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FTDI NPN Transistor Polarity — Parallax Forums

FTDI NPN Transistor Polarity

SRLMSRLM Posts: 5,045
edited 2013-01-05 12:10 in Propeller 1
I noticed something interesting today: the polarity of the collector and emitter on the NPN transistor circuit for the FTDI chip does not seem to matter. I've built a board with this circuit twice: once as shown, and once with C and E reversed (by flipping the transistor SOT-23 package upside down). In both cases, I am able to download via propeller-load a .binary image, and it seems to work correctly.

Does anybody know why this is?

attachment.php?attachmentid=98452&d=1357355242
transistor_direction.png
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Comments

  • Beau SchwabeBeau Schwabe Posts: 6,566
    edited 2013-01-04 19:41
    See "reverse active" ...

    http://en.wikipedia.org/wiki/Bipolar_junction_transistor
  • RaymanRayman Posts: 14,666
    edited 2013-01-05 05:21
    makes me wonder if we really need the transistor...
  • Duane C. JohnsonDuane C. Johnson Posts: 955
    edited 2013-01-05 06:50
    Rayman wrote: »
    makes me wonder if we really need the transistor...
    You need the transistor, reversed or not, to invert the signal. No transistor, no inversion.

    Duane J
  • RaymanRayman Posts: 14,666
    edited 2013-01-05 08:06
    It'd be interesting to try... A capacitively coupled pulse should give short pulses of both polarities on the other side...
  • Toby SeckshundToby Seckshund Posts: 2,027
    edited 2013-01-05 08:18
    Being upside down or the wrong sort (PNP/NPN) was amongst the many things I suspected when I was trying to get my AVR USB->Prop interface going.

    I gave up (for now) and used a FT232 and all was OK. Dr_A tried the various edges as the reset so there must be another timing issue.

    Having the transistor there does keep the differentiated spikes out of the Prop's innards, as well as preserving the open collector feed for the reset pin.
  • Duane C. JohnsonDuane C. Johnson Posts: 955
    edited 2013-01-05 08:21
    Rayman wrote: »
    It'd be interesting to try... A capacitively coupled pulse should give short pulses of both polarities on the other side...
    I have tried that when using generic USB/RS-232 adapters. While it does work usually, it didn't work reliably with all my Props.
    ( I found that adding an inverter on the generic adapters, so they look like PropPlugs, is more reliable though.)
    Note! If you use the capacitor method add a pair of protection diodes to the reset pin so the voltage doesn't go excessively below ground or above VDD.

    Duane J
  • Brian FairchildBrian Fairchild Posts: 549
    edited 2013-01-05 08:24
    There are plenty of non-Prop (ie AVR) modules out there that just couple DTR to /RESET with a 100n cap. Just the cap and a pull-up on the /RESET line.
  • RaymanRayman Posts: 14,666
    edited 2013-01-05 09:30
    Interesting... Only possible problem is any time delay if reset is triggered by the end of the pulse...

    BTW: I remember having a backwards transistor too, so I just found my old post:
    http://forums.parallax.com/showthread.php/109821-My-first-Prop-SMT-board?p=780049&highlight=transistor#post780049

    Looks like it works if base and emitter are backwards too :)
  • Duane C. JohnsonDuane C. Johnson Posts: 955
    edited 2013-01-05 10:56
    Rayman wrote: »
    Interesting... Only possible problem is any time delay if reset is triggered by the end of the pulse...

    BTW: I remember having a backwards transistor too, so I just found my old post:
    http://forums.parallax.com/showthread.php/109821-My-first-Prop-SMT-board?p=780049&highlight=transistor#post780049

    Looks like it works if base and emitter are backwards too :)
    With the base now grounded and the emitter now connected to the capacitor the circuit in now "common base" type.
    This is a non-inverted configuration. It would pull the collector down on the trailing edge of the reset pulse.

    Wow!! You guys with the oddly connected parts are giving me a headache %^)

    Keep up the, aah, good work.

    Duane J
  • CircuitsoftCircuitsoft Posts: 1,166
    edited 2013-01-05 12:10
    SRLM wrote: »
    ... once with C and E reversed (by flipping the transistor SOT-23 package upside down).
    Every SOT-23 transistor I've sen, by flipping upside-down, will swap the emitter and base, not emitter and collector. The only time I've seen that not to be the case was with a JFET, and that sort of makes sense.
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