General
Review and verify the following for the custom
schematic design:
- Reviewed above "Common
checklist for all sections" section of the user's guide.
- MCU_PORz input connection, L->H delay after supply ramps.
- MCU_PORz input IO level and
fail-safe capability.
- MCU_PORz input state during processor supplies ramp.
- Reset inputs follow the slew rate requirements (FS RESET, LVCMOS) as per the
processor-specific data sheet.
- Slew rate when open-drain
output type reset signal (nRSTOUT0) from PMIC or discrete DC/DC or discrete
LDO is connected to MCU_PORz input.
- RESET_REQz input and MCU_RESETz input voltage level and connection.
- Connection of warm reset inputs when not used.
Schematic Review
Follow the below list for the custom
schematic design:
- MCU_PORz input is held low
during power supply ramp-up or ramp-down.
- Cold reset input (MCU_PORz) deassertion hold time (9.5ms (9500000ns)
minimum) after all supplies ramps is provided as per the processor-specific
data sheet requirement.
- Cold and warm reset inputs slew rate requirements have been considered and
required buffers are added. Slow slew rate can glitch the reset
internally.
- Slew rate when open-drain output type reset signal (nRSTOUT0) from PMIC or
discrete DC/DC or discrete LDO is connected directly to the reset input.
Lesser slew is better (<100ns). The recommendation is to connect through
fast rise time discrete push-pull output type buffer.
- MCU_PORz (POR) input is 3.3V tolerant and fail-safe. The threshold follows
the 1.8V IO level (VDDS_OSC0).
- Provision for a glitch filter (capacitor) is provided at the MCU_PORz reset
input (add 22pF (place holder) capacitor provision).
- IO levels of MCU and MAIN domain warm reset input RESET_REQz follows the
VDDSHV0 supply (1.8V or 3.3V) and MCU domain reset input MCU_RESETz follows
the VDDSHV_MCU supply (1.8V or 3.3V).
- Connection of push button warm reset inputs through debouncing circuit
(Schmitt trigger buffer output).
- The recommendation is to
connect the warm reset inputs when not used as per pin connectivity
requirements (a pullup is recommended).
Additional
- MCU_PORz input has slew rate requirement specified. When connecting
PMIC_POWERGOOD (open-drain output type signal) to MCU_PORz input is the only
available option, adjust the pullup to optimize the rise time (~ 100ns).
- The processor is required to restart (release reset) only after the voltages
ramp down below 300mV during power-down (There is no time or tolerance
associated with the ramp down requirement. Each power rail is recommended to
decay below 300mV before any power rail is allowed to ramp up).
- Not connecting a valid MCU_PORz input causes unpredictable and random behavior,
since processor does not get a valid reset input and the internal circuits are
in random states. Slow ramp reset input causes internal processor reset circuit
to glitch.
- LVCMOS inputs have slew rate requirements specified. A schmitt trigger based
debouncing circuit is recommended for the slow ramp push button output signal
connected to the processor warm reset inputs. Schmitt trigger based debouncing
circuit is recommended when using a push button or an RC as reset input.
- Provision for external ESD protection for manual (push button) reset input added
near to the reset signal.
- Fail-safe operation (MCU_RESETz input and RESET_REQz input) when connected to
external reset inputs. Applying an external input signal to the processor reset
inputs before the processor supply ramps can cause voltage feed and affects the
board performance.
- Reviewed MCU_RESETz input related silicon errata.
- The recommendation is to follow the reset requirements including slew rate and
MCU_PORz input hold time after supplies ramp when a non-TI power architecture is
considered.