General
Review and verify the following for
the custom schematic design:
- The sections above, including
relevant application notes and FAQ links
- Pin attributes, signal
description, and electrical specifications
- Electrical characteristics
and any additional available information
- All BOOTMODE pins have
external pulls or a circuit to drive the desired boot mode. Leaving any of
the boot mode inputs unconnected is not allowed or recommended
- Connecting the boot mode
inputs directly to supply or VSS is not recommended. Shorting of multiple
boot mode inputs together and connecting a common resistor is not
recommended. (Users can have problems with the firmware configuration, where
the LVCMOS GPIOs that are intended as inputs are mistakenly configured as
outputs, driving a logic high signal instead of remaining in a
high-impedance state.) Boot mode outputs are connected to the processor
using resistor divider or through buffers as per the SK implementation
- Boot mode configuration using
dip switches or resistors. When only resistors are used, a resistor divider
is optional. A pullup or pulldown can be used
- IO compatibility (1.8V or
3.3V referenced to VDDSHV3, boot mode inputs are not fail-safe)
- The boot mode inputs are
stable before cold reset status output is pulled high
- Boot mode pins connected to
alternate functions through 0Ω for isolation or testing
Schematic Review
Follow the below list for the custom
schematic design:
- Use a common resistor value
(10kΩ or similar) when dip switch is not used for boot mode
configuration
- Use 470Ω and 47kΩ resistors
when dip switches are used to configure the boot
- Series resistor 1kΩ is used
at the output of the buffer when boot mode is implemented with buffers or
driven by external control signals
- Boot mode configuration for
PLL clock, primary and secondary boot
Additional
- BOOTMODE pins do not have
internal pullup or pulldown resistors that are active during power reset.
- For early designs, recommend that
all boot mode pins are brought out to an optional PU/PD pair with pop and no-pop
options, depending on the required boot mode. See processor-specific TRM for
complete boot mode definitions.
- Boot values are latched at the
release of power-on reset. If the boot mode pins are redefined for another
purpose during operation, boot mode pins must be released/set back to the proper
levels to select the boot mode whenever the device enters the power-on reset
state. Boot mode pin configuration specifically is a concern if signal is driven
from external peripheral.
- Add external ESD protection in
case the boot mode switches are set in an uncontrolled environment.
- Boot mode inputs are not
fail-safe. No input can be applied before the processor IO supplies ramp.
Applying an external input before supply ramps can cause voltage feed and affect
the processor performance.
- Boot mode buffers are
optional.
- When using buffers or logic gates
to configure the boot mode, verify the device used has OE (output enable
feature).