Am Mon, 15 Oct 2018 19:02:48 +0200 hat "H. Nikolaus Schaller" hns@goldelico.com geschrieben:
Hi,
It still means to me: If we disable DLL, then self-refresh should still work - and unless we don't run into any issues when using the RAM normally with DLL switched off, we shouldn't have any downsides, or should we? As DLL becomes more important the faster the RAM clock is: 532MHz isn't that fast (DDR3 supports way higher clock rates) which is the reason DLL off works fine...?
I have thought a little about the key purpose of the DLL - it is to compensate for different wiring lengths between clocks and data lines, especially on bigger DDR3 DIMMs for PCs. There, each DRAM can have a different distance from the connector and hence a different delay, especially if there are 8 or 16 chips on one board, there may be some cm. Each m distance on some PCB means roughly 10 ns delay. So a memory board with 5cm length can have 500ps delay between both ends which is a lot for a 1GHz signal (half sine wave). And this is compensated by the DLL. But our CPU bord has a much nearer connection of at most 2cm and mostly parallel data and clock lines to each chip. So this could be the reason why the DLL doesn't seem to be ultimately necessary.
Interesting. While this doesn't explain why this fails for our system, it is good to know we might not need it.
Are these memory chips the same that are being used on normal DDR3 memory DIMMS? Because in that case, especially, with the additional connectors of the slot, this seems to be very important.
Hm. One thing: Could it be DLL causes issues when the distance from the RAM chip to the CPU is TOO short?
I know that with some video / audio cables (and fibre glass) there's a MINIMUM cable length as well, otherwise it's overpowered (or has reflections).
I know the SIGNAL lines have some autolevelling and a driver impedance for those short traces.