i.MX Processors中的主题 Re: imx6 LPDDR2 parameter clarification

imx6 LPDDR2 parameter clarification

tomcoggio — Wed, 17 Feb 2016 02:44:49 GMT

reg 021b_0018 /021b_4018

WALAT

00000000

WALAT: Write Additional latency. Recommend to clear these bits. Proper board design should ensure that the DRAM devices are placed close enough to the MMDC to ensure the shew between CLK and DQS is less than 1 cycle.

I have the clock 65mm (~434pS) from the processor and the DQS lines are 81mm from the processor ( ~540pS) to the memory chip.

The memory is Micron MT42L256M64D4LM-25WT (400MHz LPDDR2)

Can someone tell me what the statement above " MMDC to ensure the shew between CLK and DQS is less than 1 cycle. " is referring to, because if both distances are less than 1cycle (2500pS) and yet the system wont run at 400MHz, it is ok at 380 MHz.

By setting the WLAT to 1, the system passes the memory test at 400 with flying colours.

My deduction from this is that the statement is referring to the return flight path ??? Could someone please explain if possible.

Thanks!

PS: prior to the WLAT of 1, the memory test would not pass at 1GHz processor speed. now it works with 1GHz also.

Re: imx6 LPDDR2 parameter clarification

Yuri — Wed, 17 Feb 2016 03:06:28 GMT

Hello,

Generally RALAT / WALAT are used to compensate internal delays in order to provide

proper timings for internal output buffer enable signals, controlling internal FIFOs.

Really optimal values for RALAT / WALAT are board dependent and should be found

experimentally. Also, please refer to MMDCx_MDMIC[WALAT] description in the i.MX6

Reference Manual for more details, in particular, regarding to an issue, when the DQS signal

(delayed internally from SDCLK) may be cropped. To avoid it WALAT should be set 1.

Please check if for Your board :

- Address and control signals shorter than the clocks ;

- Longest clock trace must be <= 3 inches.

Have a great day,
Yuri

-----------------------------------------------------------------------------------------------------------------------
Note: If this post answers your question, please click the Correct Answer button. Thank you!
-----------------------------------------------------------------------------------------------------------------------