Revisions Affected

0, A

Details

Each GPIO can be configured to an open-drain mode using the GPxODR register. However, an internal device timing issue may cause the GPIO to drive a logic-high for up to 0–10 ns during the transition into or out of the high-impedance state.

This undesired high-level may cause the GPIO to be in contention with another open-drain driver on the line if the other driver is simultaneously driving low. The contention is undesirable because it applies stress to both devices and results in a brief intermediate voltage level on the signal. This intermediate voltage level may be incorrectly interpreted as a high level if there is not sufficient logic-filtering present in the receiver logic to filter this brief pulse.

Workaround

If contention is a concern, do not use the open-drain functionality of the GPIOs; instead, emulate open-drain mode in software. Open-drain emulation can be achieved by setting the GPIO data (GPxDAT) to a static 0 and toggling the GPIO direction bit (GPxDIR) to enable and disable the drive low. For an example implementation, see the code below.

           
void main(void)
{ ...
  
  // GPIO configuration
     EALLOW;                               // disable pullup
     GpioCtrlRegs.GPxPUD.bit.GPIOx = 1;    // disable open-drain mode
     GpioCtrlRegs.GPxODR.bit.GPIOx = 0;    // set GPIO to drive static 0 before
                                           // enabling output
     GpioDataRegs.GPxCLEAR.bit.GPIOx = 1;
     EDIS;
  ...
  
  // application code
  ...
  
  // To drive 0, set GPIO direction as output
     GpioCtrlRegs.GPxDIR.bit.GPIOx = 1;
  
  // To tri-state the GPIO(logic 1),set GPIO as input
     GpioCtrlRegs.GPxDIR.bit.GPIOx = 0;
}