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

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

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andrewparlane2
Contributor III

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

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igorpadykov
NXP Employee
NXP Employee

Hi Andrew

data should be used for actual operating speed

Best regards
igor

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igorpadykov
NXP Employee
NXP Employee

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
Contributor III

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

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igorpadykov
NXP Employee
NXP Employee

Hi Andrew

data should be used for actual operating speed

Best regards
igor

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andrewparlane2
Contributor III

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 

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igorpadykov
NXP Employee
NXP Employee

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

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andrewparlane2
Contributor III

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

Thanks for your help,

Andrew

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sachinshetty
Contributor I

Hi Andew,

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

Thanks,

Sachin

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