From: MitchAlsup <MitchAlsup@aol.com>
Newsgroups: alt.comp.hardware.pc-homebuilt, comp.arch,
Subject: Re: Memory Latency Technical Specifications
Date: Sun, 13 Jan 2008 11:17:20 -0800 (PST)
On Jan 11, 7:46 pm, pg <pen...@catholic.org> wrote:
> al cl wl rl bl ddr ddr2 ddr3 ... RL = AL + CL / WL = RL -1 ...
> It years past, when SDRAM was all we had, memory latency was quoted
> with the CL figure - CL2 was supposed to be better than CL3
> Nowadays, with DDR2, DDR3 and even DDR4 flooding the markets, memory
> latency specs, to me at least, suddenly exploded !
> CL is not enough, now they have al cl wl rl bl and in one of the
> search listing that I found, from ieee.org, they even have this
> formula ---- RL = AL + CL / WL = RL -1 ...
> Now my head starts to ache.
> Can anyone here please enlighten me, in plain everyday English words,
> what all those things stand for? What are their significants? How does
> that effect the memory performance ?
AL address latency. After sending one address this is the number of
cycles one muust delay a second address before sending.
WL write latency. This is the number of cycles after a write has been
started before you are allowed to deselect that bank.
CL command latency.
What is going on is that the external interface is getting faster, but
the internal arrays are not. So back in SDRAM days these were all 1
(or smaller) and since you could only do one thing per cyle on the
pins, all was good (e.g. invisible).
When a bank is selected (ACTIVATE) there is an power surge from firing
all of the bank decoders, the word line driver, all of the bank sense
amps, and then a time is needed for the sense amps to re-establish the
stored DRAM cell charge. At the same time there are various goings on
over at the pins also causing minor surges. You really do not want the
power to be surging at the moment when the sense amp fires! (hint:
sense amps read the wrong data) When the time between requests was 12
ns all of this was hidden, at 8-ish ns it showed up as an additional
cycle, but since the very vast majority of controlers were doing line
reads/writes, it remained invisible (through DDR 2). Now we are
entering the realm where the surge can last longer than the transmit
time of a line. So inorder to make DRAM reliable, one has to avoid
doing analog-ish-like things during these current surges. Another way
to address the problem would be to dedicate more DRAM pins for power
and ground, but the industry is not going that direction due to their
cost-driven basis (pins cost real money and power routing might add
another layer of metal to the part).
WL This is the latency for write data to actually get into the DRAM
storage cell and allow that bank to enter a precharge state. You
really do not want the data only to get to the sense amp and then de-
select a bank, because it is unclear if the data has made it to the
DRAM cell or not.