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I've found older SDRAM sources with lower speed grades (133Mhz) are getting more difficult to find, while the higher speed grades (166Mhz+) are more readily available.

Not being a memory expert, I'm not sure if the speed difference is just related to access times inside the chip or if it has anything to do with the data output edge rates.

If, for example, I have a board that works with 133Mhz SDRAM and meets radiated emission limits, should I be concerned about switching over to a 166Mhz speed grade chip (even though I won't be changing the bus speed)?

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  • \$\begingroup\$ for SDRAM with speeds >= 166Mhz, better to go for DDR/DDR2 variant. \$\endgroup\$
    – user19579
    Dec 22, 2013 at 8:36
  • \$\begingroup\$ I'm definitely not going to re-spin the board and use a different micro to use faster memory or even change the bus speed of the current design. The question is more to the difference (or not) in data output edge rates of SDRAM of different speed grades. \$\endgroup\$
    – bt2
    Dec 22, 2013 at 12:30
  • \$\begingroup\$ You might need to check whether your device datasheet specifies the minimum clock frequency at which it can be operated. If it is below 133MHz, then it is safe to use. Also, it will be dependent on your SDRAM controller also. We had tried operating a 266MHz DDR2 memory interfaced to an FPGA down to 100MHz clock frequency and found that it worked (without the advantage of being DDR2, of course). \$\endgroup\$
    – Avin
    Jan 9, 2014 at 11:34
  • \$\begingroup\$ Avin, I checked this a long time back and standard SDRAM will operate correctly down to extremely low clock rates. I can't remember the actual minimum but I think it was around 15kHz! \$\endgroup\$
    – bt2
    Jan 11, 2014 at 15:42

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OP mentioned it in the comments, but it is worth repeating here that yes the rated speed is the maximum speed and the minimum speed for plain SDRAM (not DDR) is very low.

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