Rainbow Electronics MAX6901 - Manual (Page 16)

MAX6901
There are similar considerations for 32KHZ if it is
placed in its high-impedance state. For lowest time-
keeping current, it should not be allowed to float. Force
it high or low, or terminate it with a pullup or pulldown
resistor.
PC Board Layout Considerations
The MAX6901 uses a very-low-current oscillator to mini-
mize supply current. This causes the oscillator pins, X1
and X2, to be relatively high impedance. Exercise care
to prevent unwanted noise pickup.
Connect the 32.768kHz crystal directly across X1 and
X2 of the MAX6901. To eliminate unwanted noise pick-
up, design the PC board using these guidelines (Figure
7): place the crystal as close to X1 and X2 as possible
and keep the trace lengths short; place a guard ring
around the crystal, X1 and X2 traces (where applica-
ble), and connect the guard ring to GND; keep all sig-
nal traces away from beneath the crystal, X1, and X2.
Finally, an additional local ground plane can be added
under the crystal on an adjacent PC board layer. The
plane should be isolated from the regular PC board
ground plane, and tied to ground at the MAX6901
ground pin. Restrict the plane to be no larger than the
perimeter of the guard ring. Do not allow this ground
plane to contribute significant capacitance between X1
and X2.
Chip Information
TRANSISTOR COUNT: 26,214
PROCESS: CMOS
3-Wire Serial RTC in a TDFN
16 ______________________________________________________________________________________
Figure 7. Printed Circuit Board Layout for Crystal Connections
SM WATCH CRYSTAL
0.1 µF
SM CAP
*
*
*
*
*
*
* *
*
*
*LAYER 1 TRACE
*
**
**
GROUND PLANE
VIA CONNECTION
GUARD RING
GROUND PLANE
VIA CONNECTION
GROUND PLANE
VIA CONNECTION
V CC PLANE
VIA CONNECTION
**
** LAYER 2 LOCAL GROUND PLANE
CONNECT ONLY TO PIN 6
GROUND PLANE VIA
MAX6901