[SI-LIST] DDR2/3 and tQH/tQHS

  • From: Conrad Herse <conrad.herse@xxxxxxxxxxxxxxxxxx>
  • To: si-list@xxxxxxxxxxxxx
  • Date: Thu, 24 Jan 2013 10:23:37 -0600

We've been having some internal discussions on DDR tQH/tQHS that I'm 
hoping some (memory) experts can provide some insight on.

Some background: DDR2 specs provide a tQHS number (data hold from next 
DQS strobe). From this one can derive a tQH number (DQS to first DQ 
invalid) using the following formula (ignoring the inclusion of jitter 
for this discussion):

tQH = tHP - tQHS

For DDR3 the specs provide tQH (expressed as a percentage of tCK). 
Though not explicitly stated in the specs I would (dangerous word here) 
"presume" one could use the same formula above to derive a tQHS value if 
one were so inclined.

Here's the question: what happens when a DDR device is operated at a 
frequency that's slower than it's bin rating? For DDR2 tQHS remains 
constant and tQH (expressed as a percentage of tCK) varies. Following 
strict interpretation of the DDR3 spec it's tQH (expressed as a 
percentage of tCK) that remains constant and tQHS that varies, the 
opposite of DDR2.

I am skeptical of this for DDR3. I would expect that it's really tQHS 
which remains constant across different operating frequencies for a 
given speed grade, but am not familiar enough with DDR internal 
implementation to be certain.

Can anyone shed some insight into this apparent discrepancy?

Thanks,
-- 
Conrad Herse
Alcatel-Lucent

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  • » [SI-LIST] DDR2/3 and tQH/tQHS - Conrad Herse