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Centering a DDR Strobe in the Middle of a Data Packet

The Orion CEV Northstar ASIC (application-specific integrated circuit) project required a DDR (double data rate) memory bus driver/receiver (DDR PHY block) to interface with external DDR memory. The DDR interface (JESD79C) is based on a source synchronous strobe (DQS\) that is sent along with each packet of data (DQ). New data is provided concurrently with each edge of strobe and is sent irregularly. In order to capture this data, the strobe needs to be delayed and used to latch the data into a register.

A circuit solves the need for training a DDR PRY block by incorporating a PVT-compensated delay element in the strobe path. This circuit takes an external reference clock signal and uses the regular clock to calibrate a known delay through a data path. The compensated delay DQS signal is then used to capture the DQ data in a normal register. This register structure can be configured as a FIFO (first in first out), in order to transfer data from the DDR domain to the system clock domain. This design is different in that it does not rely upon the need for training the system response, nor does it use a PLL (phase locked loop) or a DLL (delay locked loop) to provide an offset of the strobe signal.

The circuit is created using standard ASIC building blocks, plus the PVT (process, voltage, and temperature) compensated delay line. The design uses a globally available system clock as a reference, alleviating the need to operate synchronously with the remote memory. The reference clock conditions the PVT compensated delay line to provide a pre-determined amount of delay to any data signal that passes through this delay line. The delay line is programmed in degrees of offset, so that one could think of the clock period representing 360° of delay. In an ideal environment, delaying the strobe 1/4 of a clock cycle (90°) would place the strobe in the middle of the data packet. This delayed strobe can then be used to clock the data into a register, satisfying setup and hold requirements of the system.

This work was done by Michael Johnson, Dave Nelson, James Seefeldt, Weston Roper, and Craig Passow of Honeywell for Johnson Space Center. For further information, contact the JSC Innovation Partnerships Office at (281) 483-3809.

Title to this invention has been waived under the provisions of the National Aeronautics and Space Act {42 U.S.C. 2457(f)}, to Honeywell. Inquiries concerning licenses for its commercial development should be addressed to:

    Honeywell
    P.O. Box 52199
    Phoenix, AZ 85072-2199

MSC-24931-1