MAX6909 MAXIM [Maxim Integrated Products], MAX6909 Datasheet - Page 11

• Manufacturer Part Number: MAX6909
• Description: I2C-Compatible Real-Time Clocks with uP Supervisor and NV RAM Controller
• Manufacturer: MAXIM [Maxim Integrated Products]

A power-fail comparator is available to monitor other
system voltages through PFI and report the status
through PFO. If the MAX6909/MAX6910 are in reset,
PFO is low; otherwise, it is high as long as PFI is
greater than 1.27V (typ).
The piezo transducer drive output (PZT) has register-
selectable frequencies of 1.024kHz, 2.048kHz,
4.096kHz, or 8.192kHz. This output can be selected to
become active when the alarm is triggered or can be
independently controlled through the configuration regis-
ter. When activated, the PZT outputs a frequency with an
attention-getting 1Hz duty cycle of 50% on and 50% off.
An on-chip crystal oscillator maintaining circuit, for use
with a 32.768kHz crystal, provides the clock for time-
keeping functions. A crystal fail output (CX FAIL) alerts
the user when the 32.768kHz crystal oscillator has
failed for 30 cycles (typ), resulting in conditions that
produce invalid timekeeping data. The crystal fail func-
tion can also be polled by reading the status bit in the
CX status register.
A 32.768kHz crystal is connected to the MAX6909/
MAX6910 through pins 9 and 10 (X1 and X2). The crys-
tal selected for use should have a specified load capac-
Figure 2. Recommended Crystal Layout
I
2
______________________________________________________________________________________
C-Compatible Real-Time Clocks with µP
GUARD RING
Supervisor and NV RAM Controller
Crystal Selection
VIA CONNECTION
GROUND PLANE
*
SM WATCH CRYSTAL
*
*LAYER 1 TRACE
**
**
VIA CONNECTION
GROUND PLANE
itance (C
in the MAX6909/MAX6910. When designing the PC
board, keep the crystal as close to the X1 and X2 pins
as possible. Keep the trace lengths short and small and
place a guard ring around the crystal and connect the
ring to GND to reduce capacitive loading and prevent
unwanted noise pickup. Keep all signals out from
beneath the crystal and the X1 and X2 pins to prevent
noise coupling. Finally, an additional local ground plane
on an adjacent PC board layer can be added under the
crystal to shield it from unwanted pickup from traces on
other layers of the board. This plane should be isolated
from the regular PC board ground plane, should be no
larger than the perimeter of the guard ring, and con-
nected to the GND pin of the MAX6909/MAX6910.
Ensure that this ground plane does not contribute to sig-
nificant capacitance between signal line and ground on
the connections that run from X1 and X2 to the crystal.
Figure 2 shows the recommended crystal layout.
Some crystal manufacturers and part numbers for their
SMT, 32.768kHz watch crystals that require 6pF loads
are listed in Table 1. In addition, these manufacturers
offer other package options depending upon the spe-
cific application considerations.
**LAYER 2 LOCAL GROUND PLANE
GROUND PLANE VIA CONNECTION
*
*
*
CONNECT ONLY TO PIN 8
**
L
) of 6pF where the capacitive load is included
*
*
10 11
*
*
*
*
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