NetNews Usenet Archive 1992 #31/NN_1992_31.iso/spool/alt/lang/asm/513

Internet Message Format | 1992-12-29 | 1.8 KB

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From: [e-mail] (robert.k.nichols)
Subject: Re: DRAM refresh rates
Message-ID: <[e-mail]>
Summary: Extremely sensitive to temperature
Sender: [e-mail]
Organization: AT&T
References: <[e-mail]> <[e-mail]> <[e-mail]>
Date: Tue, 29 Dec 1992 01:29:57 GMT
Lines: 26

In article <[e-mail]> [e-mail] (Chuck Strouss) writes:
...
>It is surprising how long some DRAMs can go without refresh.  For
>production systems, you must believe the manufacturers worst-case
>specs, but DRAMs rarely need to be refreshed that often.
...

The length of time that a DRAM cell can hold its content without
refresh is inversely proportional to the leakage current in the
device.  The leakage current, in turn, is an *exponential* function of
temperature.  What this boils down to is that a device with cells that
must be refreshed every 2 ms at max rated temperature will hold
information for over 20 SECONDS at a junction temperature of 25 C.
One particularly nasty consequence of this is that it becomes very
difficult to tell whether refresh is working correctly in your system,
and even harder to devise a test which will reliably detect memory
cells that might be marginal (from a refresh standpoint) at high
temperatures.

My apologies if the numbers aren't quite correct.  It's been some time
since I've worked in this area (since the days of 2 ms refresh
requirements, in fact), and my memory is a bit hazy.

Bob Nichols
AT&T Bell Laboratories
[e-mail]