FM31278/276/274/272 - 5V I2C Companion
V
TP
3.9V
4.4V
VTP
0
1
Manual Reset
The /RST pin is bi-directional and allows the
FM3127x to filter and de-bounce a manual reset
switch. The /RST input detects an external low
condition and responds by driving the /RST signal
low for 100 ms.
MCU
RST
FM3127x
The watchdog timer can also be used to assert the
reset signal (/RST). The watchdog is a free running
programmable timer. The period can be software
programmed from 100 ms to 3 seconds in 100 ms
increments via a 5-bit nonvolatile register. All
programmed settings are minimum values and vary
with temperature according to the operating
specifications. The watchdog has two additional
controls associated with its operation, a watchdog
enable bit (WDE) and timer restart bits (WR). Both
the enable bit must be set and the watchdog must
timeout in order to drive /RST active. If a reset event
occurs, the timer will automatically restart on the
rising edge of the reset pulse. If WDE=0, the
watchdog timer runs but a watchdog fault will not
cause /RST to be asserted low. The WTR flag will be
set, indicating a watchdog fault. This setting is useful
during software development and the developer does
not want /RST to drive. Note that setting the
maximum timeout setting (11111b) disables the
counter to save power. The second control is a nibble
that restarts the timer preventing a reset. The timer
should be restarted after changing the timeout value.
The watchdog timeout value is located in register
0Ah, bits 4-0, and the watchdog enable is bit 7. The
watchdog is restarted by writing the pattern 1010b to
the lower nibble of register 09h. Writing this pattern
will also cause the timer to load new timeout values.
Writing other patterns to this address will not affect
its operation. Note the watchdog timer is free-
running. Prior to enabling it, users should restart the
timer as described above. This assures that the full
timeout period will be set immediately after enabling.
The watchdog is disabled when V
DD
is below V
TP
.
The following table summarizes the watchdog bits. A
block diagram follows.
Watchdog timeout
Watchdog enable
Watchdog restart
WDT4-0
WDE
WR3-0
0Ah, bits 4-0
0Ah, bit 7
09h, bits 3-0
Reset
Switch
Switch
Behavior
RST
FM3127x
drives
100 ms (min.)
Figure 4. Manual Reset
Note that an internal weak pull-up on /RST
eliminates the need for additional external
components.
Reset Flags
In case of a reset condition, a flag will be set to
indicate the source of the reset. A low V
DD
reset or
manual reset is indicated by the POR flag, register
09h bit 6. A watchdog reset is indicated by the WTR
flag, register 09h bit 7. Note that the flags are
internally set in response to reset sources, but they
must be cleared by the user. When the register is
read, it is possible that both flags are set if both have
occurred since the user last cleared them.
Early Power Fail Comparator
An early power fail warning can be provided to the
processor well before V
DD
drops out of spec. The
comparator is used to create a power fail interrupt
(NMI). This can be accomplished by connecting the
PFI pin to the unregulated power supply via a resistor
divider. An application circuit is shown below.
Regulator
VDD
100 ms
clock
Timebase
WR3-0 = 1010b to restart
Counter
Watchdog
timeout
/RST
FM3127x
WDE
PFI
To MCU CAL/PFO
NMI input
+
-
1.2V ref
Figure 3. Watchdog Timer
Figure 5. Comparator as Early Power-Fail Warning
Rev. 2.0
Dec. 2007
Page 5 of 25
RAMTRON [ RAMTRON INTERNATIONAL CORPORATION ]
