1337GCSRGI datasheet(5/28 Pages) RENESAS

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IDT1337G

REAL-TIME CLOCK WITH I2C SERIAL INTERFACE

RTC

IDT® REAL-TIME CLOCK WITH I2C SERIAL INTERFACE

IDT1337G

REV M 073013

Note: IDT1337CSRI integrates a standard 32.768 kHz

crystal in the package and contributes an additional

frequency error of 10ppm at nominal VCC (+3.3 V) and

TA=+25°C.

ESR (Effective Series Resistance)

Choose the crystal with lower ESR. A low ESR helps the

crystal to start up and stabilize to the correct output

frequency faster compared to high ESR crystals.

Frequency Tolerance

The frequency tolerance for 32 kHz crystals should be

specified at nominal temperature (+25°C) on the crystal

manufacturer datasheet. The crystals used with IDT1337G

typically have a frequency tolerance of +/-20ppm at +25°C.

Specifications for a typical 32 kHz crystal used with our

device are shown in the table below.

PCB Design Consideration

• Signal traces between IDT device pins and the crystal

must be kept as short as possible. This minimizes

parasitic capacitance and sensitivity to crosstalk and

EMI. Note that the trace capacitances play a role in the

effective crystal load capacitance calculation.

• Data lines and frequently switching signal lines should be

routed as far away from the crystal connections as

possible. Crosstalk from these signals may disturb the

oscillator signal.

• Reduce the parasitic capacitance between X1 and X2

signals by routing them as far apart as possible.

• The oscillation loop current flows between the crystal and

the load capacitors. This signal path (crystal to CL1 to

CL2 to crystal) should be kept as short as possible and

ideally be symmetric. The ground connections for both

capacitors should be as close together as possible.

Never route the ground connection between the

capacitors all around the crystal, because this long

ground trace is sensitive to crosstalk and EMI.

• To reduce the radiation / coupling from oscillator circuit,

an isolated ground island on the GND layer could be

made. This ground island can be connected at one point

to the GND layer. This helps to keep noise generated by

the oscillator circuit locally on this separated island. The

ground connections for the load capacitors and the

oscillator should be connected to this island.

PCB Layout

PCB Assembly, Soldering and Cleaning

Board-assembly production process and assembly quality

can affect the performance of the 32 KHz oscillator.

Depending on the flux material used, the soldering process

can leave critical residues on the PCB surface. High

humidity and fast temperature cycles that cause humidity

condensation on the printed circuit board can create

process residuals. These process residuals cause the

insulation of the sensitive oscillator signal lines towards

each other and neighboring signals on the PCB to decrease.

High humidity can lead to moisture condensation on the

surface of the PCB and, together with process residuals,

reduce the surface resistivity of the board. Flux residuals on

the board can cause leakage current paths, especially in

humid environments. Thorough PCB cleaning is therefore

highly recommended in order to achieve maximum

performance by removing flux residuals from the board after

assembly. In general, reduction of losses in the oscillator

circuit leads to better safety margin and reliability.

Parameter

Symbol

Min

Typ

Max Units

Nominal Freq.

32.768

kHz

Series Resistance

ESR



Load Capacitance

7pF

1337G

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