Crystal Considerations with Da

Drive Level

DL

1

μW

Note 1: Some devices allow higher ESR values, check the datasheet for specific requirements.

Table 2. Crystal Suppliers, cylinder-type (ESR=45KΩ)

Manufacturer	Part	Frequency Tolerance (ppm)	ESR (KΩ)	Drive Level max (μW)	CL-pF	Alternate CL?	Temp Range (°C)	Surface or Thru-Hole	Package Dimensions (mm)	Manufacturer Ordering Number
Citizen	CFS-145	±20	40	1.0	8.0	yes	-10to+60	TH	1.5 x 5.1
Citizen	CFS-206	±20	35	1.0	12.5	yes	-10to+60	TH	2.1 x 6.2
Citizen	CMR-200T	±20	35	1.0	12.5 or 6.0	yes	-40to+85	SMT	2.0 X 6.0	CMR200TB32.768KDZFTR or CMR200TB32.768KDZBTR
ECS, Inc.	ECS-3X8	±20	35	1.0	12.5	?	-10to+60	TH	3.1 x 8.2
ECS, Inc.	ECS-2X6	±20	35	1.0	12.5	?	-10to+60	TH	2.1 x 6.2
ECS, Inc.	ECS-1X5	±20	35	1.0	8	?	-10to+60	TH	1.5 x 5.1
KDS/Daiwa	DT-26	±20 or ±30	40	1.0	12.5	yes	-10to+60	TH	2.0 x 6.0	1TB602G00
KDS/Daiwa	DT-38	±20 or ±30	30	1.0	12.5	yes	-10to+60	TH	3.0 x 8.0
Pletronics	WX15	±20	40	1.0	8.0	yes	-10to+60	TH	1.5 x5.1	WX15-32.768k-6pF
Pletronics	WX26	±20	40	1.0	12.5	6.0	-10to+60	TH	2.1 x 6.2	WX26-32.768k-6pF
Fox	NC-38		35	1.0	12.5	6.0	-20to+60	TH	3.0 x 8.3
Seiko	C-001R	±20	45	1.0	12.5	6	-10to+60	TH	3.1 x 8.0
Seiko	C-2	±20	35	1.0	12.5	6	-10to+60	TH	2.0 x 6.0

Note: Cylinder-type dimensions are barrel diameter and length, and exclude leads. All dimensions approximate.

Table 3. Crystal Suppliers, Surface Mount

Manufacturer	Part	Frequency Tolerance (ppm)	ESR (kΩ)	Drive Level max (μW)	CL-pF	Alternate CL?	Temp Range (°C)	Dimensions (mm) approximate, including leads
Seiko	SP-T3	±10, ±20	55	1.0	12.5	yes	-40 to +85	7.3x4.3.x1.8
Seiko	SP-T2	±20	50	1.0	12.5	yes	-40 to +85	8.7x3.7x2.5
EPSON	MC-306	±20	50	1.0	12.5	yes	-40 to +85	8.0x3.8x2.54
Citizen	CM200S	±20	50	1.0	12.5	yes	-40 to +85	8.0x3.8x2.5
KDS	DMX-26S	±30	50	1.0	12.5	yes	-40 to +85	8.0x3.8x2.4

Manufacturer Links

http://www.citizen.co.jp/tokuhan/quartz/Catalog1/Crystals.htm
http://www.eea.epson.com/
http://www.kdsj.co.jp/english.html
http://www.pletronics.com/XTAL.htm#32
http://www.foxonline.com/
http://www.ecsxtal.com/thrucrys.htm

Power Consumption

Many RTCs are designed to operate from a battery supply. In a typical application, a small lithium battery can be used to run the oscillator and clock circuitry while the main supply is off. To maximize battery life, the oscillator must run using as little power as possible. To accomplish this, some design tradeoffs must be made.

Negative Resistance

For typical high-frequency oscillator circuits, it is normal for the circuit to be designed with a 5 or 10X margin for the ESR. Low-frequency crystals typically have higher ESRs. An RTC oscillator may have less than a 2X margin for negative resistance. An oscillator circuit with a low margin normally consumes less current. As a result, an RTC oscillator often is sensitive to relatively small amounts of stray leakage, noise, or an increase in ESR.

The CL of the oscillator circuit influences the power consumption. An RTC with 12.5pF internal loads consumes more power than one that has 6pF loads. However, the oscillator with 12.5pF load capacitors is usually less susceptible to noise.

Crystal Layout Guidelines

Since the crystal inputs of Dallas RTCs have very high impedance (about 109Ω), the leads to the crystal act like very good antenna, coupling high-frequency signals from the rest of the system. If a signal is coupled onto the crystal pins, it can either cancel out or add pulses. Since most of the signals on a board are at a much higher frequency than the 32.768kHz crystal, it is more likely to add pulses where none are wanted. These noise pulses get counted as extra clock "ticks" and make the clock appear to run fast.

The following steps illustrate how to determine if noise is causing the RTC to run fast:

1. Power the system up and synchronize the RTC to a known accurate clock.
2. Turn