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UM0404
System reset
C1 that produces a sufficient discharge time to permit the internal or external oscillator and /
or internal PLL and the on-chip voltage regulator to stabilize.
To ensure correct Power-On reset with controlled supply current consumption, specially if
clock signal requires a long period of time to stabilize, an asynchronous hardware reset is
required during Power-On. For this reason, it is recommended to connect the external R0-
C0 circuit shown in
to the RPD pin. At Power-On, the logical low level on RPD
pin forces an asynchronous hardware reset when RSTIN is asserted low. The external pull-
up R0 will then charge the capacitor C0. Note that an internal pull-down device on RPD pin
is turned on when RSTIN pin is low, and causes the external capacitor (C0) to begin
discharging at a typical rate of 100-200
μ
A. With this mechanism, after Power-On reset,
short low pulses applied on RSTIN produce synchronous hardware reset. If RSTIN is
asserted longer than the time needed for C0 to be discharged by the internal pull-down
device, then the device is forced in an asynchronous reset. This mechanism insures
recovery from very catastrophic failure.
Figure 205. Minimum external reset circuitry
The minimum reset circuit of
is not adequate when the RSTIN pin is driven from
the ST10F276 itself during software or watchdog triggered resets, because of the capacitor
C1 that will keep the voltage on RSTIN pin above V
IL
after the end of the internal reset
sequence, and thus will trigger an asynchronous reset sequence.
shows an example of a reset circuit. In this example, R1-C1 external circuit is
only used to generate Power-On or manual reset, and R0-C0 circuit on RPD is used for
Power-On reset and to exit from Power Down mode. Diode D1 creates a wired-OR gate
connection to the reset pin and may be replaced by open-collector Schmitt Trigger buffer.
Diode D2 provides a faster cycle time for repetitive Power-On resets.
R2 is an optional pull-up for faster recovery and correct biasing of TTL Open Collector
drivers.
RSTOUT
RPD
RSTIN
C1
a) Hardware Reset
V
CC
+
C0
R0
b) For Power-On Reset
(and Interruptible Power Down mode)
External Hardware
+
ST10F276