Are PC133 speed-tested SDRAMs backward compatible with PC100?

Yes. The AC timing specifications on a PC133 device are tested to allow a system bus to run at 133 MHz.<br />
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The PC100 and PC66 timing specifications are more relaxed on these timings.<br />
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Majority of PC133 chips should work at PC100 and PC66. In fact, a -75 device is specified for PC100 timings using CAS latency = 2.<br />
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Please refer manufacturer data sheet for AC timing table in the appropriate data sheet tCK at CL = 2.