FM31278/276/274/272 - 5V I2C Companion

Rev. 2.0

Dec. 2007

Page 3 of 25

Figure 1. Block Diagram

Pin Descriptions

Pin Name

Type

Pin Description

A0, A1

Input

Device select inputs are used to address multiple memories on a serial bus. To select

the device the address value on the two pins must match the corresponding bits

contained in the device address. The device select pins are pulled down internally.

CNT1, CNT2

Input

Event Counter Inputs: These battery-backed inputs increment counters when an edge is

detected on the corresponding CNT pin. The polarity is programmable. These pins

should not be left floating. Tie to ground if these pins are not used.

CAL/PFO

Output

In calibration mode, this pin supplies a 512 Hz square-wave output for clock

calibration. In normal operation, this is the early power-fail output.

X1, X2

I/O

32.768 kHz crystal connection. When using an external oscillator, apply the clock to

X1 and a DC mid-level to X2 (see Crystal Oscillator section for suggestions).

/RST

I/O

Active low reset output with weak pull-up. Also input for manual reset.

SDA

I/O

Serial Data & Address: This is a bi-directional line for the two-wire interface. It is

open-drain and is intended to be wire-OR’d with other devices on the two-wire bus.

The input buffer incorporates a Schmitt trigger for noise immunity and the output

driver includes slope control for falling edges. A pull-up resistor is required.

SCL

Input

Serial Clock: The serial clock line for the two-wire interface. Data is clocked out of the

part on the falling edge, and in on the rising edge. The SCL input also incorporates a

Schmitt trigger input for noise immunity.

PFI

Input

Early Power-fail Input: Typically connected to an unregulated power supply to detect

an early power failure. This pin should not be left floating.

VBAK

Supply

Backup supply voltage: A 3V battery or a large value capacitor. If no backup supply is

used, this pin should be tied to ground and the VBC bit should be cleared.

VDD

Supply

Supply Voltage

VSS

Supply

Ground

F-RAM

Array

2-Wire

Interface

SCL

SDA

RST

A1, A0

CAL/PFO

PFI

VDD

VBAK

RTC

2.5V

-

+

RTC Registers

Event

Counters

CNT1

CNT2

Special

Function

Registers

S/N

X1

X2

LockOut

LockOut

+

-

1.2V

Watchdog

LV Detect

Switched Power

512Hz

RTC Cal.

Battery Backed

Nonvolatile