SN8F2250B Series
USB 2.0 Full-Speed 8-Bit Micro-Controller
3.5 EXTERNAL RESET
External reset function is controlled by “Reset_Pin” code option. Set the code option as “Reset” option to enable
external reset function. External reset pin is Schmitt Trigger structure and low level active. The system is running when
reset pin is high level voltage input. The reset pin receives the low voltage and the system is reset. The external reset
operation actives in power on and normal running mode. During system power-up, the external reset pin must be high
level input, or the system keeps in reset status. External reset sequence is as following.
z
External reset (only external reset pin enable): System checks external reset pin status. If external reset pin is
not high level, the system keeps reset status and waits external reset pin released.
z
z
z
System initialization: All system registers is set as initial conditions and system is ready.
Oscillator warm up: Oscillator operation is successfully and supply to system clock.
Program executing: Power on sequence is finished and program executes from ORG 0.
The external reset can reset the system during power on duration, and good external reset circuit can protect the
system to avoid working at unusual power condition, e.g. brown out reset in AC power application…
3.6 EXTERNAL RESET CIRCUIT
3.6.1 Simply RC Reset Circuit
VDD
R1
47K ohm
R2
RST
MCU
100 ohm
C1
0.1uF
VSS
VCC
GND
This is the basic reset circuit, and only includes R1 and C1. The RC circuit operation makes a slow rising signal into
reset pin as power up. The reset signal is slower than VDD power up timing, and system occurs a power on signal from
the timing difference.
Note: The reset circuit is no any protection against unusual power or brown out reset.
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