Methods and apparatus are disclosed herein for providing tri-state noise
immunity for memory systems such as DDR memory systems, wherein 1) there are large
variations in read data loop delay, and 2) strobe buses have similar termination
and threshold voltages. In one embodiment, strobe receiver circuitry includes a
counter and counter control logic. The counter updates a count in response to strobe
edges of received strobe signals. The counter control logic enables the counter
before each strobe signal is received by generating control signals asynchronously
with respect to the received strobe signals. The counter control logic also resets
the counter after each strobe signal is received by receiving feedback from the
counter and, in response to the feedback, resetting the counter asynchronously
with respect to the received strobe signals. The strobe receiver circuitry may
form part of a DDR memory controller.
