The recommendation is to add a series
resistor (with a value that is use case dependent and limits the current as per the
processor-specific data sheet). When loads that draw (require) higher currents
(above the processor-specific data sheet values) are connected to the processor
GPIO, the recommendation is to buffer the processor IO before connecting to the
load.
Common processor LVCMOS IO interface
guidelines:
- A number of the processor IOs are not fail-safe. No external input is
recommended to be applied before the processor supply ramps
- Processor LVCOMOS IOs have slew rate requirements specified, applying a slow
ramp input or connecting a cap at the input is not recommended
- Connecting a cap load >22pF at the LVCMOS IO output is not recommended. DNI the
cap or perform simulations based on the use case.
- Processor IO buffers (TX (output) and RX (input) and internal pulls (pullup and
pulldown)) are turned off during Reset and after reset. A parallel pull is
recommended near to the attached device being driven by the processor IO that
can float (to prevent the attached device inputs from floating until driven by
the host).
- A parallel pull (47kΩ) is recommended for any processor IO (pad) that has a
trace connected and not being drive actively). When adding parallel pull is not
feasible, make sure the traces are routed away from noisy signals
- Connecting processor IOs that can be configured for alternate function, directly
to supply or ground is not allowed or recommended (including boot mode inputs).
Custom board can have an configuration issue with the firmware and
mis-configures these LVCMOS IOs that were intended to be inputs, to be outputs
driven logic high instead.
- Verify capacitor loading of the processor output (when any capacitor value >22pF
(use case dependent, max value) is connected, recommendation is to simulate),
slew rate of the input signal (LVCMOS input slew should be 1000ns or less)
- The recommendation is to verify IO level compatibility between the processor IOs
(inputs) and the attached device IOs (output)
- Provide provision for external ESD protection when external IOs are connected
directly to processor inputs
- Make sure ST_EN bit in the PADCONFIG register is enabled for all the IOs
- Make sure the pullup or pulldown value used for processor IOs are > 4.7kΩ
- Make sure either pullup or pulldown is populated for processor IOs, When
provision for pullup and pulldown is provided, place the resistors as
tripad