X1243 CFS-206 VT-200 CR2032 - Datasheet Archive

 

AN 121 Xicor X1243 Real Time Clock Oscillator Requirements By Carlos Martinez, March 1999 Introduction varies, the oscillator

 

Application Note AN 121 Xicor X1243 X1243 Real Time Clock Oscillator Requirements By Carlos Martinez, March 1999 Introduction varies, the oscillator frequency varies. This relationship is shown graphically in Figure 1. 1. be simple, 2. oscillate at the desired frequency and not vary in frequency over voltage or temperature, 3. readily oscillate on application of power at any operating voltage and temperature. 4. be a "clean" oscillation, with no instabilities that can be interpreted as extra clocks. 5. not consume much current. Often, meeting one requirement makes it harder to meet another. This application note looks at the Xicor Real Time Clock oscillator dircuit and examines the network requirements for maximum coverage of the desired features. Oscillator Accuracy 100 80 60 40 20 0 -20 -40 56 8 10 12 14 16 18 20 22 24 26 28 Load Capacitance (pF) Figure 1. Frequency Accuracy vs. Load Capacitance Oscillator Network The real time clock operates at 32.768 kHz, because this frequency divided by 215 equals one clock per second. The oscillator accuracy depends primarily on providing the proper capacitive load to the crystal oscillator element. A typical quartz crystal oscillator is the CFS-206 CFS-206 from Citizen. This crystal expects a 12.5 pF load for operation at exactly 32.768 kHz. If the load March, 1999 Problems arise when process variations or component tolerances change the capacitive loading, hence affecting frequency accuracy. Since this oscillator provides the timebase for the real time clock counters, any variations in frequency translates into inaccuracies in the measurement of the real time. Xicor's first Real Time Clock makes use of external components to allow adjustments for greater accuracy. Frequency Deviation (ppm) With any Real Time Clock, there needs to be a quartz crystal controlling the oscillator frequency. This is necessary, because variations of even 20 parts per million in the oscillator frequency result in a clock that is off by almost a minute a month. Typically this oscillator consists of an inverting amplifier and a 32.768 kHz crystal network. The constraints on this oscillator circuit are many. It should: The Xicor Real Time Clock requires the use of two external resistors and 2 external capacitors in addition to the crystal itself (see Figure 2) The resistors set the level of feedback in the oscillator circuit. The capacitors adjust the loading on the crystal for both stability of the oscillator and proper loading of the crystal element. www.xicor.com 1 of 4 Application Note 2.7 - 5.5 V VCC X1 10 M VBACK X2 3V Lithuim VSS 68 pF 360 K Figure 2. RTC Oscillator Network and Power Supply For the circuit of Figure 2, the crystal sees the loading of the two capacitors in series. This is represented as: 1 1 1 - = - + -Ctot C1 C2 RTC Oscillator characterization where C1 = 18 pF and C2 = 43 pF. This works out to a loading of 12.7 pF. Similar loading is achieved with C1 = 18 pF and C2 = 39 pF (12.31 pF) or C1 = 16 pF and C2 = 56 pF (12.4 pF). A 12.5 pF load capacitance yields an expected error of about 0 ppm Operating Conditions The Xicor Real Time Clock is functional over an industrial temperature range (-40°C to +85°C). The crystal in an external network, however, typically operates over a much narrower range. Seiko specifies that their crystal operates from -10°C to +60°C. Also, as the temperature varies from 25°C, the accuracy of the crystal degrades. Figure 3 shows a frequency-temperture curve for the Seiko VT-200 VT-200. 0 -10 Frequency Deviation (ppm) This curve shows that changing the temperature by plus or minus 20°C results in a 10 ppm reduction in the frequency of the crystal (and a comparable error in the real time clock reading.) In order to "widen" the temperature range over which there is less than 10 ppm change in frequency, the values of the capacitors in the external network can be changed. For example, the capacitors can be chosen to give a +10 ppm error at 25°C. This will effectively shift the curve up, so instead of ±10 ppm from 7°C to 43°C, there is an accuracy of ±10 ppm from 0°C to 50°C. Characterization of the Xicor Real Time Clock indicates that the oscillator frequency changes over temperature with a curve similar to that of the crystal, though the temperature impact is not as great in the circuit as with an individual crystal. There is also a variation in frequency that is dependent on the operating voltage applied to the device. This variation is much less significant than the temperature and is linear with respect to voltage. Characteristic curves over temperature (0°C to 60°C) and voltage are shown in Figure 4. 10 0 Frequency Deviation (ppm) 12 pF AN 121 -10 -20 5V 2.7V 1.8V -30 -40 -50 -20 -60 -30 -20 -40 -10 0 10 20 30 Temperature (oC) 40 50 60 Figure 4. RTC Oscillator Frequency vs. Temperature and Voltage -50 -60 -20 -10 0 10 20 30 Temperature (oC) 40 50 60 Figure 3. Crystal Frequency-Temperature Curve March, 1999 An alternative to using an external crystal and network is the use of an external temperature controlled crystal oscillator (TCXO) module. Typically this would be used in an application that requires operation over an indus- www.xicor.com 2 of 4 Application Note trial temperature range or more accuracy over temperature. TCXOs are highly accurate and consume little current. 2.7 - 5.5 V VCC TCXO Open X1 X2 VSS Figure 5. RTC Wiring TCX0 Current Consumption Current Consumption (uA) The current consumption of the real time clock circuit can be an important consideration in the design of a system. The Xicor Real Time Clock device itself consumes very little current. When driven by an external 32.768 kHz oscillator the RTC draws less than 1uA from the supply source. When using the external network, however, the current consumption is somewhat higher. This current consumption is greatly dependant on the power supply voltage and not by the temperature. The curves in Figure 6 show typical current consumption over operating voltage. 9.0 8.0 7.0 6.0 5.0 4.0 3.0 2.0 1.0 1.8V AN 121 the real time clock circuits automatically connect to the backup supply source. A common backup battery is a Lithium type that provides between 3V and 2V. An Energizer CR2032 CR2032 Lithium battery provides 225 mAHr (from 3V down to 2V) and would maintain the time and date in the Xicor RTC for a cummulative 10 years of main power failure. Alternatively, a supercapacitor can be used to provide voltage to the clock during main power failures. A supercap can provide several days of operation during a power outage and does not pose the environmental or manufacturing problems of lithium batteries. Since the capacitor needs to be charged, an external diode is required in the circuit as shown in Figure 7. 2.7 - 5.5 V VCC VBACK Supercap VSS Figure 7. Use of a supercap for VBACK Supercaps are available with capacities to 1 F and above. Using a standard capacitor discharge equation, with the following assumptions, Table 1 shows maximum duration of a power outage that various supercaps can maintain the RTC. This capability is often all that is required. dV i = C -dt dV = 4.3V to 1.8V i = 3.5µA (average current from 4.3V to 1.8V) 2.7V 3.6V 5V Operating Voltage Figure 6. Current Consumption vs. Operating Voltage (recommended external crystal network) Table 1: RTC operating time using a SuperCap Capacitance In a typical application (see Figure 2) the main system supply connects to VCC and a primary (non-rechargeable) battery connects to VBACK. When VCC is greater than VBACK, an internal switch from the battery is open and no current flows out of the battery. When VCC fails, March, 1999 Power Outage Duration Capacitor Physical Size[1] 12.5mm diameter, 5.5mm height 0.1 F 3.9 days 1.0 F www.xicor.com 1.8 days 0.47 F 1. 0.8 days 0.22 F 8.3 days 20mm diameter, 6mm height Panasonic SG series Gold Capacitors 3 of 4 Application Note AN 121 Board Layout Summary While Xicor's Real TIme Clock is less sensitive to board layout than some clocks, there is still reason to be cautious in the layout of the external components. The problem exists because of the internal threshold levels on the X1 input are necessarily narrow. Noise on the X1 pin can therefore cause oscillations as the input comparator tries to track the noise. These oscillations are perceived as additional clocks, so the RTC appears to run fast. While there are many ways to implement this layout, one suggestion is provided in Figure 8. This layout includes a bypass capacitor from VCC to VSS and a pull-up resistor (4.3 k) on the IRQ output. Not included are pull up resistors on the SDA and SCL lines. The Xicor Real Time Clock integrates a clock calendar, alarm, battery backup circuit, EEPROM and (in some versions) a CPU Supervisor into a single package, however the primary role of this device is to maintain the real time. To do this, a crystal controlled oscillator provides an accurate timebase. By using external components, this oscillator achieves better than ±30 seconds a month accuracy over 0°C to 50°C and consumes so little current the clock operates for up to 10 years from a single lithium battery. Vcc 12 pF 68 pF Vss 0.1 µF 4.3 K 10 M 360 K 1 X1243 X1243 SOIC Component Placement PCB Layout Figure 8. PCB Layout for X1243 X1243 March, 1999 www.xicor.com 4 of 4