pin states at power up


I am having an issue with I/O behavior on beaglebone power up. For the first 20ms or so (I am guessing until U-Boot kick in), the I/Os in question (i.e gpmc_ad8/gpio22) are driving some sort of a square wave signal until finally going low. Is there a way to force the pins to come up in a specific state at power up? I dont so much care if its high or low, but the high frequency switching that seems to occur is killing the component down the circuit.

Thanks for any advice.

Look at the processor datasheet. Check the offending pins to see what their default state is. That should tell you what their mode it. Some pins like UART can be active scanning for sources to boot form. But that should only be UART0. GPMC can be used to scan for NAND in UBOOT. It may be looking for a NAND device. I know UBOOT prints out “No NAND Device Found”

There is no way to force the pins to do anything at power up other than their default states. UBoot can set them once loaded and running.


Gerald - thanks good info. I Looked more into it and according to the datasheet, the pin should default to GPIO (mode 7), so I think you are correct with the U-Boot trying something funny. Can anyone point me in the right direction with modifying or configuring U-BOOT to prevent this GPMC access at start up? Do you think its a setting (i.e. environment variable), or will I need to tweak and recompile the source code?

Sorry - total noob when it comes to U-BOOT…

Thanks for advice.