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TM4C1290NCPDT: unused GPIO and EPI unused signals

Mike Havey
Prodigy 70 points

Part Number: TM4C1290NCPDT

Q1. According to the data sheet of the Tiva, unused GPIO may be left un-connected, but it is preferred instead to ground them (table 25-7).  We would like to simply configure them as outputs and leave them dis-connected.  Is there a problem with this plan?  Why is this not preferred?

Q2. When the EPI interface is used in SDRAM mode signals EPI0S20 thru EPI0S27 and EPI0S32 thru EPI0S35 are not used.  Paragraph 11.4.2.1 says that unused EPI controller signals can be used as GPIOs.  Can you tell me how that is done?  I assume the pins are initially configured/dedicated to the EPI function and must then be re-configured in some way.  (I am a hardware engineer and will need to communicate this to our software engineer).

Thanks,

Mike

  • 0
    TI__Guru**** 191886 points

    Hi Mike,

    Mike Havey said:
    Q1. According to the data sheet of the Tiva, unused GPIO may be left un-connected, but it is preferred instead to ground them (table 25-7).  We would like to simply configure them as outputs and leave them dis-connected.  Is there a problem with this plan?  Why is this not preferred?

    What is your rationale to configure unused pins as outputs? I don't think there is a problem to leave an output pin unconnected, but I think you will burn a bit more power since the output buffers are turned on. 

    Mike Havey said:
    Q2. When the EPI interface is used in SDRAM mode signals EPI0S20 thru EPI0S27 and EPI0S32 thru EPI0S35 are not used.  Paragraph 11.4.2.1 says that unused EPI controller signals can be used as GPIOs.  Can you tell me how that is done?  I assume the pins are initially configured/dedicated to the EPI function and must then be re-configured in some way.  (I am a hardware engineer and will need to communicate this to our software engineer).

    For these pins not to be used for EPI, you can repurpose them for GPIO or other functionalities. For example, EPI0S20 is mapped to pin 5. Pin 5 can be repurposed for PQ0 as a GPIO pin or as SSI3CLK pin. 

    If I want to use this pin a GPIO output pin, I will configure as follows.

    GPIOPinTypeGPIOOutput(GPIO_PORTQ_BASE, GPIO_PIN_0);

    GPIOPinWrite(GPIO_PORTQ_BASE,GPIO_PIN_0, GPIO_PIN_0); // Set PQ0 high

    GPIOPinWrite(GPIO_PORTQ_BASE,GPIO_PIN_0, 0); // Set PQ0 low

    If I want to use this pin as a SSI3CLK pin, I will configure as follows.

    GPIOPinConfigure(GPIO_PQ0_SSI3CLK); // Mux out SSI3CLK signal onto the pin

    GPIOPinTypeSSI(GPIO_PORTQ_BASE, GPIO_PIN_0);  // The PQ0 pin is controlled by SSI3 module

  • 0 in reply to Charles Tsai
    Prodigy 70 points

    The rationale to configure the unused GPIO as outputs is as follows.  We think in general an unused output is less susceptible to any noise interference than an input.  If the GPIO is configured as an output and is not grounded, it would be possible to solder a wire to the pin to test future development ideas.  Does a static output consume much current?  Is one state (high or low) better than the other?

    Thanks,

    Mike

  • 0 in reply to Mike Havey
    TI__Guru**** 191886 points
    Mike Havey said:
    Is one state (high or low) better than the other?

    I'm not sure if there is much difference one vs. another. I will drive low as n-transistor is more dominant than p-transistor. If you are concerned with noise, I guess driving low is better. 

  • 0 in reply to Charles Tsai
    Prodigy 70 points

    Hi Charles .. I didn't see anything in the data sheet that specifies the current or power consumption of a GPIO output that is in the static low state.  I would assume the current needed to drive an output in the low state is not more than a microamp or two.  Is that correct?

    Thanks,

    Mike

  • +1 in reply to Mike Havey
    TI__Guru**** 191886 points
    Mike Havey said:
    Hi Charles .. I didn't see anything in the data sheet that specifies the current or power consumption of a GPIO output that is in the static low state.  I would assume the current needed to drive an output in the low state is not more than a microamp or two.  Is that correct?

    Hi Mike,

      Without load, it will be very small, mostly leakage current. I tend to think it will be lower than uA. 

  • 0 in reply to Charles Tsai
    Prodigy 70 points

    Thanks Charles .. this issue is resolved