SN8F2250B Series
USB 2.0 Full-Speed 8-Bit Micro-Controller
3.4.3 BROWN OUT RESET IMPROVEMENT
How to improve the brown reset condition? There are some methods to improve brown out reset as following.
z
z
z
z
LVD reset
Watchdog reset
Reduce the system executing rate
External reset circuit. (Zener diode reset circuit, Voltage bias reset circuit, External reset IC)
Note:
1.
The “ Zener diode reset circuit”, “Voltage bias reset circuit” and “External reset IC” can
completely improve the brown out reset, DC low battery and AC slow power down conditions.
2. For AC power application and enhance EFT performance, the system clock is 4MHz/4 (1 mips)
and use external reset (“ Zener diode reset circuit”, “Voltage bias reset circuit”, “External reset
IC”). The structure can improve noise effective and get good EFT characteristic.
LVD reset:
VDD
VSS
LVD Detect Voltage
Power
Power is below LVD Detect
Voltage and System Reset.
System Normal Run
System Stop
System Status
Power On
Delay Time
The LVD (low voltage detector) is built-in Sonix 8-bit MCU to be brown out reset protection. When the VDD drops and
is below LVD detect voltage, the LVD would be triggered, and the system is reset. The LVD detect level is different by
each MCU. The LVD voltage level is a point of voltage and not easy to cover all dead-band range. Using LVD to
improve brown out reset is depend on application requirement and environment. If the power variation is very deep,
violent and trigger the LVD, the LVD can be the protection. If the power variation can touch the LVD detect level and
make system work error, the LVD can’t be the protection and need to other reset methods. More detail LVD information
is in the electrical characteristic section.
Watchdog reset:
The watchdog timer is a protection to make sure the system executes well. Normally the watchdog timer would be clear
at one point of program. Don’t clear the watchdog timer in several addresses. The system executes normally and the
watchdog won’t reset system. When the system is under dead-band and the execution error, the watchdog timer can’t
be clear by program. The watchdog is continuously counting until overflow occurrence. The overflow signal of
watchdog timer triggers the system to reset, and the system return to normal mode after reset sequence. This method
also can improve brown out reset condition and make sure the system to return normal mode.
If the system reset by watchdog and the power is still in dead-band, the system reset sequence won’t be successful
and the system stays in reset status until the power return to normal range.
Reduce the system executing rate:
If the system rate is fast and the dead-band exists, to reduce the system executing rate can improve the dead-band.
The lower system rate is with lower minimum operating voltage. Select the power voltage that’s no dead-band issue
and find out the mapping system rate. Adjust the system rate to the value and the system exits the dead-band issue.
This way needs to modify whole program timing to fit the application requirement.
External reset circuit:
The external reset methods also can improve brown out reset and is the complete solution. There are three external
reset circuits to improve brown out reset including “Zener diode reset circuit”, “Voltage bias reset circuit” and “External
reset IC”. These three reset structures use external reset signal and control to make sure the MCU be reset under
power dropping and under dead-band. The external reset information is described in the next section.
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