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 processor IOs are not
fail-safe. No external input is recommended to be applied before processor supplies
ramp.
- Processor LVCMOS IOs have slew rate
requirements (LVCMOS input slew is <1000ns), specified, applying a slow ramp input or
connecting a capacitor directly at the input is not recommended.
- Connecting a capacitor load > 22pF
at the output is not recommended. DNI capacitor or perform simulations (based on the use
case).
- Processor IO buffers are (TX (Output)
and RX (Input) and internal pulls (pullup and pulldown)) turned off during reset and
after reset. A 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 but not being driven actively.
When adding parallel pull is not feasible, the recommendation is to route the traces
away from noisy signals.
- The recommendation is to verify IO
level compatibility, and fail-safe operation between the processor IOs and attached
devices.