ISL12059IBZ【PDF 2/11 页】IC RTC/CALENDAR I2C-BUS 8-SOIC

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Sheet目录2001

1页 [2页]3页 4页 5页 6页

FN6757.0

June 15, 2009

Application Section

Oscillator Crystal Requirements

The ISL12059 uses a standard 32.768kHz crystal. Either

through hole or surface mount crystals can be used. Table 6

lists some recommended surface mount crystals and the

parameters of each. This list is not exhaustive and other

surface mount devices can be used with the ISL12059 if

their specifications are very similar to the devices listed.

The crystal should have a required parallel load capacitance

of 12.5pF and an equivalent series resistance of less than

50k. The crystal’s temperature range specification should

match the application. Many crystals are rated for -10°C to

+60°C (especially through-hole and tuning fork types), so an

appropriate crystal should be selected if extended

temperature range is required.

Layout Considerations

The crystal input at X1 has a very high impedance, and

oscillator circuits operating at low frequencies such as

32.768kHz are known to pick up noise very easily if layout

precautions are not followed. Most instances of erratic clocking

or large accuracy errors can be traced to the susceptibility of

the oscillator circuit to interference from adjacent high speed

clock or data lines. Careful layout of the RTC circuit will avoid

noise pickup and insure accurate clocking.

Figure 10 shows a suggested layout for the ISL12059 device

using a surface mount crystal. Two main precautions should

be followed:

Do not run the serial bus lines or any high speed logic lines

in the vicinity of the crystal. These logic level lines can

induce noise in the oscillator circuit to cause misclocking.

Add a ground trace around the crystal with one end

terminated at the chip ground. This will provide termination

for emitted noise in the vicinity of the RTC device.

In addition, it is a good idea to avoid a ground plane under

the X1 and X2 pins and the crystal, as this will affect the load

capacitance and therefore the oscillator accuracy of the

circuit. If the FT/OUT pin is used as a clock, it should be

routed away from the RTC device as well. The traces for the

VDD pins can be treated as a ground, and should be routed

around the crystal.

FIGURE 9. MULTIPLE BYTES READ SEQUENCE

SIGNALS

FROM THE

MASTER

SIGNALS FROM

THE SLAVE

SIGNAL AT

SDA

IDENTIFICATION

BYTE WITH

R/W = 0

ADDRESS

BYTE

IDENTIFICATION

BYTE WITH

R/W = 1

LAST READ

DATA BYTE

FIRST READ

DATA BYTE

1000

10 00

R/W BIT = “0”

R/W BIT = “1”

TABLE 3. SUGGESTED SURFACE MOUNT CRYSTALS

MANUFACTURER

PART NUMBER

Citizen

CM200S

MicroCrystal

MS3V

Raltron

RSM-200S

SaRonix

32S12

Ecliptek

ECPSM29T-32.768K

ECS

ECX-306

Fox

FSM-327

FIGURE 10. SUGGESTED LAYOUT FOR ISL12059 AND

ISL12059

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