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AM4378: Starter Kit PCB layout questions

Gregory Duperon
Expert 8450 points

Part Number: AM4378

I'm working with a customer to integrate aspects of the Start Kit into a design.  Customer has a few questions regarding potential changes to DDR3 routing.

Can you help with the following questions:

1) The Starter Kit uses blind vias in a number of places.  In many cases, it is clear from the congestion a through via would have impacted routing channels needed on other layers.  In other instances, a blind via was used where a through via could have been used.  Based on the size and layers of the board, we can save 20-30% on our PCBs if blind or backdrilled vias are not necessary.  Once we remove nets not needed by our design, we now have a much larger number of vias that could be converted to through style.  

a) Were blind vias just used for signal integrity and can we convert these to through vias?

b) If for signal integrity, can we move the DDR3 routing from layer 3 to layer 6 so that the resulting via stub is very small?

2) The Starter Kit uses 4 mil traces in several areas for the DDR3 routing.  Shifting these to 5 mil would give us more production flexibility.  We understand the need to ensure impedance and clearance spacing requirements are met.  Can we shift to 5 mil?

3) The Starter Kit uses 0 Ohm series resistors on address, clock, and a number of control strobes.  We'd like to eliminate these and any associated vias added to bring the signal to the top.  This seems to be very favorable because it only improves the routing, but we'd like to know if there was any purpose to these parts being added.  They seem contrary to the supplied guidelines.

4) The Starter Kit uses Micron DDR3L. These parts are currently on allocation so it is possible they will not be available when we are ready to build.  We are looking at second sourcing parts from ISSI as in every aspect I can see the specifications are the same.  During the development phase we will migrate to lower density parts as we feel only 512MB is needed for our system.

Memory Part Numbers:

Micron Original Starter Kit part:  DDR3L  MT41K256M16HA-125  (-1600, 256Mx16)

Micron Prototype DDR3L  MT41K256M16TW-093:P  (-2133, 256Mx16)

ISSI Prototype  DDR3L  IS43TR16256AL-15HBL  (-1333, 256Mx16)

Micron Production DDR3L  MT41K128M16JT-125:K  (-1600, 128Mx16)

  • 0
    TI__Guru**** 393215 points
    1. Please refer to this document for better understanding of high speed interfaces routing: www.ti.com/.../spraar7g.pdf
    2. That's entirely up to you. Impedance and spacing requirements must be met.
    3. Yes, you can remove these. EVMs are early designs that are used for device validation and they have a number of components, which are not required in end designs.
    4. The minimum requirement is DDR3-1600 to be used for AM437x designs. See Table 5-51 in the AM437x Datasheet Rev. D.

    You should also be aware that the AM437x Starter Kit uses a balanced-T topology for the DDR layout. This has been done for evaluation of this topology, but is not included in the DDR3 design guidelines at this time. It can be implemented, but your customer should have a lot of experience in DDR3 layout and SI analysis should be performed on the design before committing to PCB.