FM3135 Integrated RTC/Alarm/FRAM & Embedded Crystal
Overview
Real-Time Clock Operation
The FM3135 device combines a serial nonvolatile
The real-time clock (RTC) is a timekeeping device
that can be battery or capacitor backed for
permanently-powered operation. It offers a software
calibration feature that allows high accuracy.
RAM with a real-time clock (RTC) and alarm. These
complementary but distinct functions share
a
common interface in a single package. Although
monolithic, the product is organized as two logical
devices, the F-RAM memory and the RTC/alarm.
From the system perspective, they appear to be two
separate devices with unique IDs on the serial bus.
The RTC consists of an oscillator, clock divider, and
a register system for user access. It divides down the
32.768 kHz time-base and provides a minimum
resolution of seconds (1Hz). Static registers provide
the user with read/write access to the time values. It
includes registers for seconds, minutes, hours, day-
of-the-week, date, months, and years. A block
diagram (Figure 2) illustrates the RTC function.
The memory is organized as a stand-alone 2-wire
nonvolatile memory with a standard device ID value.
The real-time clock and alarm are accessed with a
separate 2-wire device ID. This allows clock/calendar
data to be read while maintaining the most recently
used memory address. The clock and alarm are
controlled by 15 special function registers. The
registers are maintained by the power source on the
VBAK pin, allowing them to operate from battery or
backup capacitor power when VDD drops below a set
threshold. Each functional block is described below.
The user registers are synchronized with the
timekeeper core using R and W bits in register 00h
described below. Changing the R bit from 0 to 1
transfers timekeeping information from the core into
holding registers that can be read by the user. If a
timekeeper update is pending when R is set, then the
core will be updated prior to loading the user
registers. The registers are frozen and will not be
updated again until the R bit is cleared to ‘0’. R is
used to read the time.
Memory Operation
The FM3135 integrates a 64Kb F-RAM. The
memory is organized in bytes, 8192 addresses of 8
bits each. The memory is based on F-RAM
technology. Therefore it can be treated as RAM and
is read or written at the speed of the two-wire bus
with no delays for write operations. It also offers
effectively unlimited write endurance unlike other
nonvolatile memory technologies. The two-wire
interface protocol is described further on page 12.
Setting the W bit to ‘1’ locks the user registers.
Clearing it to a ‘0’ causes the values in the user
registers to be loaded into the timekeeper core. W is
used for writing new time values. Users should be
certain not to load invalid values, such as FFh, to the
timekeeping registers. Updates to the timekeeping
core occur continuously except when locked. All
timekeeping registers must be initialized at the first
powerup or when the LB bit is set. See the
description of the LB bit on page 11.
The memory array can be write-protected by
software. Two bits (WP0, WP1) in register 0Eh
control the protection setting as shown in the
following table. Based on the setting, the protected
addresses cannot be written and the 2-wire interface
will not acknowledge any data to protected addresses.
The special function registers containing these bits
are described in detail below.
Backup Power
The real-time clock/calendar is intended to be
permanently powered. When the primary system
power fails, the voltage on the VDD pin will drop.
When VDD is less than VSW, the RTC will switch to
the backup power supply on VBAK. The clock
operates at extremely low current in order to
maximize battery or capacitor life. However, an
advantage of combining a clock function with F-
RAM memory is that data is not lost regardless of the
backup power source.
Table 1. F-RAM Write-Protect
Write-Protect Range
WP1
WP0
None
0
0
1
1
0
1
0
1
Bottom 1/4
Bottom 1/2
Full array
If a battery is applied without a VDD power supply,
the device has been designed to ensure the IBAK
current does not exceed the 1µA maximum limit.
The WP bits are battery-backed. On a powerup
without a backup source, the WP bits are cleared to a
‘0’ state.
Trickle Charger
To facilitate capacitor backup the VBAK pin can
optionally provide a trickle charge current. When the
Rev. 1.2
Feb. 2009
Page 3 of 21
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