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MAX6900
I2C-Bus-Compatible Real-Time Clock
in a SOT23
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13
PARAMETER
SYMBOL
MIN
TYP
MAX
UNITS
Frequency
f
32.768
kHz
Equivalent Series Resistance (ESR)
Rs
40
60
k
Parallel Load Capacitance
CL
11.2
12.5
13.7
pF
Q Factor
Q
40,000
60,000
Table 4. Quartz Crystal Parameters
MANUFACTURER
PART NO.
TEMP.
RANGE
CL (pF)
+25
°C FREQUENCY
TOLERANCE (ppm)
Abracon Corporation
ABS25-32.768-12.5-B-
-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
Table 5. Crystal Manufacturers
than 31 bytes could cause erroneous data in the
MAX6900.
Power-On Reset (POR)
The MAX6900 contains an integral POR circuit that
ensures all registers are reset to a known state on
power-up. Once VCC rises above 1.6V (typ), the POR
circuit releases the registers for normal operation.
When VCC drops to less than 1.6V (typ), the MAX6900
resets all register contents to the POR defaults.
Oscillator Startup
The MAX6900 oscillator typically takes 5s to 10s to
begin oscillating. To ensure the oscillator is operating
correctly, the software should validate proper time-
keeping. This is accomplished by reading the Seconds
register. Any reading of 1s or more from the POR is a
validation of proper startup.
Reserved Register
This is reserved for factory testing ONLY. Do not write
to this register. If inadvertent Writes are done to this
register, cycle power on the MAX6900.
Crystal Selection
Connect a 32.768kHz watch crystal directly to the
MAX6900 through pin 2 and pin 3 (X1, X2). Use a crys-
tal with a specified load capacitance (CL) of 12.5pF.
See Table 4 for a list of crystal parameters.
Table 5 lists some crystal manufacturers and part num-
bers for their surface-mount 32.768kHz watch crystals
that require 12.5pF. In addition, these manufacturers
offer other package options depending upon the spe-
cific application considerations.
Frequency Stability Over Temperature
Timekeeping accuracy of the MAX6900 is dependent
on the frequency stability of the external crystal. To
determine frequency stability, use the parabolic curve
in Figure 10 and the following equations:
Rf
Rd
Cd
25pF
Cg
25pF
EXTERNAL
CRYSTAL
X1
X2
MAX6900
Figure 9. Oscillator Functional Schematic