FM33256/FM3316 SPI Companion w/ FRAM
both sources indicated if both have occurred since the
user cleared them.
counter increments on a rising edge of CNT. The
polarity bit CP is nonvolatile.
Power Fail Comparator
CP
16-bit Counter
An analog comparator compares the PFI input pin to
an onboard 1.5V reference. When the PFI input
voltage drops below this threshold, the comparator
will drive the PFO pin to a low state. The comparator
has 100 mV of hysteresis (rising voltage only) to
reduce noise sensitivity. The most common
application of this comparator is to create an early
warning power fail interrupt (NMI). This can be
accomplished by connecting the PFI pin to an
upstream power supply via a resistor divider. An
application circuit is shown below. The comparator is
a general purpose device and its application is not
limited to the NMI function.
CNT
Figure 7. Event Counter
There is also a control bit that allows the user to
define the counter as nonvolatile or battery-backed.
The counter is nonvolatile when the NVC bit
(register 0Dh, bit 7) is logic 1 and battery-backed
when the NVC bit is logic 0. Setting the counter
mode to battery-backed allows counter operation
under VBAK (as well as VDD) power. The lowest
operating voltage for battery-backed mode is 2.0V.
When set to “nonvolatile” mode, the counter operates
only when VDD is applied and is above the VTP
voltage.
VDD
Regulator
The event counter may be programmed to detect a
tamper event, such as the system’s case or access
door being opened. A normally closed switch is tied
to the CNT pin and the other contact to the case
chassis, usually ground. The typical solution uses a
pullup resistor on the CNT pin and will continuously
draw battery current. The FM33xx chip allows the
user to invoke a polled mode, which occasionally
samples the pin in order to minimize battery drain. It
internally tries to pull the CNT pin up and if open
circuit will be pulled up to a VIH level, which will trip
the edge detector and increment the event counter
value. Setting the POLL bit (register 0Dh, bit 1)
places the CNT pin into this mode. This mode allows
the event counter to detect a rising edge tamper event
but the user is restricted to operating in battery-
backed mode (NVC=0) and using rising edge
detection (CP=1). The CNT pin is polled once every
125ms. The additional average IBAK current is less
than 5nA. The polling timer circuit operates from the
RTC, so the oscillator must be enabled for this to
function properly.
FM33xx
+
PFO
To MCU
NMI input
-
1.5V ref
Figure 6. Comparator as a Power-Fail Warning
If the power-fail comparator is not used, the PFI pin
should be tied to either VDD or VSS. Note that the
PFO output will drive to VDD or VSS as well.
Event Counter
The FM33xx offers the user a nonvolatile 16-bit
event counter. The input pin CNT has
a
programmable edge detector. The CNT pin clocks the
counter. The counter is located in registers 0E-0Fh.
When the programmed edge polarity occurs, the
counter will increment its count value. The register
value is read by setting the RC bit (register 0Dh, bit
3) to 1. This takes a snapshot of the counter byte
allowing a stable value even if a count occurs during
the read. The register value can be written by first
setting the WC bit (register 0Dh, bit 2) to 1. The user
then may clear or preset the counter by writing to
registers 0E-0Fh. Counts are blocked when the WC
bit is set, so the user must clear the bit to allow
counts.
Vbak
FM33xx
< 100pF
CNT
125ms
Figure 8. Polled Mode on CNT pin Detects
Tamper to System Case
The counter polarity control bit is CP, register 0Dh
bit 0. When CP is 0, the counter increments on a
falling edge of CNT, and when CP is set to 1, the
In the polled mode, the internal pullup circuit can
source a limited amount of current. The maximum
Rev. 1.0
Dec. 2006
Page 6 of 28
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