General
Review and verify the following for the custom schematic design:
- Reviewed above "Common checklist for all sections" section of the
user's guide.
- Connection of reset inputs (cold and warm reset)
- PORz input connection and L->H delay after all the processor supplies ramp
(ramp-up and ramp-down)
- PORz H->L delay after processor supplies ramp down (low before supplies ramp
down)
- PORz input IO level and fail-safe capability
- Reset (Cold, Warm) input follows the slew rate requirements (Fail-Safe Reset
(FS RESET) Electrical Characteristics) as per the processor-specific data
sheet
- Addition of pulldown and glitch filter at the input of PORz
- PORz input state during processor supplies ramp
- PORz input slew rate when open-drain output type reset signal (nRSTOUT0)
from PMIC or discrete DC/DC or discrete LDO is connected as PORz input
- Connection of external input to warm reset input RESETz (IO level)
- RESETz slew rate when push button input is connected
- Addition of glitch filter at the input of RESETz
- Connection of warm reset input when not used
Schematic Review
Follow the below list for the custom schematic
design:
- A valid input that goes high
after all the supplies ramp plus the required delay for clock start-up
- PORz input is held low during power supply ramp-up or ramp-down.
- PORz input deassertion hold time (9.5ms (9500000ns) minimum) after all processor
supplies ramps is provided as per the processor-specific data sheet requirement.
- Cold and warm reset input slew rate requirements have been considered and
required push-pull output type discrete buffers are added. Slow slew 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 reset input through fast rise time discrete
push-pull output type buffer.
- PORz input is 3.3V tolerant and fail-safe. The threshold follows the 1.8V IO
level (VDDS_OSC0)
- Provision for a pulldown 10K and
a glitch filter (capacitor) is provided at the PORz reset input (add 22pF (place
holder) capacitor provision)
- IO level of warm reset input
RESETz follows the VDDSHV1 supply (Fixed 1.8V or 3.3V)
- Connection of push button for
warm reset input through debouncing logic (Schmitt trigger buffer output).
- The recommendation is to connect
the warm reset input when not used as per pin connectivity requirements (a
pullup is recommended)
Additional
- PORz input has internal hysteresis enabled and slew rate requirement specified.
When connecting PMIC_POWERGOOD (open-drain output type signal) to 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 all the supply
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 PORz input can cause 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 RESETz input has slew rate
requirements specified. A schmitt trigger based debouncing logic (circuit) is
recommended for the slow ramp push button output signal connected to the
processor warm reset input RESETz. Schmitt trigger based debouncing logic
(circuit) is recommended when using an RC as reset input.
- Provision for external ESD protection for manual (push button) reset input is
recommended to be added near to the reset signal.
- Fail-safe operation when connected to external reset input for warm reset.
Applying an external input before the processor supply ramps can cause voltage
feed and affects the board performance.
- The recommendation is to follow the reset requirements including slew rate and
PORz input hold time after supplies ramp when a non-TI power architecture is
considered.