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

Daniel Serpell daniel.serpell at gmail.com
Tue Aug 25 00:31:37 BST 2015


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:

The schematic is here:

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.


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