SI4021-A1-FT【PDF 25/34 页】IC TX FSK 915MHZ 5.4V 16-TSSOP

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Si4021

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 Vdd has reached the correct level and stabilized

? Power glitch reset: Transients present on the V

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 Vdd and the internal

reset-ramp signal is higher than the reset threshold voltage, which is 600 mV (typical). As long as the Vdd voltage is less than 1.6V (typical)

the chip stays in reset mode regardless the voltage difference between the Vdd and the internal ramp signal.

The reset event can last up to 150ms supposing that the V

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

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

has a rising

rate greater than 100mV/ms and the voltage difference between the internal ramp signal and the V

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 Vdd 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 Vdd 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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