Crystal Considerations with Da

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 (CL) greater than the RTC-specified load capacitance is used. The severity of the inaccuracy is dependent on the value of the CL. For example, using a crystal with a CL of 12pF on an RTC designed with a 6pF CL 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 VDD, 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 VDD.
   
   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 CL is less than the CL of the RTC. The severity of the inaccuracy is dependent on the value of the CL.
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 PC board 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 CH (clock halt) or EOSC (enable oscillator) bit has not been set or cleared, as required. Many Dallas 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 PC board. 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.