FM4005
Counter 2 Polarity is C2P, bit 1; the Cascade Control
is CC, bit 2; and the Read Counter bit is RC bit 3.
registers. The registers are frozen and will not be
updated again until the R bit is cleared to 0. R is used
for reading the time.
C1P
16-bit Counter
Setting the W bit to 1 locks the user registers.
Clearing it to 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.
CNT1
C2P
CNT2
16-bit Counter
CC
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 drops below 2.5V, the RTC (and event
counters) will switch to the backup power supply on
VBAK. The clock uses very little current which
maximizes battery or capacitor life.
Figure 6. Event Counter
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.
When inserting a battery into a system board,
higher-than-normal battery drain may occur. It is
recommended that your system power-up
procedure complies with one of the following:
Serial Number
A memory location to write a 64-bit serial number is
provided. It is a writeable nonvolatile register that
can be locked by the user once the serial number is
set. The serial number registers can be written an
unlimited number of times. 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.
Scenario #1
a) Apply Vdd to board.
b) Insert battery. At this point, IBAK is zero.
c) When VDD is powered down, the IBAK current
will be less than 1µA.
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.
Scenario #2
a) Insert battery without power to board (VDD is
off). At this point, the IBAK current may be
much higher than the 1µA spec limit. An
extended period of time (days) in this state
could significantly reduce battery life.
b) Apply VDD to board. IBAK goes to zero.
c) When VDD is powered down, the IBAK current
will be less than 1µA.
Real-time Clock Operation
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.
Trickle Charger
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 7) illustrates the RTC function.
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 15 µA until VBAK
reaches VDD or 3.75V whichever is less. In 3V
systems, this charges the capacitor to VDD without
an external diode and resistor charger. In 5V systems,
it provides the same convenience and also prevents
the user from exceeding the VBAK maximum
voltage specification.
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 while R is set, then the
core will be updated prior to loading the user
! Note: systems using lithium batteries should clear
the VBC bit to 0 to prevent battery charging. The
VBAK circuitry includes an internal 1 KΩ series
resistor as a safety element.
Rev. 2.3
Oct. 2006
Page 6 of 23
RAMTRON [ RAMTRON INTERNATIONAL CORPORATION ]
