FM31L278/L276/L274/L272 - 3V I2C Companion
Crystal Oscillator
The crystal oscillator is designed to use a 12.5pF
crystal without the need for external components,
such as loading capacitors. The FM31L27x device
has built-in loading capacitors that match the crystal.
FM31L27x
X1 X2
R1
R2
If a 32.768kHz crystal is not used, an external
oscillator may be connected to the FM31L27x.
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.
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.
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.
VDD
SCL
SDA
X2
VDD
SCL
SDA
X2
X1
X1
PFI
PFI
VBAK
VBAK
Layout for Surface Mount Crystal
(red = top layer, green = bottom layer)
Layout for Through Hole Crystal
(red = top layer, green = bottom layer)
Rev. 3.0
Feb. 2009
Page 8 of 25
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