ... -15-13.pdf. The links were to the datasheet of Cypress's SRAM chip I use, and to the CMOS static-handling precautions ap. notes I referenced.

I do wish that people would not move web pages. If they really must re-structure the website and they can't keep the old locations in addition to the new, then there ought to be an automatic re-direct. I have no idea how long the links I gave have been dead.

Later edit: Use this one now: http://wilsonminesco.com/WM-1_4Mx8SRAMmodule4-23-20.pdf . (The old link should redirect though.)

[GARTHWILSON]

The price has gone up and down with the price of the ICs. I've been getting the ICs in quantity from a legitimate Cypress distributor in the U.S. for less than half of what you would probably have to pay for just a few. (I won't use Chinese or eBay distributors, as I've been burned before.) In the first week of April of 2020 however, I went to buy more, and the price had more than doubled, and although it was still in production, there was no stock anywhere which makes me think some contractor is probably buying them up faster than they can be made. The ISSI IS61C5128AL-10KLI was recommended as a substitute. After comparing the data sheets in detail, I concluded that the several differences (mostly improvements over the Cypress part) were too minor to affect the hobbyists who would be buying the module. Further, the ISSI part is much cheaper than the Cypress part would cost me now, so I've gone to the ISSI part. I still need to revise the module data sheet. [Edit, 4/23/20: Done. The link on the front page of my site goes to the revised data sheet.] The ISSI SRAM's data sheet is here (.pdf). The ISSI part is about $0.75 more per piece that I had been paying for the Cypress one though, so I'm raising the price for a fully loaded module (ie, with 8 ICs) from $69 to $75. Add postage of course, and sales tax if going to a California address. (Add $10 more if you want .300" row spacing, because of the extra labor involved in improvising the connector.) Partially loaded modules are available for less. Pictures and additional details are on the front page of my website.

[BigDumbDinosaur]

GARTHWILSON wrote:

The ISSI IS61C5128AL-10KLI was recommended as a substitute.

I've been using ISSI's 128KB SRAM in POC V1. It was a choice between theirs and Cypress'.  Cost was the deciding factor, as all electrical and mechanical characteristics are the same in both cases.

[BigEd]

I'd be interested to know how many of your modules are out there by now, Garth.

[GARTHWILSON]

BigEd wrote:

I'd be interested to know how many of your modules are out there by now, Garth.

Looks like 28.

[BigEd]

Ta!