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OMAPL138 Boot[0:7] Multiplex Use.

WENBO LI
Intellectual 440 points
Other Parts Discussed in Thread: OMAPL138

Hi,

On OMAPL138, the Boot[0:7] pins are multiplexed with signals like GPIO_7[0:7] etc. At power-up, I think I must set the Boot[0:7] pins to 0x02 for NOR boot. But once the boot finishes, I would like to use Boot[0:7] pins as GPIO_7[0:7]. What's the simplest way to do this?

I think an 8-ch 2:1 analog switch may do the job. At power up, Boot[0:7] pins connect to boot straps (e.g. 0x02) through the analogh switch from one branch, and after the boot, the Boot[0:7] pins go to another branch of the analog switch, in other words, Boot[0:7] are not tied to to boot straps any more after the boot. Thus, they can be programed and to be used as GPIO_7[0:7] after the boot. Is this the proper way to do so? Is there any applicaiton document/note on how to use the multiplexed Boot[0:7] pins?

Thank you very much in advance.

  • 0
    TI__Guru** 116170 points

    Could you elaborate why you want to use the GPIO muxed with the boot pins in your application? I generally wouldn`t advice going down this route as it would complicate your design and might has potential to cause boot issues if not handled appropriately. There are several other GPIO pins on the device that are available for user configuration so you could explore other options. If you have exhausted all GPIO pins then we might have to look at other ways to implement this design.

    Regards,

    Rahul

  • 0 in reply to Rahul Prabhu
    Intellectual 440 points

    Thanks Rahul.

    Yes, I have exhausted all GPIO pins. Any idea to implement this design?

    A thought here:

    By default, these pins Boot[0:7] are inputs. In normal operation after the reset, if we use these pins as outputs only, is that going to be better. In this case, we just to use a 20k ohm pull up resistor for logic high, and a 20k ohm pull-down resistor for logic low to satisfy the boot mode logic states, and after the reset is released, then the software is able to configure them as an outupt to drive the external logic control. Not sure this solution is clean.

    The question is, with a pull down resistor 20k, can OMAP read it as a logic low reliably as an input (boot mode selection), and can the OMAP still drive the pin to a logic high reliably when it's configured to an output? In other words, what's the maximum resistor value for the pull down resistor that the OMAP can still read it as a logic low at boot selection mode (Boot[0:7] as inputs).  

    Any suggestions?

    Thanks,

    Wenbo