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.