Q&A: What is gpio configuration sampling in mx6 boot?

[#1]

Question:

To eliminate the need for pull-ups AND  pull-downs on the gpio boot override pins to save board space, does anyone know when in the boot rom that the GPIO pins are sampled and whether the internal Pull Downs are already enabled?

Is this true for all the GPIO override pins? To just be able to provide an external Pull-up to specify boot options and NOT both pull-up and pull-down option.

Can this be done?

Answer:

The pull-ups and pull-downs are active while POR_B is low (as defined in the datasheet). The pins are sampled on the 2^nd rising edge of RTC_XTALO before the rising edge of POR_B.

As a caution, on-chip pull-up/downs are very weak and are primarily intended for controlling the pin (keeping it from logically wiggling) if the pin is not connected at all. If a pin is connected out to a trace, relaying on the on-chip PU/PD alone could be a little risky. Since the PU/PD is weak, it's possible to have noise coupled onto the pin. [Similar to Brett's comment above].