General
Review and verify the following for
the custom schematic design:
- Above sections, 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 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 greater than 22pF
(approximation for place holder) is used).
- All IO pins on each IO supply group VDDSHVx or
VDDSHV_MCU or VDDSHV_CANUART connect to one voltage
level. Each IO has an associated supply voltage used
to power the IO cell (VDDSHVx). If VDDSHVx is
sourced from 3.3V (1.8V) supply, all IO referenced
to the rail operate at 3.3V (1.8V) levels.
- No input voltage applied to the processor IOs
before the supply ramp for VDDSHVx (excluding
fail-safe IOs). Most processor IOs are not
fail-safe. Do not apply the voltage to the IOs while
the corresponding IO power domain (VDDSHVx) is off.
Fail-safe IOs include MCU_PORz, WKUP_I2C0_SCL,
WKUP_I2C0_SDA, MCU_I2C0_SCL, MCU_I2C0_SDA, EXTINTn,
and USB0-1_VBUS, when a recommended VBUS divider is
used.
- One of the common use cases
for the IO interface is driving LEDs for indication. The designer can review
the LED current sourced and sinked 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 prohibited or discouraged,
including boot mode inputs. The user can have errors with the firmware and
miss-configure the LVCMOS GPIOs that are intended as inputs, to be outputs
driven logic high instead.
Schematic Review
Follow the below 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 group VDDSHVx 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 group (VDDSHVx). Pulling a signal to the wrong
IO voltage causes voltage leakage between the IO rails of the device.
- IO level compatibility for
externally applied inputs from a different board or through connector.
- Supply rails connected follow
the ROC.
Additional
- Common processor LVCMOS IO
interface guidelines, refer to Section 7.5.3.2.
- Most of the processor IOs
are not fail-safe. Do not apply an input before supply ramps.
- Processor LVCMOS IOs have
slew rate requirements specified, applying a slow ramp input or
connecting a capacitor at the input is not recommended.
- Connecting a capacitor
load 22pF (ball park for place holder) 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 required near to the attached device being
driven by the processor IO that can float.
- Any processor IO that has a trace
connected needs a parallel pull. When adding pull is not feasible, place the
traces away from noisy signals. Processor IO buffers are off during reset and
power-up. 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 level compatibility for externally applied
inputs from a add-on or carrier board or through an external
connector.
- 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 any capacitor value is greater than 22pF (approximation
for place holder) is connected, designer must simulate), slew rate 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.