General
Review and verify the following for the custom schematic design:
- Above sections, including relevant application notes and FAQ links
- Pin attributes, signal description, and electrical specifications
- Electrical characteristics and any additional available information
- All BOOTMODE pins (depending on the BOOTMODE pin mapping option used) have external pulls or a circuit to drive the required boot mode. Leaving any of the boot mode inputs unconnected is not recommended or allowed
- 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. (Board designers 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 inputs are connected to the processor using resistor divider or through buffers as per the EVM 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 VDDSHV0, 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
- 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 reconfigured for alternate function during operation, boot mode pins are required to be released/set back to the proper configuration to select the boot mode whenever the device enters the power-on reset state. Boot mode configuration specifically is a concern if signal is driven from external peripheral.
- Add external ESD protection in case the boot mode switches are configured 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 can affect the custom board functions.
- Boot mode buffers are optional and are provided on the EVM for test automation
- When using buffers or logic gates to configure the boot mode, verify the device used has OE (output enable feature).