器件3D模型

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DS1337S+C01 开发手册 - Maxim Integrated（美信）

制造商:

Maxim Integrated（美信）

分类:

实时时钟芯片

封装:

SOIC-8

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3D模型

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封装尺寸在P3

技术参数、封装参数在P2

应用领域在P5

型号编号列表在P3

导航目录

封装尺寸在P3
技术参数、封装参数在P2
应用领域在P5
型号编号列表在P3

DS1337S+C01数据手册

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1. Noise coupling into the crystal from adjacent signals. This problem has been extensively covered above. Noise coupling usually causes an RTC to be grossly

inaccurate.

2. Wrong crystal. An RTC typically runs fast if a crystal with a specified load capacitance (C

) greater than the RTC-specified load capacitance is used. The severity of

the inaccuracy is dependent on the value of the C

. For example, using a crystal with a C

of 12pF on an RTC designed with a 6pF C

causes the RTC to be about 3

to 4 minutes per month fast.

Slow Clocks

The following are the most common scenarios that cause a crystal-based RTC to run slow.

1. Overshoots on RTC input pins. It is possible to cause a RTC to run slow by periodically stopping the oscillator. This can be inadvertently accomplished by noisy input

signals to the RTC. If an input signal rises to a voltage that is greater than a diode drop (~0.3V) above V

, the ESD protection diode for the input pin will forward

bias, allowing the substrate to be flooded with current. This, in turn, stops the oscillator until the input signal voltage decreases to below a diode drop above V

This mechanism can cause the oscillator to stop frequently if input signals are noisy. Therefore, care should be taken to ensure there is no overshoot on input signals.

Another situation that is common to overshoot problem is having an input to the RTC at 5V when the RTC is in battery-backup mode. This can be a problem in systems

that systematically shut down certain circuits but keep others powered up. It is very important to ensure there are no input signals to the RTC that are greater than the

battery voltage (unless stated otherwise in the device data sheet) when the device is in battery-backup mode.

2. Wrong crystal. A RTC typically runs slow if a crystal with a specified C

is less than the C

of the RTC. The severity of the inaccuracy is dependent on the value of

the C

3. Stray capacitance. Stray capacitance between the crystal pins and/or to ground can slow an RTC down. Therefore, care must be taken when designing the PCB layout

to ensure the stray capacitance is kept to a minimum.

4. Temperature. The further the operating temperature is from the crystal turnover temperature, the slower the crystal oscillates. See Figures 3 and 4.

Clock Does Not Run

The following are the most common scenarios that cause a RTC to not run.

1. The single most common problem when the clock does not run is that the clock halt (CH) or enable oscillator (EOSC) bit has not been set or cleared, as required.

Many Maxim RTCs include a circuit that keeps the oscillator from running when power is first applied. This allows a system to wait for shipment to the customer,

without drawing power from the backup battery. When the system is powered for the first time, the software/firmware must enable the oscillator and prompt the user for

the correct time and date.

2. Surface mount crystals may have some N.C. (no connect) pins. Make sure that the correct pins from the crystal are connected to the X1 and X2 pins.

Crystal Manufacturing Issues

Tuning fork crystals should not be exposed to ultrasonic cleaning. They are susceptible to damage from resonant vibration.

Crystals should not be exposed to temperatures above their maximum ratings. Exposure to excessive temperatures may damage the crystal, and usually increase the ESR.

Crystal "cans" should not be soldered to a PCB. This is sometimes done to ground the case of the crystal. Soldering directly to the case of the crystal usually subjects the

unit to excessive temperatures.

RTCs should generally be used in noncondensing environments. Moisture forming around the oscillator conductors can cause leakage, which can cause the oscillator to

stop. Conformal coatings can be used to protect the circuit, however, conformal coating may by itself cause problems.

Some conformal coatings, especially epoxy-based materials, can have unacceptable levels of ionic contamination. In addition, conformal coatings can, if the PC board

surface is not sufficiently cleaned prior to conformal coating, cause contaminants to concentrate around leads and traces.

Solder flux residue can cause leakage between pins. RTC oscillator circuits are especially sensitive to leakage because of their low-power operation. Leakage between the

oscillator input and output, or leakage to ground, often keep the oscillator from running.

References

1. John R. Vig, Quartz Crystal Resonators and Oscillators for Frequency Control and Timing Applications, www.rakon.com/

Advanced Crystal Technology, www.actcrystals.com/act/

3. Fox Electronics, www.foxonline.com/

4. ECS, Inc. International, www.ecsxtal.com/

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