FM3104-G データシートの表示（PDF） - Ramtron International Corporation

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FM3104-G

(FM3104 - FM31256) Integrated Processor Companion with Memory

Ramtron International Corporation 

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.

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 2.5V, the RTC (and event counters)

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 FRAM

memory is that data is not lost regardless of the

backup power source.

The IBAK current varies with temperature and voltage

(see DC parametric table). The following graph

shows IBAK as a function of VBAK. These curves are

useful for calculating backup time when a capacitor

is used as the VBAK source.

Figure 7. IBAK vs. VBAK Voltage

The minimum VBAK voltage varies linearly with

temperature. The user can expect the minimum VBAK

voltage to be 1.23V at +85°C and 1.90V at -40°C.

The tested limit is 1.55V at +25°C. The minimum

VBAK voltage has been characterized at -40°C and

+85°C but is not 100% tested.

FM3104/16/64/256

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 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.

In the case where no battery is used, the VBAK pin

should be tied according to the following conditions:

 For 3.3V systems, VBAK should be tied to VDD.

This assumes VDD does not exceed 3.75V.

 For 5V systems, attach a 1 µF capacitor to VBAK

and turn the trickle charger on. The VBAK pin

will charge to the internal backup voltage which

regulates itself to about 3.6V. VBAK should not

be tied to 5V since the VBAK (max) specification

will be exceeded. A 1 µF capacitor will keep

the companion functions working for about 1.5

second.

Although VBAK may be connected to VSS, this is not

recommended if the companion is used. None of the

companion functions will operate below about 2.5V.

 Note: systems using lithium batteries should clear

the VBC bit to 0 to prevent battery charging. The

VBAK circuitry includes an internal 1 K series

resistor as a safety element.

Figure 8. VBAK (min.) vs. Temperature

Rev. 2.1

Sept. 2011

Page 7 of 26

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