[Arm-netbook] help needed with a ttl logic circuit

[Daniel Serpell]

Hi!

On Sat, Aug 22, 2015 at 2:21 PM, Luke Kenneth Casson Leighton
<lkcl at lkcl.net> wrote:
> hi folks could someone please help analyse this circuit to see if it is correct?
>
> http://hands.com/~lkcl/ttl_logic_circuit.png
>
> the requirement:
>
> * an STM32F's "BOOT0" is, by default "pulled to GND" in order that
> "normal" operation happens
>
> * the boot operation needs to be selectable by a GPIO from a main CPU
> (TTL, with a different voltage)
>
> * if however the main CPU's GPIO is *DISCONNECTED*, "normal" operation
> of the STM32F is required.
>
> so, the main things here are:
>
> * the STM32F is run from a 3.3v supply
>
> * the CPU can be a totally different TTL voltage level from the
> STM32F.  it could be 5.0V, it could be 3.3v, it could be 3.0v, it
> could be even 1.8v.
>
> * the default behaviour needs to be "pull to GND", even if the input
> is floating.
>

A little late to the discussion, but I would use something like:
  https://github.com/sparkfun/Logic_Level_Bidirectional

The schematic is here:
  https://cdn.sparkfun.com/datasheets/BreakoutBoards/Logic_Level_Bidirectional.pdf

To assure that the output is low when the LV input is disconnected,
you could add a
resistance from the input to ground, I would use a 2k resistor, so
that at 1.8V the
gate has 1.5V, setting a low state in the output.

This circuit is cheap, and you would use only 3 resistors and one FET.

    Daniel.