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

[Ralf-Peter Rohbeck]

Yes that's what I'd use. You'll still need a 100k pull-up from EXT_BOOT0 
to VCC to keep the transistors on when EXT_BOOT0 is disconnected or high.
You don't need the high beta of the combined transistors, just the 
increased VBE, so you could replace Q1 with a silicon diode with the 
anode on EXT_BOOT0 and the cathode on B of Q3.

On 08/23/2015 07:52 AM, Luke Kenneth Casson Leighton wrote:
> ok so i redid it, using a darlington pair, because yes the voltage
> definitely needs to be below 0.69v at the output:  also, forget about
> the "disconnected" operation, that can be done with a jumper.
>
> http://hands.com/~lkcl/ttl_logic_circuit2.png
>
> still don't feel completely happy (esp. with the schottky diode), i
> feel another transistor might be needed especially because VREFTTL
> (for the EOMA68_BOOT0 TTL level) could be *lower* than 3.3v.
>
> l.