FM31278/276/274/272 - 5V I2C Companion
The calibration setting is stored in F-RAM so is not
lost should the backup source fail. It is accessed with
bits CAL.4-0 in register 01h. This value only can be
written when the CAL bit is set to a 1. To exit the
calibration mode, the user must clear the CAL bit to a
0. When the CAL bit is 0, the CAL/PFO pin will
revert to the power fail output function.
Crystal Oscillator
The crystal oscillator is designed to use a 12.5pF
crystal without the need for external components,
such as loading capacitors. The FM3127x device has
built-in loading capacitors that match the crystal.
If a 32.768kHz crystal is not used, an external
oscillator may be connected to the FM3127x. Apply
the oscillator to the X1 pin. Its high and low voltage
levels can be driven rail-to-rail or amplitudes as low
as approximately 500mV p-p. To ensure proper
operation, a DC bias must be applied to the X2 pin.
It should be centered between the high and low levels
on the X1 pin. This can be accomplished with a
voltage divider.
In the example, R1 and R2 are chosen such that the
X2 voltage is centered around the X1 oscillator drive
levels. If you wish to avoid the DC current, you may
choose to drive X1 with an external clock and X2
with an inverted clock using a CMOS inverter.
FM3127x
X1 X2
Vdd
R1
R2
Figure 8. External Oscillator
Layout Requirements
The X1 and X2 crystal pins employ very high
impedance circuits and the oscillator connected to
these pins can be upset by noise or extra loading. To
reduce RTC clock errors from signal switching noise,
a guard ring must be placed around these pads and
the guard ring grounded. SDA and SCL traces should
be routed away from the X1/X2 pads. The X1 and X2
trace lengths should be less than 5 mm. The use of a
ground plane on the backside or inner board layer is
preferred. See layout example. Red is the top layer,
green is the bottom layer.
VDD
SCL
SDA
X2
X1
PFI
VBAK
VDD
SCL
SDA
X2
X1
PFI
VBAK
Layout for Surface Mount Crystal
(red = top layer, green = bottom layer)
Layout for Through Hole Crystal
(red = top layer, green = bottom layer)
Rev. 2.0
Dec. 2007
Page 8 of 25
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