7
Data Sheet
STC803/809/810-811/812/-823/824/825-6342/6343/6344/6345
µµµµµP Supervisor Circuits
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STC
Ver1.0
Functional Description
Reset Output
A microprocessor’s (µP’s) reset input starts the µP in a known
state. Whenever the µP is in an unknown state, it should be
held in reset. The supervisory circuits assert reset during power-
up and prevent code execution errors during power-down or
brownout conditions.
On power-up, once Vcc reaches about 1.0V, RESET is a
guaranteed logic low of 0.4V or less. As Vcc rises, RESET stays
low. When Vcc rises above the reset threshold, an internal timer
releases RESET after about 200ms. RESET pulses low whenever
Vcc drops below the reset threshold, i.e. brownout condition.
If brownout occurs in the middle of a previously initiated reset
pulse, the pulse continues for at least another 200ms. On power-
down, once Vcc falls below the reset threshold, RESET stays
low and is guaranteed to be 0.4V or less until Vcc drops below
1.0V. Fig 4 shows the timing relationship.
The active-high RESET output is simply the inverse of the
RESET output, and is guaranteed to be valid with Vcc down to
1.0V.
Watchdog Timer
The STC823/824 watchdog circuit monitors the
µP’s activity. If
the
µP does not toggle the watch-dog input (WDI) within 1.6s,
reset asserts. As long as reset is asserted or the WDI input is
toggled, the watchdog timer will stay clear and will not count.
As soon as reset is released, the timer will start counting. WDI
input pulses as short as 50ns can be detected.
Disable the watchdog function by leaving WDI unconnected
or by three-stating driver connected to WDI.
Manual Reset
The manual-reset input (MR) allows reset to be triggered by a
push button switch. MR has an internal pullup resistor, so it
can be left open when not used.
Power-Fail Comparator
The power-fail comparator can be used for various purposes
because its output and noninverting input are not internally
connected. The inverting input is internally connected to a
1.25V reference.
Figure 3. Typical Application Circuit
DC Linear
Regulator
Vcc
Supervisory
PFI
Circuit
WDI
µ
µµ
µP
Vcc
RESET
I/O Line
Interrupt
IN
RESET
PFO
MR
µ
µµ
µP
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