On Fri, Sep 9, 2016 at 2:56 AM, Eric Smith <space...@gmail.com> wrote: > Note that the speed grade of a DRAM only guarantees that it is at > least as fast as the grade; it may be faster. For instance, an > MK4116-2 is rated for maximum 150 ns access, while an MK4116-3 is > rated for maximum 200 ns access. If a machine is designed to work > with the MK4116-3 (maximum 200 ns access), either the MK4116-2 or > MK4116-3 will work, and the machine cannot distinguish them, as it's > entirely possible and likely that many 4116-3 chips are under typical > conditions actually just as fast as the 4116-2. This is true whether > the mix of -2 and -3 parts are in the same bank or different banks.
I once worked on an ISA board that was perfect. The driver worked, we got good data. We could control the system we needed to based on a stream of measurements from the board. We moved on to other projects, but maybe 6 or 7 years later another customer had a need almost identical to the first. We found we didn't have enough parts left over from before so we had to buy new FIFOs to build new boards. The FIFOs we used were 200ns parts (I forget the number), so we bought a bunch and set to building new boards. In test we found the card was misbehaving once in a while (like maybe once a day). Of course, the driver was blamed (I hated that, as the driver writer). Long story short: the hardware design had some asynchronous elements that depended on the 200ns timing. The FIFOs were actually a faster 100ns version that was marked for 200ns because the times were a maximum and it was easier to make one speed grade and sell it into many different legacy markets. Once we corrected the asynchronous behavior in the circuit, everything worked again. So even getting an identical part number won't guarantee an identical part due to changes in process :( Warner
