FM32276 datasheet(6/21 Pages) RAMTRON

6 / 21 page

FM32278/276/274/272 - 5V I2C Companion

Rev. 3.0

Feb. 2009

Page 6 of 21

The voltage on the PFI input pin is compared to an

onboard 1.2V 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 (max) of hysteresis to reduce noise sensitivity,

only for a rising PFI signal. For a falling PFI edge,

there is no hysteresis.

The comparator is a general purpose device and its

application is not limited to the NMI function.

Note: The maximum voltage on the comparator input PFI

is limited to 3.75V under normal operating conditions.

Event Counter

The FM3227x offers the user two battery-backed

event counters. Input pins CNT1 and CNT2 are

programmable edge detectors. Each clocks a 16-bit

counter. When an edge occurs, the counters will

increment their respective registers. Counter 1 is

located in registers 0Dh and 0Eh, Counter 2 is

located in registers 0Fh and 10h. These register

values can be read anytime VDD is above VTP, and

they will be incremented as long as a valid VBAK

power source is provided. To read, set the RC bit

four counter bytes allowing a stable value even if a

count occurs during the read. The registers can be

written by software allowing the counters to be

cleared or initialized by the system. Counts are

blocked during a write operation. The two counters

can be cascaded to create a single 32-bit counter by

setting the CC control bit (register 0Ch). When

cascaded, the CNT1 input will cause the counter to

increment. CNT2 is not used in this mode.

Figure 6. Event Counter

The control bits for event counting are located in

Counter 2 Polarity is C2P, bit 1; the Cascade Control

is CC, bit 2; and the Read Counter bit is RC bit 3.

The polarity bits must be set prior to setting the

counter value(s). If a polarity bit is changed, the

counter may inadvertently increment. If the counter

pins are not being used, tie them to ground.

Serial Number

A memory location to write a 64-bit serial number is

provided. It is a writeable nonvolatile memory block

that can be locked by the user once the serial number

is set. The 8 bytes of data and the lock bit are all

accessed via the device ID for the processor

companion. Therefore the serial number area is

separate and distinct from the memory array. The

serial number registers can be written an unlimited

number of times, so these locations are general

purpose memory. However once the lock bit is set the

values cannot be altered and the lock cannot be

removed. Once locked the serial number registers can

still be read by the system.

The serial number is located in registers 11h to 18h.

The lock bit is SNL, register 0Bh bit 7. Setting the

SNL bit to a 1 disables writes to the serial number

registers, and the SNL bit cannot be cleared.

Backup Power

The event counter and battery-backed registers may

be powered with a backup power source. When the

primary system power fails, the voltage on the VDD

pin will drop. When VDD is less than 2.5V, the event

counters and battery-backed registers will switch to

the backup power supply on VBAK.

A battery may be inserted into a system board

without any concern for excessive current draw on

the FM3227x’s VBAK pin.

Trickle Charger

To facilitate capacitor backup, the VBAK pin can

optionally provide a trickle charge current. When the

VBC bit, register 0Bh bit 2, is set to ‘1’, the VBAK pin

will source approximately 80 µA until VBAK reaches

3.75V. In 5V systems, this charges the capacitor to

VDD without an external diode and resistor charger

and also prevents the user from exceeding the VBAK

maximum voltage specification. There is a Fast

Charge mode which is enabled by the FC bit (register

0Bh, bit 5). In this mode the trickle charger current is

set to approximately 1 mA, allowing a large backup

capacitor to charge more quickly.

In the case where no backup source is used, the VBAK

pin should be tied to VSS. VBAK should not be tied to

5V since the VBAK (max) specification will be

exceeded. Be sure to turn off the trickle charger

(VBC=0), otherwise charger current will be shunted

to ground from VDD.

! Note: systems using lithium batteries should clear the

VBC bit to 0 to prevent battery charging. The VBAK

circuitry includes an internal 1 K

Ω series resistor as a

safety element. The trickle charger is UL Recognized.

16-bit Counter

CNT1

CNT2

C1P

C2P

16-bit Counter

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