General
Review and verify the following for the custom schematic design:
- The sections above, including relevant
application notes and FAQ links.
- Pin connectivity requirements and pin attributes.
- Electrical characteristics and any additional available information.
- Input signal applied to the processor LVCMOS inputs follow the slew rate requirements. Connecting a capacitor directly at the input increases the signal slew and is not recommended.
- Connection of capacitor load directly to the processor output for control or enabling of attached device is not allowed (recommend simulation when capacitor load > 22pF (place holder) is used).
- All IO pins referenced to VDDSHVx, VDDSx or
VDDS_WKUP connect to one voltage level. Each IO has an associated supply
voltage used to power the IO cell (VDDSHVx or VDDSx or VDDS_WKUP). If
VDDSHVx or VDDSx or VDDS_WKUP is sourced from 3.3V (1.8V) supply, then all
IO referenced to VDDSHVx, VDDSx or VDDS_WKUP rail operate at 3.3V (1.8V)
levels.
- No input voltage applied to the processor IOs
before the VDDSHVx, VDDSx or VDDS_WKUP supply ramps (excluding fail-safe
IOs). Most processor IOs are not fail-safe. Applying voltage to the IOs is
not recommended or allowed, while the corresponding IO supply for IO group
(VDDSHVx or VDDSx or VDDS_WKUP) is off. Fail-safe IOs include PORz, I2C2
(only when using the selected device package pins with I2C OD FS
buffer type. For example, B8, D8 for ANB package), EXTINTn, and USBn_VBUS (n
= 0-1), when a recommended VBUS divider is used.
- One of the common use case for the IO interface is driving LEDs for indication. The designer can review the LED source or sink current and the effect on the voltage level and adjust the LED current accordingly.
- Shorting of multiple IOs together directly is not recommended.
- Pad configuration based on the required IO direction.
- Directly connecting processor IOs with alternate functions to supply or VSS is not recommended or allowed, including boot mode inputs. The board designer can have errors with the firmware and miss-configure the LVCMOS GPIOs that are intended as inputs, to be outputs driving logic high instead.
Schematic Review
Follow the list for the custom schematic
design:
- Pulls are added for any of
the processor or attached device IOs that can float.
- Pullups are connected to the
same IO supply for IO group VDDSHVx or VDDSx or VDDS_WKUP referenced by the
IOs.
- The supply voltage for all
pullups that are connected to processor IOs matches the voltage applied to
the corresponding IO supply for IO group (VDDSHVx or VDDSx or VDDS_WKUP).
Pulling a signal to a different IO voltage can cause voltage leakage.
- IO level compatibility for
externally applied inputs from a add-on or carrier board or through an
external connector.
- Supply rails connected follow
the ROC.
Additional Information
- For common processor LVCMOS IO interface
guidelines, refer to Section 7.5.2.2.
- Most of the processor IOs
are not fail-safe. Applying input before supply ramps is not recommended
or allowed.
- Processor LVCMOS IOs have
slew rate requirements specified; applying a slow ramp input or
connecting a capacitor directly at the input is not recommended.
- Connecting a capacitor
load > 22pF (placeholder) at the output is not recommended. DNI
capacitor or perform simulations based on the use case.
- Processor IO buffers are
off during reset. A pull is recommended near to the attached device
being driven by the processor IO that can float.
- A parallel pull is recommended for any processor
IO pad that has a trace connected. When adding pull is not feasible, route the
traces away from noisy signals. Processor IO buffers are off during reset. A
pullup is recommended near to the attached device to hold the attached device IO
inputs that can float in a known state. Use of pulls are attached-device
dependent.
- IO compatibility and fail-safe operation between the processor IOs and attached devices connected through IOs.
- Fail-safe operation when connected to external signals. Applying an external input before supply ramps cold causes voltage feed and affects the processor performance.
- Capacitor loading of the processor output (when
capacitor value > 22pF (place holder) is connected; designer is expected to
simulate), slew of the input signal (LVCMOS input slew is 1000ns or less).
- IO current sink or source follows the data sheet recommendations.
- External ESD protection is provided when the IOs connect directly to external interface signals.