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As I'm going to design my first high-speed PCB, I'm confused with some mismatches between what i see in high-speed PCB design tutorials and what i see in real world designs. in the PCB handbook named "RIGHT THE FIRST TIME" by "LEE W. RITCHEY", it's discussed that the transmission lines higher than 1/4 TEL will fail from reflection. In some boards like raspberry pi 3 and banana pi BPI-M64, i don't see any termination resistor between Processor and RAM IC(s). As the frequency of the Allwinner A64 and 4 512 DDR3 RAMS in BPI-M64 is about 800MHz i think 1/4 TEL is less than 1 Centimeter but in the board I see the routes are longer than 2 on 3 Cm without any termination. the The question is:

Is it realy possible to do such designs without termination resistors or I am calculating in a wrong way?

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  • \$\begingroup\$ Careful, the Transmission Electrical Length has nothing(!) to do with your clock rate. It has to do with the highest significant frequency component in your rising edge. \$\endgroup\$
    – pfabri
    Aug 25, 2021 at 9:10

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DDR3 has integrated (on-die) termination for the data lines, and Vtt termination for other lines. Therefore, not all lines require external termination resistors. You will find that several high-speed IO standards have integrated termination, as this avoid the parasitics, extra space, and extra routing of "regular" discrete termination.

I suggest you read this Altera DDR3 termination & layout guidelines document as well as this Wikipedia article: On-die termination and this presentation on DDR3/DDR3L/DDR4.

DDR3 termination is a very complicated subject, as different bits of the interface have different termination situations, as well as on-die termination which can also be dynamic (register settings can modify the value of the on-die termination). To better understand the nuances, you'll likely want to look at several DDR3 controller/DRAM datasheets and reference designs.

Finally, see TIDEP0012 for an example of a DDR3 reference design that does not use Vtt termination.

To sum up, DDR3 is somewhat of a special case when it comes to "conventional" high-speed digital designs, due to integrated features that make it easier to use. Ritchey's book is a good one, I would also suggest reading Howard Johnson's and Henry Ott's texts for more high-speed digital design considerations. Also, without looking at the schematics, it's difficult to fully understand the circuit at hand. Last I checked, the Raspi 3 was still closed hardware. Have you got a schematic? You'll have a far better time designing circuits by reading the datasheet and looking at reference designs than by trying to infer designs from closed hardware.

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  • \$\begingroup\$ Thanks a lot for your complete answer. That's all I need to know at this time. \$\endgroup\$
    – Milad
    Apr 16, 2017 at 5:25
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    \$\begingroup\$ Also see LS1012ARDB for an example of a DDR3L reference design without any "off-die" termination. \$\endgroup\$
    – Luka
    Sep 20, 2019 at 13:19
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    \$\begingroup\$ Fun fact: Dr Howard Johnson was a student of Lee Ritchey. \$\endgroup\$
    – pfabri
    Aug 25, 2021 at 9:14

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