Xicor X1243 Real Time Clock Oscillator Requirements Introduction

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Xicor X1243 Real Time Clock Oscillator Requirements Introduction

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Xicor X1243 Real Time Clock Oscillator Requirements
Introduction
With Real Time Clock, there needs quartz crystal controlling oscillator frequency. This necessary, because variations even parts million oscillator frequency result clock that almost minute month. Typically this oscillator consists inverting amplifier 32.768 crystal network. constraints this oscillator circuit many. should: simple, oscillate desired frequency vary frequency over voltage temperature, readily oscillate application power operating voltage temperature. "clean" oscillation, with instabilities that interpreted extra clocks. consume much current. Often, meeting requirement makes harder meet another. This application note looks Xicor Real Time Clock oscillator dircuit examines network requirements maximum coverage desired features. varies, oscillator frequency varies. This relationship shown graphically Figure Problems arise when process variations component tolerances change capacitive loading, hence affecting frequency accuracy. Since this oscillator provides timebase real time clock counters, variations frequency translates into inaccuracies measurement real time. Xicor's first Real Time Clock makes external components allow adjustments greater accuracy.
Frequency Deviation (ppm)
Load Capacitance (pF)
Figure Frequency Accuracy Load Capacitance
Oscillator Accuracy
real time clock operates 32.768 kHz, because this frequency divided equals clock second. oscillator accuracy depends primarily providing proper capacitive load crystal oscillator element. typical quartz crystal oscillator CFS-206 from Citizen. This crystal expects 12.5 load operation exactly 32.768 kHz. load
Oscillator Network
Xicor Real Time Clock requires external resistors external capacitors addition crystal itself (see Figure resistors level feedback oscillator circuit. capacitors adjust loading crystal both stability oscillator proper loading crystal element.
March, 1999
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VBACK Lithuim
Figure Oscillator Network Power Supply
circuit Figure crystal sees loading capacitors series. This represented
-Ctot
This curve shows that changing temperature plus minus 20°C results reduction frequency crystal (and comparable error real time clock reading.) order "widen" temperature range over which there less than change frequency, values capacitors external network changed. example, capacitors chosen give error 25°C. This will effectively shift curve instead from 43°C, there accuracy from 50°C.
Oscillator characterization
Characterization Xicor Real Time Clock indicates that oscillator frequency changes over temperature with curve similar that crystal, though temperature impact great circuit with individual crystal. There also variation frequency that dependent operating voltage applied device. This variation much less significant than temperature linear with respect voltage. Characteristic curves over temperature (0°C 60°C) voltage shown Figure
where This works loading 12.7 Similar loading achieved with (12.31 (12.4 pF). 12.5 load capacitance yields expected error about
Operating Conditions
Xicor Real Time Clock functional over industrial temperature range (-40°C +85°C). crystal external network, however, typically operates over much narrower range. Seiko specifies that their crystal operates from -10°C +60°C. Also, temperature varies from 25°C, accuracy crystal degrades. Figure shows frequency-temperture curve Seiko VT-200.
Frequency Deviation (ppm)
2.7V 1.8V
Frequency Deviation (ppm)
Temperature (oC) Temperature (oC)
Figure Oscillator Frequency Temperature Voltage
Figure Crystal Frequency-Temperature Curve
alternative using external crystal network external temperature controlled crystal oscillator (TCXO) module. Typically this would used application that requires operation over indus-
March, 1999
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trial temperature range more accuracy over temperature. TCXOs highly accurate consume little current.
TCXO
real time clock circuits automatically connect backup supply source. common backup battery Lithium type that provides between Energizer CR2032 Lithium battery provides mAHr (from down would maintain time date Xicor cummulative years main power failure. Alternatively, supercapacitor used provide voltage clock during main power failures. supercap provide several days operation during power outage does pose environmental manufacturing problems lithium batteries. Since capacitor needs charged, external diode required circuit shown Figure
VBACK Supercap
Open
Figure Wiring TCX0
Current Consumption
current consumption real time clock circuit important consideration design system. Xicor Real Time Clock device itself consumes very little current. When driven external 32.768 oscillator draws less than from supply source. When using external network, however, current consumption somewhat higher. This current consumption greatly dependant power supply voltage temperature. curves Figure show typical current consumption over operating voltage.
Current Consumption (uA)
Figure supercap VBACK
1.8V 2.7V 3.6V
Supercaps available with capacities above. Using standard capacitor discharge equation, with following assumptions, Table shows maximum duration power outage that various supercaps maintain RTC. This capability often that required.
4.3V 1.8V 3.5µA (average current from 4.3V 1.8V)
Operating Voltage
Figure Current Consumption Operating Voltage (recommended external crystal network)
Table operating time using SuperCap
Capacitance Power Outage Duration days days days days Capacitor Physical Size[1] 12.5mm diameter, 5.5mm height 20mm diameter, height
typical application (see Figure main system supply connects primary (non-rechargeable) battery connects VBACK. When greater than VBACK, internal switch from battery open current flows battery. When fails,
0.22 0.47
Panasonic series Gold Capacitors
March, 1999
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Board Layout
While Xicor's Real TIme Clock less sensitive board layout than some clocks, there still reason cautious layout external components. problem exists because internal threshold levels input necessarily narrow. Noise therefore cause oscillations input comparator tries track noise. These oscillations perceived additional clocks, appears fast. While there many ways implement this layout, suggestion provided Figure This layout includes bypass capacitor from pull-up resistor (4.3 output. included pull resistors lines.
Summary
Xicor Real Time Clock integrates clock calendar, alarm, battery backup circuit, EEPROM some versions) Supervisor into single package, however primary role this device maintain real time. this, crystal controlled oscillator provides accurate timebase. using external components, this oscillator achieves better than seconds month accuracy over 50°C consumes little current clock operates years from single lithium battery.
X1243 SOIC
Layout
Component Placement
Figure Layout X1243
March, 1999
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Xicor X1243 Real Time Clock Oscillator Requirements Introduction

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