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AM335x eMMC external pull-ups

tzvi reznick
Prodigy 130 points
Other Parts Discussed in Thread: AM3359

Hello,

We are working on a custom board using am335x processor.

According to the hardware design guides there was a change in the requirements regarding EMMC.

We did not put 10k pullups on emmc lines following this guideline:

when connecting to a card cage, include 10K pullups on all data signals. These pullups are not needed for embedded device connections, such as eMMC memory devices.

Now we saw that the instructions changed.

We see some instability with emmc, sometimes it is not recognized when Linux kernel boots.

Do we have to put pullups on emmc ?

See the information from the hardware design guides below: (and relevant emmc info)

This is the original hardware guide  we used:

processors.wiki.ti.com/index.php

MMC

·         include a 22ohm series resistor on MMCx_CLK (as close to the processor as possible). This signal is used as an input on read transactions and the resistor will eliminate possible signal reflections on the signal which can cause false clock transitions.

·         this also requires you to set RXACTIVE=1 in the pinmux configuration for the MMC_CLK signal.

·         when connecting to a card cage, include 10K pullups on all data signals. These pullups are not needed for embedded device connections, such as eMMC memory devices.

After 26-Jan-2016

processors.wiki.ti.com/.../AM335x_Schematic_Checklist

MMC

 

·         include a 22ohm series resistor on MMCx_CLK (as close to the processor as possible). This signal is used as an input on read transactions and the resistor will eliminate possible signal reflections on the signal which can cause false clock transitions.

Thanks 

Tzvi.

  • 0
    TI__Guru**** 393215 points
    Hi,

    External pullups are required by eMMC specification. In your case do you have the internal pullups on the CMD and DATA lines enabled?
  • 0 in reply to Biser Gatchev-XID
    Prodigy 130 points
    Hi,
    Thank you for your swift reply.
    Yes I have internal pullups on CMD and DATA lines enabled, emmc was not working without them.
    most of the time everything works fine.
    do you think that the internal pullups should be enough ?

    Regards
    Tzvi.
  • 0 in reply to tzvi reznick
    TI__Guru**** 393215 points
    They are quite weak, typical value is 33kOhm at 3.3V, with a very large min/max range - approx. +/-50%. So external resistors are definitely the preferred solution.
  • 0 in reply to Biser Gatchev-XID
    Prodigy 190 points

    Does someone know the reason for the external pull up?(previously there was no need for eMMC)

    In previous schematics checklist there was no need for pull ups while using eMMC now there is a change in the schematics checklist and 10K resistors are needed.

    My board does not have external pull ups but has internal pull up  both in the eMMC (Micron) and the  in the am3359.

    Is it not enough? why they are required? in the eMMC data sheet  it said it is to prevent bus floating but bus does not float (internal pull ups)? I know from other posts designers operates it without the external resistors is it safe?

    Can someone arrange the information and the explain this issue from top to bottom? is it related to emmc  components which are not comply with the standard or.....

    Regards

  • 0 in reply to Liron Rodavsky
    TI__Guru**** 393215 points
    If your board functions normally then you don't need external pullups. They are recommended per eMMC specification and especially if you have long PCB traces to the eMMC.