fm3808 Ramtron Corporation, fm3808 Datasheet - Page 18

• Manufacturer Part Number: fm3808
• Description: 256kb Bytewide Fram W/ Real-time Clock
• Manufacturer: Ramtron Corporation

Crystal Selection
The second passive component needed for the RTC
function is the timekeeping crystal. A 32.768 kHz
time-base is required, and the FM3808 is designed to
accept a low cost crystal. The major parameters
associated with the crystal are timekeeping accuracy
and backup current. The FM3808 is designed to
accept a crystal with a characteristic capacitance of 6
pF. Deviations from this specification will lead to
different accuracy and IBAK from the specified
values. Though accuracy is unlikely to improve, the
Rev. 1.3 (EOL)
Feb. 2006
0.0E+00
2.5E-06
2.0E-06
1.5E-06
1.0E-06
5.0E-07
-100
-120
-140
-160
-180
-200
-20
-40
-60
-80
4.7
0
4.5
Figure 10. Typical Crystal Error vs. Temperature
Figure 9. Backup Current vs. Voltage
4.25
IBAK vs VBAK
4
Ambient Temperature
3.75
VBAK
C
IBAK may go up or down from the specified value as
a function of the capacitive load.
The timekeeping accuracy is also a strong function of
the operating temperature due to errors in crystal
frequency. Temperature behavior of timekeeping
crystals is well known and it follows a curve like the
one shown below. The specific crystal manufacturer
should be consulted for the behavior of their specific
device. Note the error in frequency ppm. One ppm is
roughly 2.6 seconds per month in timekeeping error.
3.5
3.25
3
2.75
2.5
Page 18 of 27
FM3808