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

[Ralf-Peter Rohbeck]

You could use two transistors in a darlington configuration or add a 
diode and a resistor to the base of the transistor. Either would cause 
an effective 1.4V VBE needed to turn the transistor on. OTOH with a 
Germanium or Schottky diode you should be OK as long as EXT_BOOT0 is a 
CMOS output because it'll go to almost 0V if the current is low. Not 
sure what the levels might be. Even a nominally TTL output should go 
much lower than 0.7V at low current.

On 08/22/2015 10:21 AM, Luke Kenneth Casson Leighton 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.
>
>
> R28 ensures that the transistor is, by default, ON, so that R2 is
> connected to GND and thus BOOT0 operates in "normal" mode.
>
> however current will flow from EXT_BOOT0 if it is "high", which would
> start fighting with VCC_SYS_3V3 if EXT_BOOT0 is over 3.3v, so D1 stops
> that from happening.
>
> i think i'm stuck though when it comes to pulling EXT_BOOT0 down to
> "GND" because actually it will be TTL 0.7v (or so), which, when the
> diode is taken into account (another 0.7v), i don't think it's enough
> to switch off the transistor.
>
> ... so.... help!  what do i do?
>
> tia,
>
> l.
>
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