Silicon Laboratories SI4421 - Manual (Page 34)

Si4421
RESET MODES
The chip will enter into reset mode if any of the following conditions are met:
 Power-on reset: During a power up sequence until the V
dd
has reached the correct level and stabilized
 Power glitch reset: Transients present on the V
dd
line
 Software reset: Special control command received by the chip
Power-on reset
After power up the supply voltage starts to rise from 0V. The reset block has an internal ramping voltage reference (reset-ramp
signal), which is rising at 100mV/ms (typical) rate. The chip remains in reset state while the voltage difference between the actual
V
dd
and the internal reset-ramp signal is higher than the reset threshold voltage, which is 600 mV (typical). As long as the V
dd
voltage
is less than 1.6V (typical) the chip stays in reset mode regardless the voltage difference between the V
dd
and the internal ramp
signal.
The reset event can last up to 100ms supposing that the V
dd
reaches 90% its final value within 1ms. During this period, the chip
does not accept control commands via the serial control interface.
Power-on reset example:
Power glitch reset
The internal reset block has two basic mode of operation: normal and sensitive reset. The default mode is sensitive, which can be
changed by the appropriate control command (see Related control commands at the end of this section). In normal mode the power
glitch detection circuit is disabled.
There can be spikes or glitches on the V
dd
line if the supply filtering is not satisfactory or the internal resistance of the power supply
is too high. In such cases if the sensitive reset is enabled an (unwanted) reset will be generated if the positive going edge of the V
dd
has a rising rate greater than 100mV/ms and the voltage difference between the internal ramp signal and the V
dd
reaches the reset
threshold voltage (600 mV). Typical case when the battery is weak and due to its increased internal resistance a sudden decrease of
the current consumption (for example turning off the power amplifier) might lead to an increase in supply voltage. If for some reason
the sensitive reset cannot be disabled step-by-step decrease of the current consumption (by turning off the different stages one by
one) can help to avoid this problem.
Any negative change in the supply voltage will not cause reset event unless the V
dd
level reaches the reset threshold voltage (250mV
in normal mode, 1.6V in sensitive reset mode).
If the sensitive mode is disabled and the power supply turned off the V
dd
must drop below 250mV in order to trigger a power-on reset
event when the supply voltage is turned back on. If the decoupling capacitors keep their charges for a long time it could happen that
no reset will be generated upon power-up because the power glitch detector circuit is disabled.
Note that the reset event reinitializes the internal registers, so the sensitive mode will be enabled again.
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