Datasheet

DQ V
REF
Training
The DRAM internal DQ VREF specification parameters are operating voltage range, stepsize, VREF step time,
VREF full step time and VREF valid level.
The voltage operating range specifies the minimum required VREF setting range for DDR4 DRAM devices.
The minimum range is defined by VREFmax and VREFmin as depicted in the following figure.
VREF
Range
VDDQ
VREFmax
Vswing Small
Vswing Large
System Variance
Total Range
VREFmin
Figure 44. V
REFDQ
Operating Range (V
REFmin
, V
REFmax
)
The VREF stepsize is defined as the stepsize between adjacent steps. VREF stepsize ranges from 0.5% VDDQ
to 0.8% VDDQ. However, for a given design, DRAM has one value for VREF step size that falls within the range.
The VREF set tolerance is the variation in the VREF voltage from the ideal setting. This accounts for
accumulated error over multiple steps. There are two ranges for VREF set tolerance uncertainty. The range of
VREF set tolerance uncertainty is a function of number of steps n.
The VREF set tolerance is measured with respect to the ideal line which is based on the two endpoints.
Where the endpoints are at the min and max VREF values for a specified range. An illustration depicting an
example of the stepsize and VREF set tolerance is below.
VREF
Stepsize
VREF Set
Tolerance
Actual VREF
Output
Straight Line
(endpoint Fit)
VREF
Digital Code
Figure 45. Example of V
REF
set tolerance (max case only shown) and stepsize
AS4C256M16D4
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Rev.1.0 Aug.2019