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why the PB6 and PB7 pins have 2.2 volt as output, but rest have 3.3?

sivaprasad sm
Intellectual 520 points
Other Parts Discussed in Thread: CC3200

Hai,

I interfaced a character LCD display with TIVA C launchpad and it was successfully worked but now its fails with same code and connection on another launchpad i set all the pins of port B as out put ,and write 0xff, but PB6 and PB7 have only up to 2.2 volts as output voltage. on other hand its 3.3 volts in another launchpad...i think it may be the reason for the improper working of the LCD.please let me know hows this happened?  how its possible to set back to 3.3 volts as output...is it any manufacturing defect or programmatic error?

  • 0
    Expert 2415 points
    It's a "feature" of the TM4C123 LaunchPad. There are two 0-ohm resistors (R9 & R10) linking PD0<->PB6 and PD1<->PB7 and you probably have pulldown setting on the PD pins while the PB pins try to output 3.3V, or something like that.

    I can't remember the official reason for the resistors being there (something to do with MSP430 LP compatibility perhaps?), but if you're not concerned with that, you can simply remove the aforementioned resistors with a fine-tipped soldering iron.

    Or if you don't need the PD pins as outputs, just configure them as inputs with no pullups or pulldowns active - then you should have normal functionality on the PB pins.
  • 0 in reply to Veikko Immonen
    Guru 117855 points

    Let the record show that (ever gentle) cb1 was first to characterize such unwanted/unwelcome behavior as, "feature" of these esteemed "lunch-pads!"  What launch?

    Repetition is one form of flattery - yet attribution proves always "wise & proper..."

    Beyond "tweaking" vendor - this reporter has long (yelled) for "proper" (i.e. some) highlight of this failed design choice.   True - if user "squints" and digs into mice-type - he might note the sparse, unremarkable notation of this (extremely poor) default pin choice...

    Far better to supply those 0-ohm "gotchas" in small bag - for the (extremely few) who may actually seek past/lesser MCU board compatibility...

    Alas - vendor "duly notes" - and another hundred or so, "crash/burn!"

  • 0 in reply to cb1_mobile
    Expert 2415 points
    No need to attribute to anyone something I've come up on my own, before I ever set foot on these forums. I "burnt" myself (luckily only lightly) with that "feature" and learned about it way back then. I must say that you sometimes make equally much noise about yourself than the actual issue at hand.
  • 0 in reply to Veikko Immonen
    Guru 27285 points

    Veikko Immonen said:
    No need to attribute to anyone something I've come up on my own, before I ever set foot on these forums. I "burnt" myself (luckily only lightly) with that "feature" and learned about it way back then.

     I was burnt too when I landed here.. But that wasn't the worst issue, I solved myself and changed launchpad, the old one is still operating with some pin nonworking and slight warm.

     The worst issue I was involved with was double stack setting and as experienced first thing where in place was measuring max stack... So I was feeling that was a stack failure but no stack size was solving issue... Stack was set forever @512Bytes! and browsing one to one the IDE setting I found that then inspecting map file real issue was hit....

     No help my previous experience I got burn on a silly double stack setup...

     So I think R9 R10 has to be in some way alerted.

    Veikko Immonen said:
    . I must say that you sometimes make equally much noise about yourself than the actual issue at hand.

     I don't like this, it sound quite offending and still is one of common failures, biggest failure we are still pointing we fell as uncontrolled flooding.

  • 0 in reply to Veikko Immonen
    Guru 27285 points

    Veikko Immonen said:

    I can't remember the official reason for the resistors being there (something to do with MSP430 LP compatibility perhaps?),

     First launchpad was msp430 G series sold for 4.3$ shipped. This launchpad was simply a 1 to 1 uC pin to connector so changing processor some feature where not on same pin.

     Before to redefine Launchpad/Boosterpak a lot of old fashion BP where released. To maintain compatibility second launchpad (LX120 Stellaris launchpad) was released and to map both I2C and SPI this odd bridge was in place... Why not two 330Ohm resistors?

     New TIVA Launchpad is just same LX120 Stellaris with few changes to silk screen and processors no more LM120->TM4G12xxx but new full featured TM4C123GH6, pin to pin compatible so new feature like QEI, CANBUS and USB OTG where added on new lp.

     New redesigned LP/BP address this issue and new MSP430/C2000/CC3200/TIVA also have compatibility to the new layout, Never TI corrected nor highlighted this issue on new TM4C123GXL!

     New series with jumper Like TM4C1294XL, DK-4Cxxx, CC3200 and other BP can be next release of this worst issue... Its enough reversing both pin pairs to select I2C or SPI.