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Power-On Reset and Brownout Detector
5.1
Power-On Reset and Brownout Detector
The device includes a power-on reset (POR), providing correct initialization during device power on. It also
includes a brownout detector (BOD) operating on the regulated 1.8-V digital power supply only. The BOD
protects the memory contents during supply voltage variations which cause the regulated 1.8-V power to
drop below the minimum level required by digital logic, flash memory, and SRAM.
When power is initially applied, the POR and BOD hold the device in the reset state until the supply
voltage rises above the power-on-reset and brownout voltages.
The cause of the last reset can be read from the register bits
SLEEPSTA.RST
. It should be noted that a
BOD reset is read as a POR reset.
5.2
Clock-Loss Detector
The clock-loss detector can be used in safety-critical systems to detect that one of the XOSC clock
sources (32-MHz XOSC or 32-kHz XOSC) has stopped. This can typically happen due to damage to the
external crystal or supporting components. When the clock-loss detector is enabled, the two clocks
monitor each other continously. If one of the clocks stops toggling, a clock-loss detector reset is generated
within a certain maximum time-out period. The time-out depends on which clock stops. If the 32-kHz clock
stops, the time-out period is 0.5 ms. If the 32-MHz clock stops, the time-out period is 0.25 ms. When the
system comes up again from reset, software can detect the cause of the reset by reading
SLEEPSTA.RST[1:0]
. After a reset, the internal RC oscillators are used. Thus, the system is able to start
up again and can then be powered down gracefully. The clock-loss detector is enabled/disabled with the
CLD.EN
bit. It is assumed that the 32-MHz XOSC is selected as system clock source when using the
clock-loss detector. The 32-kHz clock can be 32-kHz RCOSC (should be calibrated for accurate reset
timeout) or 32-kHz XOSC.
In power modes 1 and 2, the clock-loss detector is automatically stopped and restarted when the clocks
start up again.
Before entering power mode 3, switch to the 16-MHz RCOSC and disable the clock-loss detector. When
entering active mode again, turn on the clock-loss detector and then switch back to the 32-MHz XOSC.
CLD (0x6290)
–
Clock-Loss Detection
Bit
Name
Reset
R/W
Description
7:1
–
0000 000
R0
Reserved
0
EN
0
R/W
Clock-loss detector enable
74
Reset
SWRU191C
–
April 2009
–
Revised January 2012
Copyright
©
2009
–
2012, Texas Instruments Incorporated