I'm looking at this data sheet: T67M-512Mb-Mobile-Lpddr-Sdram (targeting speed grade -75) and trying to understand the timing requirements for performing a read, specifically when to sample the DQ input data lines. It gives a formula on page 27 for Data valid output window (DVW) being
tQH - tDQSQ
Writing down the min and max values as
<min,max> in unit of nanoseconds, I find
tDQSQ on page 26 as
<-,0.6> . I assume if the minimum is not stated, the worst case is 0?
A formula is given for
tQH on page 27 as
tQHS is on page 27 and given as
<-,0.75> while yet another formula is given for
tCH,tCL. Not sure what a comma means here.
tCL are both found on page 26 with values of
Plugging in all my values, I get a final formula of:
(<0.45, 0.55> - <0.0, 0.75>) - <0.0, 0.6>
This formula confuses me as there are values within the ranges that can result in an overall valid window time of zero, or even a negative value.
Am I calculating/interpreting this correct? Do these values means that there are no gaurantees that the output data will be valid?