Setting I/O's

GoogleGroups
1

Dear Reader,

I have a question about the following problem.
On the PCB I designed I use the BBB for setting some pumps and valves.
When the system starts up the first seconds the IO pins are not logic 1 or 0.
Is it possible to set the GPIO’s in U-boot before starting up the kernel and how can I do this?

Thanks,

Maarten

2

Read the datasheet for the default state of each pin on power up.

http://www.ti.com/product/am3358

These cannot be changed. Once the SW is loaded, it can set the pins to whatever function it supports.

Gerald

3

Dear Gerald,

Isn’t it possible to set the IO’s in the U-boot?
before the kernel is started.
This would be fast enough.

Regards,

Maarten

4

Possibly. Not sure if Linux would like that or not. I would think it would have to be a custom UBoot just for your use in that case.

Gerald

5

From the top google hit on "uboot set gpio":

http://e2e.ti.com/support/arm/sitara_arm/f/791/t/248181.aspx

...it's even for a TI Sitara processor.

Executive summary:
Use u-boot commands to directly access memory to turn on the GPIO clock
and setup pinmux and gpio values as desired.

And here's the u-boot documentation page on memory commands:
http://www.denx.de/wiki/view/DULG/UBootCmdGroupMemory#Section_5.9.2.6.

6

All your programmable I/O pins are going to startup in tristate mode. That means you can just add a weak pullup or pulldown resistor (EG. 10K) to each pin’s circuit to make your external devices behave properly until your software programs the pins.

7

That is not correct. Please. Read the datasheet.

Gerald

8

Of course I would not assume that every I/O pins starts in tristate, but he said earlier:

On the PCB I designed I use the BBB for setting some pumps and valves.
When the system starts up the first seconds the IO pins are not logic 1 or 0.
That sounds to me like the particular pins he used do start in tristate.

Reading the datasheet is always the first thing to do, but once you confirm that an I/O pin is tristated then what would be wrong about using a weak pullup or pulldown resistor to keep those pumps and valves from doing some damage before you get the I/Os programmed?