Rainbow Electronics MAX6902 - Manual (Page 15)

Timekeeping Current
When DOUT is high impedance (CS = high or during a
DIN transfer segment), there is a potential for increased
timekeeping current (up to 100x) if DOUT is allowed to
float. If minimum timekeeping current is desired, then
ensure DOUT is not allowed to float. The microcon-
troller port pin attached to DOUT could be configured
as an input with a weak pullup. An alternate solution is
to use a 100k
Ω, or less, pulldown or pullup resistor (for
microcontroller port pins with
≤1µA input leakage).
Timekeeping Current—Backup Battery
Systems
Often a real-time clock (RTC) is operated in a system
with a backup battery. A microprocessor supervisory
circuit with backup battery switchover, or other switch-
ing arrangement, is used to switch power from V
CC to
V BATT when V CC falls below a set threshold. Most of
these systems leave only the RTC and some SRAM to
run from V
BATT . The microcontroller that communicates
with the RTC is powered only from V CC . When the
microcontroller is put into reset, its ports typically
become high impedance. This essentially floats DIN,
CS, DOUT, and SCLK. There is a potential for
increased timekeeping current (up to x100) as V
CC falls
through the linear region of the gates for DIN, CS,
DOUT, and SCLK. Duration of this effect depends on
the discharge rate of V
CC . To minimize current draw
from V BATT in such systems, ensure that V CC falls
rapidly at power down. One option is a V CC discharge
resistor of 100k Ω or less from V
CC to ground. This also
ensures sufficient impedance, back through the micro-
controller’s ESD protection, on V
CC when it is gone to
keep DIN, CS, DOUT, and SCLK from floating, which
can cause excessive timekeeping current. Alternately,
a 100k
Ω pulldown (for microcontroller port pins with
≤1µA input leakage) on each pin (DIN, CS, DOUT, and
SCLK) ensures that timekeeping current specifications
are met during the power switchover.
Power-On Reset
The MAX6902 contains an integral POR circuit that
ensures all registers are reset to a known state on
power-up. Once V
CC rises above 1.6V (typ), the POR
circuit releases the registers for normal operation. When
V
CC drops to less than 1.6V (typ), the MAX6902 resets
all register contents to the POR defaults (Figure 2).
RESERVED Register
Address/Command 17h is reserved for factory testing
ONLY. Do not write to this register. If inadvertent writes
are done to this register, cycle power to the MAX6902.
Power-Supply Considerations
For most applications, a 0.1µF capacitor from V CC to
GND provides adequate bypassing for the MAX6902. A
series resistor can be added to the supply line for oper-
ation in extremely harsh or noisy environments.
PC Board Layout Considerations
The MAX6902 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 MAX6902. To eliminate unwanted noise pickup,
design the PC board using these guidelines (Figure 7):
1) Place the crystal as close to X1 and X2 as possible
and keep the trace lengths short.
2) Place a guard ring around the crystal, X1 and X2
traces (where applicable), and connect the guard
ring to GND; keep all signal traces away from
beneath the crystal, X1, and X2.
3) 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
MAX6902 ground pin.
4) Restrict the plane to be no larger than the perimeter of
the guard ring. Do not allow this ground plane to con-
tribute significant capacitance between X1 and X2.
Chip Information
TRANSISTOR COUNT: 26,418
PROCESS: CMOS
MAX6902
SPI-Compatible RTC in a TDFN
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