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TM4C1294NCPDT: TM4C1294NCPDT PJ1 pin as U3Tx

Part Number: TM4C1294NCPDT

Hi there,

I'm planning to use PJ1 with alternative function U3Tx, but reading the datasheet:

"■ Slow GPIO pads: These pads provide 2-mA drive strength and are designed to be sensitive to
voltage inputs. The following GPIOs port pins are designed with Slow GPIO Pads:
– PJ1"

I have to drive a 6N137 diode, so my question is if that  2-mA limitation also applies to PJ1 configured as U3Tx or only if used as PJ1 GPIO. Should I use PA5 instead? (alternative U3Tx pin with 12-mA capability).

Thanks,

B

  • Ciao Bruno,
    UART transmission should go into a dedicated driver, or directly to another receiver that uses TTL level - which typically need very little current. I'm not familiar with the solution you are proposing for UART Tx, but it might be risky (unreliable) if you really need high currents for such.
    The limitation on a pad will apply to any of its uses, not only GPIO.
    Now, as for "should you use PA5"? If you can, and after considering the above, sure use a pad with less restrictions. There are 100+ pins on the device, so it is a good idea to leave the special/restricted ones as a last resort.
    Cheers
    Bruno Saraiva
  • Hi,

    6N137 is an optocoupler, so you must dimension driving current taking into account CTR degradation over the device life. So, higher current is mandatory. Another problem is this device is near the end of production, some providers already not recommending this for new projects.

    But the CTR degradation problem remain for any other optocoupler.

  • It is easy to add cost to other people's BOM, but in that case it might be a good idea to use an extra transistor to drive the opto, isn't it? And take Petrei's hint and go find one that isn't NRND.
  • Thanks for the information, guys. Never saw this uC NRND (where is TI showing it?  and who's the recommended new replacement?). I already have a product running with this controller, I'm just expanding the current hardware.

    On the other side, I may add a 74HC04 to act as inverter and feed the optocoupler diode from it's output instead of directly from the uC pin. I currently have resistors limiting the current of the 6N167 diode, to about 12mA. The limit of the pins, but inside the parameters to have 10 safe years of working.

  • You may alter your rhythm by noting that your MCU is NOT under "NRND" status. (other poster was referring to the opto-coupler as NRND.)
    Your use of the HC device appears wise - iirc you may (even) parallel outputs to provide (even) more current drive...
  • Fascendini,
    Petrei referred that the 6N137 is NRND, not the MCU!
    As for the pin driving capacity, note that there are other things involved: while there is the possibility for using ONE pin for 12mA as you well mentioned, there are limits to the total current being output per side of the MCU, and for the total current out of all GPIO's. So it is never a good practice to count on those 12mA. If you are aiming for those 10 years, go ahead and add the transistor.
    Cheers
    Bruno
  • Oh, I did read wrong that sentence.. thought he was talking about the uC being NRND... :D I'm adding a 74HC04 to the design and that will make the trick. Thanks for all the information!