What values do I use for tRCD, tRC and tRAS in the IMX6UL DDR3 script aid?

[andrewparlane2]

We are building a board with the IMX6UL and a MT41K128M16JT-125 DDR3L chip.

The datasheet for the DDR3L chip states that for speed grade 125 the data rate is 1600 MT/s. However the max speed of the DDR bus on the IMX6UL is stated as "up to 400MHz (800 MT/s)" (table 33-1 in the TRM).

In table 1 of the DDR3L chip it states that tRCD is 13.75ns for speed grade 125 at 1600 MT/s. There is not an entry for 800 MT/s. In table 29 it has entries for DDR3L-800 which are speed grades 25 / 25E, and it lists tRCD values for that in terms of clock ticks, giving 5 ticks for grade 25E and 6 ticks for grade 25, where 1 tick is 2.5ns, so 12.5ns for grade 25E or 15ns for grade 25.

That's three options I have to use for tRCD. The same applies for tRC and tRAS. I'm guessing I want to use one of the values for the DDR3L-800 but not sure if to use the grade 25 or grade 25E. I could try both, but I'd prefer to understand how this works and not just go with the faster one because it works on one board.

Thanks for any advice.

Andy

[igorpadykov]

Hi Andrew

seems the same part is used on i.MX6DL Sabre SD (max. ddr freq. 400MHz) board

and one can look at its settings in ddr tester (../script/mx6dl)

i.MX6/7 DDR Stress Test Tool V2.80 

Best regards
igor
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[andrewparlane2]

Hi Igor,

Thanks for your reply. Unfortunately while the Saber SD uses a similar part it is different.

Sabre SD: MT41J128M16HA-15E

Mine: MT41K128M16JT-125

They're pretty similar but MT41K and MT41J have different data sheets. Additionally I'm using speed grade -125, whereas the Sabre SD uses -15E.

In addition, I'm more interested in the theory of how to choose the correct numbers rather than copying existing values. I'm sure I can get my board to work, since there's only three possible options, and I can try all three (starting with the fastest).

So to rephrase my question: When using a DDR chip at a slower frequency than it's max, do you use the tRCD, tRC, and tRAS:

a) The speed bin for the listed speed grade of the chip even though you aren't running it at that speed. In my case this would be using speed grade -125 (1600 MT/s).

b) The speed bin for the frequency I'm actually using even though that's not for the speed bin of the chip. In my case this would be using speed grade -25 or -25E (800 MT/s).

c) Something different?

Thanks,

Andrew

[igorpadykov · Accepted solution]

Hi Andrew

data should be used for actual operating speed

Best regards
igor

[andrewparlane2]

Hi Igor,

Great thanks.

Follow up question:

For the speed of 800 MT/s there are two speed grades listed in table 29 of the DDR chip's datasheet. There's -25 and -25E. Now obviously if I were using a DDR chip with speed grade -25 or -25E it's obvious which value I should use. However since I'm using a -125 chip, how do I choose between the -25 and -25E values.

The clock period for both is 2.5ns (400MHz), however tRCD is 5 clock cycles for -25E = 12.5ns and 6 clock cycles for -25 = 15ns which is a pretty big difference.

The datasheet doesn't seem to indicate the difference between speed grades with an E and those without, it just lists them as different speed grades.

Again I could just try both options, but I'd prefer to understand how to make this decision.

Thanks,

Andrew 

[igorpadykov]

Hi Andrew 

for "-25 and -25E" it is more appropriate address to ddr vendor micron.

Current DDR3 Script Aid supports up to 1600Mbps ("-125") and its usage is described in
presentation (for I.MX6UL DDR3 Script Aid  there is example for "-125" and tRCD is 13.75ns )
I.MX6UL DDR3 Script Aid 

DES-N1936 i.MX 6UltraLite DDR Tools Overview and Hardware Design Considerations.pdf 

Best regards
igor

[andrewparlane2]

Ok Thanks. I'll contact micron for assistance about choosing which speed bin to use.

Thanks for your help,

Andrew

[sachinshetty]

Hi Andew,

 I have the same question, did you figure out which table in the datasheet to refer to?

Thanks,

Sachin