Rainbow Electronics MAX6901 User Manual
Page 15
V
CC
drops to less than 1.6V (typ), the MAX6901 resets
all register contents to the POR defaults (Table 2).
RESERVED Registers
Addresses/Commands 90h, 91h, 96h, and 97h are
reserved for factory testing ONLY. Do not write to these
registers. If inadvertent Writes are done to any of these
registers, cycle power to the MAX6901.
Power-Supply Considerations
For most applications, a 0.1µF capacitor from V
CC
to
GND provides adequate bypassing for the MAX6901. A
series resistor can be added to the supply line for oper-
ation in extremely harsh or noisy environments.
Timekeeping Current—Normal Operation
When I/O is high impedance (CS = low, or after each
rising-clock edge for a data output transfer), there is a
potential for increased timekeeping current (up to 100x)
if the I/O is allowed to float. If minimum timekeeping
current is desired, the microcontroller port pin should
be configured as an input with a weak pullup.
Alternatively, use a 100k
Ω or less pulldown or pullup
resistor (for microcontroller port pins with
≤1µA input
leakage).
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.
Timekeeping Current—Battery Backup
Systems
Often times, an RTC is operated in a system with a
backup battery. A microprocessor supervisory circuit
with backup battery switchover, or other switching
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 reset, its port pins typically become
high impedance. This essentially floats I/O, CS, and
SCLK on the MAX6901. There is a potential for
increased timekeeping current (up to x100) as V
CC
falls
through the linear region of the input gates for I/O,
SCLK, and CS. The 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 when V
CC
is gone back
through the microcontroller’s ESD protection, to keep
I/O, SCLK, and CS from floating. Alternately, a 100k
Ω
pulldown (for microcontroller port pins with
≤1µA input
leakage) on each pin (I/O, SCLK, and CS) ensures that
timekeeping current specifications are met during the
power switchover.
MAX6901
3-Wire Serial RTC in a TDFN
______________________________________________________________________________________
15
Table 4. 32.768kHz Surface-Mount Watch Crystals
MANUFACTURER
MANUFACTURER
PART NO.
TEMP. RANGE
C
L
(pF)
+25
°C FREQUENCY
TOLERANCE (ppm)
Abracon Corporation
ABS25-32.768-12.5-B-2-T
-40°C to +85°C
12.5
±20
Caliber Electronics
AWS2A-32.768kHz,
-20°C to +70°C
12.5
±20
ECS INC International
ECS-.327-12.5-17
-10°C to +60°C
12.5
±20
Fox Electronics
FSM327
-40°C to +85°C
12.5
±20
M-tron
SX2010/ SX2020
-20°C to +75°C
12.5
±20
Raltron
RSE-32.768-12.5-C-T
-10°C to +60°C
12.5
±20
SaRonix
32S12A
-40°C to +85°C
12.5
±20
Figure 6. Typical Temperature Curve for 32.768kHz Watch
Crystal
TYPICAL TEMPERATURE CHARACTERISITICS
(k = 0.035ppm/
°C
2
, T
O
= +25
°C)
∆
f (ppm)
TEMPERATURE (
°C)
-250
-200
-150
-100
-50
0
-50 -40 -30 -20 -10
0
10
20
25
30
40
50
60
70
80
90