i.MX6 DDR3 Write Post Amble Delay (tWPST) too short.

Document created by Mark Middleton Employee on Jun 5, 2013
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Issue: During DDR3 Burst Write, the DQS strobe signal must be driven low for a minimum of 0.3 x cycle period on the last data clock cycle before it is released. This ensures sufficient time for the write to be strobed correctly. When measuring this timing parameter, it has often been found to be too short. This may be contributing to write errors on customer boards, depending on the signal layout used by the board.

 

Root Cause: The internal DQS strobe enable signal is controlled by the MMDC, which is tied to the SDCLK clock signal. But the DQS strobe signal can be delayed in the MMDC to match different SDCLK trace lengths by using Write Leveling parameters to ensure the the DQS strobe edge reaches the DDR3 device at the same time the SDCLK edges reaches the device. If the write level delay is too long, the MMDC can crop the end of the DQS strobe signal too short, causing a violation of the Write Post Amble Delay timing specification and potentially leading to  write errors.

 

How much delay in the Write Leveling parameter would cause this problem? The Reference Manual states that a delay around half a cycle may cause problems, but testing on some boards indicates that delays even as short as 1/4 a cycle could cause violations of the Write Post Amble Delay.

 

Solution: The MMDC was designed with the ability to add extra time to the strobe enable period during write procedures. This parameter is referred to as Write Additional Latency. It is found in the MMDCx_MDMISC register and the field is labeled as WALAT. Incrementing the value of this register field by one adds a full clock cycle delay to the Write Post Amble period, and ensures enough time at the end of a burst write to guarantee a correct write. There is no maximum value to Write Post Amble Delay. Setting WALAT = 1 (or larger if WL parameters are larger) will cause a small hit in overall performance, but will add to the reliability of write operations, particularly on boards that require larger WL parameter settings.

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