IDT1337
REA LTIME CLOCK WITH SERIAL INTERFACE
RTC
Typical Operating Circuit
V
CC
V
CC
CRYSTAL
V
CC
2k
CPU
2k
X1
SCL
SDA
X2
V
CC
SQW/INTB
10k
10k
IDT1337
GND
INTA
Detailed Description
Communications to and from the IDT1337 occur serially
over an I
2
C bus. The IDT1337 operates as a slave device on
the serial bus. Access is obtained by implementing a START
condition and providing a device identification code,
followed by data. Subsequent registers can be accessed
sequentially until a STOP condition is executed. The device
is fully accessible through the I
2
C interface whenever VCC
is between 5.5 V and 1.8 V. I
2
C operation is not guaranteed
when VCC is below 1.8 V. The IDT1337 maintains the time
and date when VCC is as low as 1.3 V.
The following sections discuss in detail the Oscillator block,
Clock/Calendar Register Block and Serial I
2
C block.
Effective Load Capacitance
Please see diagram below for effective load capacitance
calculation. The effective load capacitance (CL) should
match the recommended load capacitance of the crystal in
order for the crystal to oscillate at its specified parallel
resonant frequency with 0ppm frequency error.
Oscillator Block
Selection of the right crystal, correct load capacitance and
careful PCB layout are important for a stable crystal
oscillator. Due to the optimization for the lowest possible
current in the design for these oscillators, losses caused by
parasitic currents can have a significant impact on the
overall oscillator performance. Extra care needs to be taken
to maintain a certain quality and cleanliness of the PCB.
Crystal Selection
The key parameters when selecting a 32 kHz crystal to work
with IDT1337 RTC are:
•
Recommended Load Capacitance
•
Crystal Effective Series Resistance (ESR)
•
Frequency Tolerance
In the above figure, X1 and X2 are the crystal pins of our
device. Cin1 and Cin2 are the internal capacitors which
include the X1 and X2 pin capacitance. Cex1 and Cex2 are
the external capacitors that are needed to tune the crystal
frequency. Ct1 and Ct2 are the PCB trace capacitances
between the crystal and the device pins. CS is the shunt
capacitance of the crystal (as specified in the crystal
manufacturer's datasheet or measured using a network
analyzer).
IDT™
REAL-TIME CLOCK WITH I
2
C SERIAL INTERFACE
3
IDT1337
REV J 111009
IDT [ INTEGRATED DEVICE TECHNOLOGY ]
