Different CL for iMX6 and DDR3 (MT41K128M16JT-125)

[mfranz]

Hey,

we've used "I.MX6Q DDR3 Script Aid V0.10.xlsx" to generate register settings for the iMX6Q and four MT41K128M16JT-125 in u-boot (using DDR3-1066 and its timings) for a custom board. The Script Aid suggests among other values:

• MMDCx_MDCFG0 (0x021b000c): 0x545979A4 (where the '4' stands for CL 7)
• MMDC0_MDSCR (0x021b001c; set MR0): 0x19308030 (where the first '3' stands for CL 7)

This doesn't work, though -- I get R/W errors and u-boot doesn't even come up after setting the board up using JLink. What makes it work is to increase the CL in MMDC0_MDSCR (0x19408030; CL 8) or to decrease the CL in MMDCx_MDCFG0 (0x545979A3; CL 6). It doesn't matter as long as the CL for the DDR3 is greater than the CL for the iMX6Q by one! This is a little irritating. Can anyone imagine what could be the reason for this behaviour? I would have assumed that the CAS read latencies should be the same for iMX6Q and DDR3, and that's what pretty much all boards in u-boot are configuring, except one.

BTW: The calibration values may also not be the default values of 0x00000000 and 0x40404040, respectively. Maybe that's related?

m.

[Yuri · Accepted solution]

Hello,

  CAS latency should be the same for the i.MX6 MMDC and DRAM (MT41K128M16JT).

Some issues in Your case may be concerned with PCB design. Please verify board, using

Chapter 3 (i.MX 6 Series Layout Recommendations)  of the Hardware Development Guide  

https://www.nxp.com/docs/en/user-guide/IMX6DQ6SDLHDG.pdf 

  Also, it makes sense to use  section 1.1 (Schematic and Layout Design Rules) of the DDR3 Porting Guide. 

In  particular, there is an Excel page named “MX6 DRAM Bus Length Check” in “HW Design Checking List  
for i.Mx6”. Designer can use it for layout self-checking. Input trace length of the design into cells in pink  
circle then, the bottom cell in same column may change to red color if layout breaks the rule.

Freescale i.MX6 DRAM Port Application Guide-DDR3 

Have a great day,
Yuri

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