On Sat, Aug 22, 2015 at 10:27 PM, Ralf-Peter Rohbeck via arm-netbook arm-netbook@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.