Power-On Reset (POR)
PRELIMINARY V1.8
107
18. Power-On Reset (POR)
18.1.
General
The power-on reset circuit provides internal SS1102C reset for most power-up
situations.
POR consists of power-up detect block, start-up timer, and reset latch. Output of the
reset latch is an internal reset (chip_reset) signal. This signal goes low when Vcc rises to
the specified level and the signal goes high after some specified interval of time. This
interval is determined by the start-up timer.
Ramping up of Vcc generates the Power-up Detect (PUD) signal. This signal does not
come until Vcc achieves a level where the logic circuits of the POR start to operate.
The POR is used only at power-up and should not be used to detect drops in power
supply voltage.
The PUD is multiplexed with Nreset (external reset signal).
The SS1102C becomes functional after the chip_reset is generated.
Figures 1 and 2 show timing diagrams.
18.2.
Operation
At power-up, the reset latch and the start-up timer are reset to appropriate state by the
PUD pulse. After some time interval (2n pulses of the high frequency clock for an n-bit
counter) the start-up timer will trigger the reset latch (if there is no Nreset active) and
thus issue the chip_reset signal.
The chip_reset signal will be generated by the internal POR circuit when Nreset signal
is held high. The Nreset can be used to override the internal reset.
18.3.
Crystal Oscillator Start-Up Time
After Vcc achieves the operational level, the crystal oscillator will require time to
stabilize. This is the crystal oscillator start-up time.
Low frequency crystals have a typical start-up time of 1-2sec. Higher frequency crystals
have shorter start-up times (1-2ms). Start-up times are voltage dependent. The SS1102C
uses the high frequency crystal for the start-up timer because it is the default clock at
reset time. The counter delay is designed to be longer than the start-up time of the high
frequency crystal so the fast clock will be stabilized before the chip is reset. The user’s
software should account for the low frequency crystal’s start-up time.
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