FEDEVEL
Platform forum

Abhi P Choksi , 08-13-2019, 05:58 AM

As per the NXP hardware design guide its stated that these voltages are required at +1.35V. In the design of Open Rex board, I observed that VDDARM_IN and VDDARM23_IN kept at +1.375V rather than +1.35V. I would like to know how the designer came to this decision?

Paul van Avesaath , 08-13-2019, 07:42 AM

maybe to counteract some losses in the design?

Abhi P Choksi , 08-13-2019, 10:46 PM

I don't get your answer. Please explain in more detail.

Paul van Avesaath , 08-13-2019, 11:49 PM

i do not know for sure here. but since it is such a small difference it might be to counter act the voltage lost in a power plane.. to explain:
if I would have two points connected to a power plane
1 point is supplying the power let say of 1V
the second point is lets say 10 cm away from the first point and i will have a 1K resistor connected from that plane and the other side to GND.

If would measure at first point with a volt meter it would say 1V
if I would measure on the second point it might say something like 0.86V

there is DC loss in you power plane.

so to fix that I could set the voltage on the first point to 1.14V to counteract the 0.14V dc loss.

if i would measure the Voltage on the second point it would now read 1V..

Abhi P Choksi , 08-14-2019, 12:16 AM

I appreciate your explanation.
Thanks.

robertferanec , 08-16-2019, 07:46 AM

The reason may be more simple. I believe, NXP is updating datasheets and they also have new iMX6 CPUs (the PLUS processors). So as time goes, they adjust some values. Maybe when you compare the old schematics and documents with the new one, there may be some differences - including this one.

PS: There is internal regulator inside of the CPU, so this input voltage can have much wider range and is adjusted. The value of VDDARM just needs to be high enough to be sure the CPU will be stable, but we do not want to have it too high as unnecessary voltage will change to heat.