fm3808 Ramtron Corporation, fm3808 Datasheet - Page 8

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

Calibration
The RTC enters calibration mode when the CAL bit
in register 7FF0h is set to 1. Interrupts are disabled in
CAL mode. The RTC is calibrated by applying a
digital correction to the RTC logic block. In CAL
mode, the INT pin is driven with a 512.00 Hz
nominal square wave. Based on the measured
deviation from this frequency, a correction value must
be written by the user into the calibration register
7FF8h. See Table 2 for the correction codes.
Positive ppm errors require a negative adjustment that
removes pulses. Negative ppm errors require a
positive correction that adds pulses. Positive ppm
adjustments have the CALS bit set to 1, whereas
negative ppm adjustments have CALS = 0. After
calibration, the clock will have a maximum error of
Rev. 1.3 (EOL)
Feb. 2006
calibrated temperature.
The calibration setting is nonvolatile and is stored in
7FF8h (bits D4-D0). This value only can be written
when the CAL bit is set to a 1. To exit calibration
mode, the user must clear the CAL bit to a 0.
When the calibration mode is entered, the user can
measure the frequency error on the INT pin. This
error expressed in ppm translates directly into
timekeeping
adjustment
correction is applied by adding or removing pulses on
a periodic basis. Therefore, the correction will not
appear on the 512 Hz output. The calibration
correction must be applied using the values shown in
Table 2. The timekeeping accuracy can be verified
by comparing the FM3808 time to a reference source.
4.34 ppm or
corrects
error.
0.19 minutes per month at the
this
An
error.
offsetting
However,
calibration
Page 8 of 27
FM3808
the