MB3793-27A Datasheet, PDF (14/30 Pages) Fujitsu Component Limited. – Power Voltage Monitoring IC with Watchdog Timer

MB3793-27A
9. Operation Sequence
9.1 Positive clock pulse input
See “1. Basic operation (positive clock pulse)” under “Diagram.”
9.2 Negative clock pulse input
See “2. Basic operation (negative clock pulse)” under “Diagram.”
The MB3793 operates in the same way whether it inputs positive or negative pulses.
9.3 Clock monitoring
To use the MB3793 while monitoring only one clock, connect clock pins CK1 and CK2.
Although the MB3793 operates basically in the same way as when monitoring two clocks, it monitors the clock signal at every
other input pulse.
See “3. Single-clock input monitoring (positive clock pulse)” under “Diagram.”
9.4 Description of Operations
The numbers given to the following items correspond to numbers (1) to (13) used in “Diagram.”
(1) The MB3793 outputs a reset signal when the supply voltage (V_CC) reaches about 0.8 V (V_CCL)
(2) If V_CC reaches or exceeds the rise-time detected voltage V_SH, the MB3793 starts charging the power-on reset hold
time setting capacitor C_TP. At this time, the output remains in a reset state. The V_SH value is about 2.76 V.
(3) When C_TP has been charged for a certain period of time T_PR (until the CTP pin voltage exceeds the threshold voltage
(V_th) after the start of charging), the MB3793 cancels the reset (setting the RESET pin to “H” level from “L” level).
The V_th value is about 2.4 V with V_CC = 3.3 V
The power-on reset hold
t_PR (ms) ^.=. A  C_TP (F)
time
t_PR
is
set
with
the
following
equation:
The value of A is about 750 with V_CC = 3.3 V and about 700 with V_CC = 3.0 V. The MB3793 also starts charging the
watchdog time setting capacitor (C_TW).
(4) When the voltage at the watchdog timer monitor time setting pin CTW reaches the “H” level threshold voltage V_H, the
C_TW switches from the charge state to the discharge state.
The value of V_H is always about 1.24 V regardless of the detected voltage.
(5) If the CK2 pin inputs a clock pulse (positive edge trigger) when the C_TW is being discharged in the CK1-CK2 order or
simultaneously, the C_TW switches from the discharge state to the charge state.
The MB3793 repeats operations (4) and (5) as long as the CK1/CK2 pin inputs clock pulses with the system logic circuit
operating normally.
(6) If no clock pulse is fed to the CK1 or CK2 pin within the watchdog timer monitor time t_WD due to some problem with the
system logic circuit, the CTW pin is set to the “L” level threshold voltage V_L or less and the MB3793 outputs a reset
signal (setting the RESET pin to “L” level from “H” level).
The value of V_L is always about 0.24 V regardless of the detected voltage.
The
t_WD
watchdog timer
(ms) ^.=. B  C_TW
monitor
(F)
time
t_WD
is
set
with
the
following
equation:
The value of B is hardly affected by the power supply voltage; it is about 1600 with V_CC = 3.0 V to 3.3 V.
Document Number: 002-08550 Rev. *A
Page 14 of 30