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IMX28 Current limit (ILIMIT) when powered on by 5v (VDD5V)

Question asked by Matthew Gingras on Jan 14, 2016
Latest reply on Jan 18, 2016 by Yuri Muhin



Ladies and gentlemen,


This is urgent for scheduling of our production run of thousands of our units.


We have hardware configured in the same fashion as Freescale's application note AN4199, Figure 10, in section 3.2.1.  We've noticed in the IMX28 Reference Manual (MCIMX28RM) in section 11.3.1 "USB Compliance Features" it states:


"...System designers must understand
that the current inrush limit during 5 V power-up places restrictions on application current
consumption until it is disabled.
Specifically, after connection to 5V, if the system draws
more current than the current limit allows, the startup sequence does not complete and the
ROM code does not execute."


We are very concerned about this limitation which has potential to prevent the processor from starting.  So our questions are as follows:


1) What specfically should we be watching out for in current consumption so not to hit this limit?  Is there a schematic or application example of what not to do?  What voltage rails should we stay away from oversourcing current from?


2) With objective evidence, such as an error code or an asserted pin, how can we determine if this current limit has been reached and is the reason the device did not start?


3) The manuals for the IMX28 does not make clear what happens when VDD5V is applied to the unit being off, and ILIMIT is set, BEFORE the ROM code executes.  We would like to know exactly what happens, in preferbly a flow chart, BEFORE the ROM code starts running.  Specifcally for the path of an ILIMIT power down, and what mechanism is making that decision so we can satisfy it 100% of the time in our design.


Thank you in advance for the help.