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

[Luke Kenneth Casson Leighton]

On Sat, Aug 22, 2015 at 10:27 PM, Ralf-Peter Rohbeck via arm-netbook
<arm-netbook at lists.phcomp.co.uk> wrote:
> You could use two transistors in a darlington configuration

 just reading about that here:
http://www.electronics-tutorials.ws/blog/relay-switch-circuit.html

> 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.

 ralf, really appreciate the response.  any chance you could find a
random image on the internet as a demo circuit, or perhaps draw it out
for me?

 at the moment i'm considering using a 2nd transistor, powered from
the other board (yes it'll be TTL - just with a different voltage).

 basically this design is split into two boards, one is the EOMA68
main PCB with the CPU Card: i'd like the CPU Card to control the boot
process (for firmware re-flashing) of the STM32F072, which is on the
2nd PCB.

 but, if the PCB is disconnected for any reason (testing purposes) i'd
like it to operate "stand-alone", powering up in "normal operation",
which requires that BOOT0 be connected to GND with e.g. a 10k
resistor.

 the different TTL voltages that the CPU card(s) can operate at is
what's the key issue, and, also, i don't want to put in another pin to
send over the TTL REF voltage over to the STM32F072 board (unless
absolutely necessary).

 what i'm currently thinking is, put one transistor on the one PCB and
another one one the other.  in effect a "buffer" circuit.

 mmmm.... also.... just checking the datasheet (DM00090510 section
6.3.14) it says that BOOT0's threshold level is "0.3 * (VIN) - 0.3".
that works out at 0.69 volts, so anything with a standard 0.7v
diode... wark-wark :)


 ARGH you know what?   i'm just going to say if normal operation is
needed in stand-alone, put a jumper across EXT_BOOT0 :)

 i'll redo the circuits in a mo....

l.