Strobe offset in bidirectional memory strobe configurations
A method and apparatus for determining correct timing for receiving, in a host in a memory system, of a normal toggle transmitted by an addressed memory chip on a bidirectional data strobe. An offset in the data strobe is established, either by commanding the addressed memory chip, in a training period, to drive the data strobe to a known state, except during transmission of a normal toggle, or by providing a voltage offset between a true and a complement phase in the data strobe, or by providing a circuit bias in a differential receiver on the host the receives the data strobe. A series of read commands are transmitted by the host to the addressed memory chip, which responds by transmitting the normal toggle. Timing of reception of the normal toggle as received by the host chip is adjusted until the normal toggle is correctly received.
1. A memory system comprising:
a bidirectional data strobe;
one or more memory chips coupled to the bidirectional data strobe;
a host coupled to the bidirectional data strobe, the host further comprising:
a receiver to receive signals from the bidirectional data strobe;
a circuit to pass an output from the receiver to circuitry on the host only when the circuit is enabled by a receiver enable;
a training control to, during a training period, transmit commands to the one or more memory chips and to determine a correct timing of the receiver enable, the training control further configured to ensure that the bidirectional data strobe has a particular voltage value during the training period;
wherein the host is configured to send a first command to a memory chip, the memory chip, in response, configured to drive the data strobe to the particular value during the training period; and
wherein the host is configured to send a first read command, subsequent to the first command, to the memory chip during the training period to cause the memory chip to transmit a normal toggle responsive to the read command.
2. The memory system of claim 1 , wherein the host is configured to send one or more additional read commands to the memory chip during the training period, each of the one or more additional read commands causing the memory chip to transmit a normal toggle responsive to the read command.
3. The memory system of claim 2 , wherein the host is configured to adjust a sample time after receiving each normal toggle until a satisfactory sample time is determined which correctly receives the normal toggle.
4. The memory system of claim 1 wherein the first command is a Mode Register Set command that sets a DQS drive bit on an addressed memory chip.
5. The memory system of claim 1 , wherein the host provides a voltage offset between a true phase and a complement phase of the bidirectional data strobe to effect the particular value during determination of correct timing of the receiver enable.
6. The memory system of claim 5 , wherein a termination on the host creates the voltage offset between the true phase and the complement phase of the bidirectional data strobe during the training period.
7. The memory system of claim 6 , the termination on the host matching a transmission line impedance of the bidirectional data strobe during the training period.