AN11247 - NXP Semiconductors
Contents
1. 8 3 Trademarks 05 9 GOnte nts inonda eee ees Please be aware that important notices concerning this document and the product s described herein have been included in the section Legal information NXP B V 2012 All rights reserved For more information please visit http www nxp com For sales office addresses please send an email to salesaddresses nxp com Date of release 17 December 2012 Document identifier AN11247_12. OFFSET 6 0 0111111 0111110 0000010 0000001 0000000 1111111 1111110 1000001 1000000 Offset value in decimal 63 62 Offset value in ppm Normal mode 136 710 134 540 4 340 2 170 o 2 170 4 340 136 710 138 880 Course mode 273 420 269 080 8 680 4 340 o 4 340 8 680 273 420 277 760 1 Default value Initial calibration In order to achieve highest accuracy it is possible to tune the frequency before any temperature compensation is implemented Thus it is possible to compensate for variations in To and fog in order to ensure that the crystal runs at or near 32 768 kHz at room temperature The RTCs mentioned in this manual all have the option to output the buffered crystal frequency to the pin CLKOUT In order to output the buffered crystal frequency at output CLKOUT the CLKOUT signal has to be enabled and the CLKOUT frequency set to 32 768 kHz The frequency can now be tuned using the electronic tuning register 9 1F supercapacitor E It 100 nF Fig 2 Oscillator tuning CLKOE CLKOUT INT CE PCF2123 SCL SDI Vss 1 This typical application diagram shows CKLOUT enabled The buffered crystal frequency is available at pin CLKOUT 001aai557 All information provided in this document is subject to legal disclaimers NXP B V 2012 All rights reserved Appl
3. 1 fast correction Offset value Eppm 4 069 3 600 gt 4 correction pulses are needed 013aaa683 1 The numbers in this example are valid only for PCF85063 and PCF8523 Fig 3 Offset calibration calculation workflow AN11247_1 All information provided in this document is subject to legal disclaimers NXP B V 2012 All rights reserved Application note Rev 1 17 December 2012 10 of 16 NXP Semiconductors AN1 1 247 Improved timekeeping accuracy using an external temperature sensor 5 Procedure to correct time deviations due to temperature variations AN11247_1 5 1 5 2 In order to implement the temperature compensation algorithm described in this application note a microcontroller the system microcontroller as well as an external temperature sensor to measure the ambient temperature are required The temperature sensor must be located such that the measured temperature is a good representation of the crystal temperature How often the temperature needs to be measured depends on the application and is a parameter to be chosen by the system designer The microcontroller has now the additional task to initiate temperature measurement determine the new tuning register value and to set this register in the RTC The RTC needs to send an interrupt to the microcontroller on regular intervals telling it to measure the temperature If it is a low power application where the microcontroller may be in standby
4. BT T AN11247_1 All information provided in this document is subject to legal disclaimers NXP B V 2012 All rights reserved Application note Rev 1 17 December 2012 3 of 16 NXP Semiconductors AN1 1 247 AN11247_1 Improved timekeeping accuracy using an external temperature sensor Here f is the actual frequency fo is the frequency at room temperature B is the parabolic coefficient T is the temperature and To is the turnover temperature marked by the top of the parabolic curve Further fo can be considered to consist of two components as fo From fog Here from is the nominal frequency as specified and for the offset from this nominal frequency which is a result of production spread both at room temperature f fom fa Lan A a The frequency deviation f Soom in ppm can now be expressed as pe Leon A f 1 ae B T T 4 i Br anr nom nom l 20 T 001aag901 requency P ol deviation Af fo vais ppm t 20 en 60 100 140 180 40 30 20 10 0 10 20 30 40 50 60 70 80 90 T C Fig1 The deviation of frequency w r t temperature of a typical 32 768 kHz crystal In equation 1 there are three variables that influence the frequency response as a function of temperature These are the parabolic coefficient B the turnover temperature To and the frequency offset f 4 at room temperature The crys
5. above or below room temperature Two examples to get a better sense of the relationship between the real time and the deviation in ppm e A clock running 1 s per day too fast has an inaccuracy of 1 3600 24 11 57 ppm e A deviation of 1 s per week means 1 65 ppm 3 Possible solutions AN11247_1 A few but not many options are available to improve the accuracy Timekeeping accuracy can for example be improved through crystal screening integrated crystals keeping the crystal RTC at a well specified temperature or using a Temperature Compensated Crystal Oscillator TXCO like NXP s PCF2127 and PCF2129 Crystal screening means to select only those crystals for which the offset for falls within a narrower range than the normal production spread This would need to be done by the crystal manufacturer and limits the vertical shift of the parabolic curve within a narrower range Besides adding to the cost it does not alter the parabolic nature of the crystal s frequency curve and only provides a small accuracy improvement at room temperature Integrating the crystal in the same package as the RTC is a way of supplying RTC s of which the performance is known Usually a crystal screening will have taken place It is also possible to measure fo during production and correct for it by programming a dedicated register in the RTC if such a register is implemented It reduces the initial deviance at room temperature but as the previous o
6. an accurate initial tuning at the turn over temperature see section 4 3 This correction method uses no feedback so it is important to have a precise table or equation since any mistakes in this input will result in false compensation values Creating a lookup table or second order equation If the datasheet of the crystal used incorporates the parabolic curve and a table with these deviations this can be used under the condition that the resolution of the data is high enough Otherwise a high resolution frequency counter is necessary to measure and record how long a nominal 1s period out of the RTC actually takes when the crystal RTC combination is subject to varying temperatures In order to cover the whole temperature range for which the RTC is specified measurements should be done from 40 C to 85 C The oscillator frequency should not be measured by attaching the frequency meter to the OSCO pin which would add capacitance to this pin and thus detune the frequency with a Af In worst case the oscillator could even stop running After the data has been collected it can be put into an Excel spreadsheet from where the equation can be generated A less time consuming method is to use the second order equation and the value for B given in the crystal s datasheet The production spread of B for a certain type of crystal is small From equation 1 on page 4 the frequency deviation for a given temperature follows and with it the correction
7. 4 ppm and 4 069 ppm are based on a nominal 32 768 kHz clock The offset value is coded in two s complement giving a range of 63 LSB to 64 LSB Table 2 Offset values in period time not frequency for PCF85063 and PCF8523 OFFSET 6 0 Offset value in decimal Offset value in ppm Normal mode Course mode 0111111 63 273 420 256 347 0111110 62 269 080 252 278 0000010 2 8 680 8 138 0000001 1 4 340 4 069 0000000 0 o of 1111111 1 4 340 4 069 1111110 2 8 680 8 138 1000001 63 273 420 256 347 1000000 64 277 760 260 416 1 Default value The correction is made by adding or subtracting clock correction pulses thereby changing the period of a single second but not by changing the oscillator frequency By adding clock pulses the RTC is sped up the crystal curve moves up By subtracting clock pulses the time slows down Thus adding or subtracting clock pulses results in moving the curve in Fig 1 up or down in order to approach ideally 0 ppm accuracy at the given temperature The offset values for PCF2123 are different and are given in Table 3 All information provided in this document is subject to legal disclaimers NXP B V 2012 All rights reserved Application note Rev 1 17 December 2012 8 of 16 NXP Semiconductors AN11247 AN11247_1 4 3 Improved timekeeping accuracy using an external temperature sensor Table 3 Offset values in period time not frequency for PCF2123
8. 5V 1 8 V 5 5V 1 8 V 5 5 V 1 6 V 5 5 V 1 6 V 5 5V Clock operating voltage 0 9V 5 5V 0 9V 5 5V 0 9V 6 0V 1 1V 55V 1 0V 5 5V Typical current consumption 270 nA 270 nA 270 nA 110 nA 150 nA at Voo 3 V Operating temperature 40 C to 85 C 40 C to 125 C 40 C to 85 C 40 C to 85 C 40 C to 85 C range Packages HWSONS8 HXSON10 HXSON10 U HVQFN16 U HVSONS TSSOP14 S08 TSSOP 14 1 Naked die AN11247_1 All information provided in this document is subject to legal disclaimers NXP B V 2012 All rights reserved Application note Rev 1 17 December 2012 7 of 16 NXP Semiconductors AN1 1 247 AN11247_1 4 2 Improved timekeeping accuracy using an external temperature sensor For more detailed information about these devices register structure and how to set them please refer to the respective datasheets which are available at www nxp com Offset register The PCF85063 PCF8523 and PCF2123 incorporate an offset register which can be used to implement several functions such as e Accuracy tuning e Temperature compensation e Ageing adjustment The offset is made once every two hours in the normal mode all types or more frequent in the course mode In course mode the offset is made for the e PCF85063 every four minutes e PCF8523 once per minute e PCF2123 once per hour For PCF85063 and PCF8523 each LSB will introduce an offset of 4 34 ppm for the normal mode and 4 069 ppm for the course mode The values of 4 3
9. AN11247 Improved timekeeping accuracy with PCF85063 PCF8523 and PCF2123 using an external temperature sensor Rev 1 17 December 2012 Application note Document information Info Content Keywords PCF85063 PCF8523 PCF2123 RTC accuracy timekeeping temperature sensor temperature compensation crystal 32 768 kHz Abstract The temperature dependent characteristics of quartz crystals prevent time keeping also with state of the art real time clocks from being highly accurate over a wide temperature range unless corrective measures are implemented This application note describes how the use of an external temperature sensor placed in a location which will be at or near the same temperature as the quartz crystal attached to the RTC can improve accuracy considerably If the application is already using a temperature sensor for some reason only some extra firmware is needed without adding to the Bill Of Material BOM NXP Semiconductors AN1 1 247 Improved timekeeping accuracy using an external temperature sensor Revision history Rev Date Description 01 20121217 Initial version first release Contact information For additional information please visit http www nxp com For sales office addresses please send an email to salesaddresses nxp com AN11247_1 All information provided in this document is subject to legal disclaimers NXP B V 2012 All rights reserved Application note Rev 1 17 Decem
10. All rights reserved Application note Rev 1 17 December 2012 14 of 16 NXP Semiconductors AN11247 Improved timekeeping accuracy using an external temperature sensor 8 Legal information 8 1 Definitions Draft The document is a draft version only The content is still under internal review and subject to formal approval which may result in modifications or additions NXP Semiconductors does not give any representations or warranties as to the accuracy or completeness of information included herein and shall have no liability for the consequences of use of such information 8 2 Disclaimers Limited warranty and liability Information in this document is believed to be accurate and reliable However NXP Semiconductors does not give any representations or warranties expressed or implied as to the accuracy or completeness of such information and shall have no liability for the consequences of use of such information In no event shall NXP Semiconductors be liable for any indirect incidental punitive special or consequential damages including without limitation lost profits lost savings business interruption costs related to the removal or replacement of any products or rework charges whether or not such damages are based on tort including negligence warranty breach of contract or any other legal theory Notwithstanding any damages that customer might incur for any reason whatsoever NXP Semico
11. ber 2012 2 of 16 NXP Semiconductors AN1 1 247 Improved timekeeping accuracy using an external temperature sensor 1 Introduction Real time Clocks RTC have been around for a long time and many electronic systems include one for functions like keeping calendar time tariff switching time stamping or waking up a system periodically to initiate certain actions for example doing some measurements Not all applications need the same level of accuracy but when high accuracy is necessary like in applications that rely on real time data where accuracy better than 5 ppm is needed a standard RTC does not fulfil these Time keeping can only be as accurate as its reference Several environmental factors like humidity vibration and pressure can influence RTC accuracy but it is mainly the inferior characteristics of a quartz crystal over temperature which result in deviations if temperature is changing Several techniques have been used to improve the timekeeping accuracy achieved using 32 768 kHz crystals This application note describes how the use of an external temperature sensor placed in a location which will be at or near the same temperature as the RTC and its crystal can improve accuracy considerably If the application is already using a temperature sensor only some extra firmware is needed without adding to the Bill Of Material BOM Implementing temperature compensation using this method is highly simplified if the RTC has an elec
12. ect to export control regulations Export might require a prior authorization from competent authorities 8 3 Trademarks Notice All referenced brands product names service names and trademarks are property of their respective owners NXP B V 2012 All rights reserved Application note Rev 1 17 December 2012 15 of 16 NXP Semiconductors AN11247 Improved timekeeping accuracy using an external temperature sensor 9 Contents 1 With ODUCT ON acess cee ieiis sa catedeecesteed can cesteeseeetecteaceaien 3 2 The issue when using an RTC without temperature Compensation ccceeseeeeeeeees 3 3 Possible Solutions c cccceesseeeeeeeeeeeeseeeees 5 4 The RTCs with a tuning register 4 1 General Description 4 2 Offset register 4 3 Initial calibration cccceceeeeeeeeeceeeeeeeteeseaeees 5 Procedure to correct time deviations due to temperature variations cccssseeeeeeeeeeees 11 5 1 Step 1 Setting the timer etree 11 5 2 Step 2 Find the time deviation ee 11 5 2 1 Creating a lookup table or second order equation EP E EA iis Moeus E A E 12 5 2 2 Example ienaa eoan eaea aa 12 5 3 Step 3 update the electronic tuning register 13 6 NXP Demonstration board OM6297 14 7 FROTCNONGCES 22 fos secede cnt feat date es case tn seni cgieee ness 8 Legal information 8 1 Definitions 0 06 8 2 Disclaimers
13. fect of flattening the parabolic curve ideally to a perfectly straight horizontal line By using an off the shelf TCXO a high accuracy can be achieved without any development or calibration effort Temperature compensation always implies measuring the temperature at which the crystal operates at regular intervals Then according to the measured temperature some action needs to be taken like adjustment of the crystal loading to the clock source as implemented in conventional TCXOs or using an algorithm which reads and updates registers in the RTC The latter is the approach used in this application note for the NXP Real Time Clocks with an electronic tuning register PCF85063 PCF8523 and PCF2123 4 The RTCs with a tuning register AN11247_1 4 1 Since this application note deals specifically with the NXP RTCs which contain an electronic tuning or offset register PCF85063 PCF8523 PCF2123 below a short introduction to these devices is given General Description The PCF85063 PCF8523 and PCF2123 are CMOS Real Time Clock calendars optimized for low power consumption They contain a set of 8 bit registers with an auto incrementing address register an on chip 32 768 kHz oscillator with two integrated oscillator capacitors a frequency divider which provides the source clock for the Real Time Clock calendar RTC a programmable clock output and optionally a timer and an alarm The Real Time Clock family PCF85063 three versions i
14. ication note Rev 1 17 December 2012 9 of 16 NXP Semiconductors AN1 1 247 Improved timekeeping accuracy using an external temperature sensor If the required accuracy is within 1 second per day a frequency counter with a resolution of at least 8 digits and an accuracy of 1 ppm is required The accuracy of 1 ppm equals 32 768 mHz and thus if the clock is running at a nominal 32 768 kHz e 1 ppm 32768 0327 Hz e 1 ppm 32767 9673 Hz 1 day has 86400 s 1s day 11 6 ppm The oscillator should be tuned while the application is at the turnover temperature T of the crystal The recommended workflow is given in Fig 3 In that example the clock runs by 14 6484 ppm too fast A negative correction would maybe be expected but the offset values in Table 2 and Table 3 refer to the period time instead of to the frequency An increased oscillator frequency causes a shortened period time Therefore the period time must be extended This results in the positive correction pulse values Example Measure the frequency on pin CLKOUT fmeas 32768 48 Hz Convert to time tmeas 1 fmeas 30 5171 us Calculate the difference to the ideal period of 1 32768 00 Dmeas 1 32768 tmeas 0 000447 us Calculate the ppm deviation compared to the measured value Eppm 1000000 x Dmeas tmeas 14 6484 ppm Calculate the offset register value Mode 0 low power Offset value Eppm 4 34 3 375 gt 3 correction pulses are needed Mode
15. mode 0 035 25 50 87 50 20 0 035 20 45 70 88 16 0 035 15 40 56 00 13 0 035 10 35 42 88 10 0 035 5 30 31 50 7 0 035 0 25 21 88 5 0 035 5 20 14 00 3 0 035 10 15 7 88 2 0 035 15 10 3 50 1 0 035 20 5 0 88 0 0 035 25 0 0 00 0 0 035 30 5 0 88 0 0 035 35 10 3 50 1 0 035 40 15 7 88 2 0 035 45 20 14 00 3 0 035 50 25 21 88 5 0 035 55 30 31 50 7 0 035 60 35 42 88 10 0 035 65 40 56 00 13 0 035 70 45 70 88 16 0 035 75 50 87 50 20 0 035 80 55 105 88 24 0 035 85 60 126 00 29 5 3 Step 3 update the electronic tuning register In the previous step the required setting for the electronic tuning register programmable offset register for compensating varying temperatures was found This value must not directly be written in the register because in section 4 3 Initial calibration the correct clock speed for the turnover temperature was set Directly writing into the offset register would overwrite this setting Therefore the value found in the look up table must be added to the already programmed initial setting and this value must be written into the offset register AN11247_1 All information provided in this document is subject to legal disclaimers NXP B V 2012 All rights reserved Application note Rev 1 17 December 2012 13 of 16 NXP Semiconductors AN1 1 247 Improved timekeeping accuracy using an external temperature sensor 6 NXP Demonst
16. nductors aggregate and cumulative liability towards customer for the products described herein shall be limited in accordance with the Terms and conditions of commercial sale of NXP Semiconductors Right to make changes NXP Semiconductors reserves the right to make changes to information published in this document including without limitation specifications and product descriptions at any time and without notice This document supersedes and replaces all information supplied prior to the publication hereof Suitability for use NXP Semiconductors products are not designed authorized or warranted to be suitable for use in life support life critical or safety critical systems or equipment nor in applications where failure or malfunction of an NXP Semiconductors product can reasonably be expected to result in personal injury death or severe property or environmental damage NXP Semiconductors accepts no liability for inclusion and or use of NXP Semiconductors products in such equipment or applications and therefore such inclusion and or use is at the customer s own risk AN11247_1 All information provided in this document is subject to legal disclaimers Applications Applications that are described herein for any of these products are for illustrative purposes only NXP Semiconductors makes no representation or warranty that such applications will be suitable for the specified use without further testing or modification Cus
17. ption it does nothing to alleviate the frequency deviation as a result of changing temperatures Keeping the crystal within a narrow temperature range can be done by mounting the crystal in a temperature controlled container but this will add considerable cost and complexity to the system Another option is to use a 32 768 kHz temperature compensated crystal oscillator TCXO as the clock source for a stand alone RTC but obviously this will add cost too The principle on which TCXOs are based is simple Every crystal is optimized for a particular load capacitance The value of this capacitance is included in the datasheet and deviations from this value will result in a shift in frequency of the oscillator A TCXO uses this characteristic to implement a compensation mechanism It includes a temperature sensor which measures the temperature at certain intervals The device includes a lookup table and the temperature measurements in combination with the All information provided in this document is subject to legal disclaimers NXP B V 2012 All rights reserved Application note Rev 1 17 December 2012 5 of 16 NXP Semiconductors AN1 1 247 Improved timekeeping accuracy using an external temperature sensor lookup table result in an output signal to apply a load capacitance value for the integrated 32 768 kHz crystal to achieve high accuracy Changing the load capacitance influences directly the oscillator frequency It has the ef
18. r low power extremely low extremely low consumption tiny consumption tiny consumption tiny power power package package package consumption consumption battery back up Type of interface C PC SPI SPI C Max interface bus speed 400 kHz 400 kHz 8 MHz 6 25 MHz 1 MHz RAM 1 byte 1 byte 1 byte no no Year leap year tracking yes yes yes yes yes yes yes yes yes yes Year counter 2 digit 2 digit 2 digit 2 digit 2 digit 99 years 99 years 99 years 99 years 99 years Electronic tuning register yes yes yes yes yes Programmable alarm and no yes yes yes yes count down timer functions Oscillator stop detector yes yes yes yes yes Battery back up switch no no no no yes CLKOUT output push pull push pull push pull open drain open drain with CLKOE with CLKOE with CLKOE Programmable CLKOUT 32 768 kHz 32 768 kHz 32 768 kHz 32 768 kHz 32 768 kHz frequencies 16 384 kHz 16 384 kHz 16 384 kHz 16 384 kHz 16 384 kHz 8 192 kHz 8 192 kHz 8 192 kHz 8 192 kHz 8 192 kHz 4 096 kHz 4 096 kHz 4 096 kHz 4 096 kHz 4 096 kHz 2 048 kHz 2 048 kHz 2 048 kHz 2 048 kHz 1 024 kHz 32 Hz 1 024 kHz 1 024 kHz 1 024 kHz 1 024 kHz 1 Hz off 1 Hz off 1 Hz off 1 Hz off 1 Hz off CLKOUT Hi Z CLKOUT LOW CLKOUT LOW CLKOUT Hi Z CLKOUT Hi Z 2 Integrated oscillator forCL 7pFor forCL 7pFor forC_ 7pFor forC _ 7pF for CL 7 pF or capacitors C 12 5 pF C 12 5 pF C 12 5 pF C 12 5 pF Supply voltage range 1 8 V 5
19. r_CLKOUT_ctrl at address OFh The generation of interrupts is controlled via bit CTBIE register Control_2 and should be set to enable Timer B Register Tmr_B_reg 13h would have to be loaded with 5 assuming a source clock frequency of 1 60 Hz which results in a clock giving one pulse per minute This can be achieved by setting the bits TBQ 2 0 in register Tmr_B_freq_ctrl at address 12h to 011 This is just an example other countdown frequencies can be selected as well as other time intervals Refer to the datasheets for the control register settings for each type Step 2 Find the time deviation In order to get the time deviation that must be compensated for a given temperature a lookup table or a second order equation parabola derived from measurements done during the development phase can be used This table equation should contain the deviation in ppm relative to the nominal conditions and is specific for the crystal used If the type of crystal is changed a new lookup table equation is necessary This can All information provided in this document is subject to legal disclaimers NXP B V 2012 All rights reserved Application note Rev 1 17 December 2012 11 of 16 NXP Semiconductors AN1 1 247 5 2 1 5 2 2 Improved timekeeping accuracy using an external temperature sensor require quite some effort Furthermore no compensation for the quartz crystal aging is included For best results it is important to have
20. ration board OM6297 A fast way to start developing an algorithm and test its working is possible by using the NXP demonstration board OM6297 This board was designed to illustrate the operation of an ultra low power SPI based real time clock PCF2123 along with an industry standard 8 character alphanumeric LCD and an LM75B temperature sensor The microcontroller chosen for this application is the NXP P89LPC932A1 which is a single chip microcontroller available in various packages In the RTC demo board the PLCC28 package is used The LPC932 is based on a high performance architecture which works at up to six times the rate of standard 80C51 devices The LPC932 has many integrated functions to reduce component count board space and system cost It offers 8KB of on chip flash code memory 768 bytes of data RAM along with 512 bytes of data EEPROM storage and supports hardware I2C bus and SPl bus for communication The LPC932 supports multiple additional I O channels counters timers PWM s and analog comparators Please see the datasheet for full details The available temperature sensor is the NXP LM75BD in a standard SO8 package The LM75B is a pin for pin replacement for the industry standard LM75 and LM75A with an improved temperature resolution of 0 125 C It has a wide supply range and an I2C bus interface Temperatures can range from 55 C to 125 C with programmable temperature thresholds The LM75B is ideal for low power design
21. s as it only uses a current of 1 0 uA in standby mode With both the RTC and temperature sensor onboard it is easy to start developing an application and implementing the algorithm as described here The LCD driver chosen is the NXP PCF8562 in a plastic 48 pin TSSOP package The PCF8562 has a wide supply voltage range an I2C bus interface for commands and control and can drive up to 128 segments using a 1 4 multiplex rate The board also contains a 10 pin header that allows the LPC932A1 to be reprogrammed using serial ICP In Circuit Programming from a PC A simple ICP programmer called USB ICP is available from Future Designs Inc to allow any PC with a USB port to easily program the LCD demo board The USBICP also supports other NXP microcontrollers that use standard ICP programming Please consult FDI s website for more details on USB ICP 7 References AN11247_1 The documents listed below provide further useful information They are available at NXP s website www nxp com a Product data sheets PCF85063 b Product data sheet PCF8523 c Product data sheet PCF2123 d AN10652 Improved timekeeping accuracy with PCF8563 using external temperature sensor Rev 1 2 November 2007 UM10301 User Manual for NXP Real Time Clocks Rev 1 23 December 2008 f UM10204 C bus specification and user manual Rev 4 13 February 2012 D All information provided in this document is subject to legal disclaimers NXP B V 2012
22. s optimized for applications where very little space is available but still contains the key features expected from a state of the art time reference Typical applications are digital cameras portable handheld gaming consoles self care medical devices and printers faxes The PCF85063TP tracks time and date Aside from electronic tuning a frequency output and an interrupt on every 30 s or 60 s can be enabled It interfaces via a Fast mode Pe bus The PCF85063ATL and PCF85063BTL feature in addition an alarm facility and a programmable countdown timer controlled via either C or SPl bus The PCF 2123 is an ultra low power Real Time Clock featuring the above mentioned features It communicates via the SPl bus with a maximum data rate of 6 25 Mbit s The PCF8523 is an ultra low power Real Time Clock which has in addition to the PCF2123 an integrated battery back up circuit It communicates via the I C bus Table 1 shows a quick comparison between the various types All information provided in this document is subject to legal disclaimers NXP B V 2012 All rights reserved Application note Rev 1 17 December 2012 6 of 16 NXP Semiconductors AN1 1 247 Improved timekeeping accuracy using an external temperature sensor Table 1 Comparison of target real time clocks Features PCF85063 family tiny low power Ultra low power PCF85063TP PCF85063ATL PCF85063BTL PCF2123 PCF8523 Unique features low power low powe
23. tal manufacturer specifies these parameters and typical values are Tp 25 C AT 5 C and fof 30 ppm The coefficient B has a very small spread for various crystals of the same type but it has the largest effect on the shape of the parabolic curve of the frequency deviation as a function All information provided in this document is subject to legal disclaimers NXP B V 2012 All rights reserved Application note Rev 1 17 December 2012 4 of 16 NXP Semiconductors AN1 1 247 Improved timekeeping accuracy using an external temperature sensor of temperature Variation in the turnover temperature To will shift the deviation curve left or right variation in the offset at room temperature will shift it up or down In practice the combination of variation in Tp and offset at room temperature easily causes an in accuracy of 30 ppm at room temperature which corresponds to a time deviation of about 15 minutes per year In addition there is the effect of temperature Typical values for B range from 0 035 ppm C to 0 04 ppm C In a real application and using B 0 04 means that a clock built using a regular 32 768 kHz tuning fork crystal will at room temperature only have the frequency deviation resulting from the variations in Tp and fy However it will lose an additional two minutes per year at 10 degrees Celsius above or below room temperature and will lose an additional eight minutes per year at 20 degrees Celsius
24. tomers are responsible for the design and operation of their applications and products using NXP Semiconductors products and NXP Semiconductors accepts no liability for any assistance with applications or customer product design It is customer s sole responsibility to determine whether the NXP Semiconductors product is suitable and fit for the customer s applications and products planned as well as for the planned application and use of customer s third party customer s Customers should provide appropriate design and operating safeguards to minimize the risks associated with their applications and products NXP Semiconductors does not accept any liability related to any default damage costs or problem which is based on any weakness or default in the customer s applications or products or the application or use by customer s third party customer s Customer is responsible for doing all necessary testing for the customer s applications and products using NXP Semiconductors products in order to avoid a default of the applications and the products or of the application or use by customer s third party customer s NXP does not accept any liability in this respect Translations A non English translated version of a document is for reference only The English version shall prevail in case of any discrepancy between the translated and English versions Export control This document as well as the item s described herein may be subj
25. tronic tuning register Such a register allows influencing the clock speed by adding or subtracting counts from the oscillator divider chain This changes the duration of the period of a single second without changing the oscillator frequency The NXP RTCs PCF85063 PCF8563 and PCF2123 offer this functionality 2 The issue when using an RTC without temperature compensation Time keeping can sometimes be done using the built in oscillator in the system microcontroller In many situations though using an RTC is unavoidable Using a stand alone RTC has several benefits e Lower power consumption e Frees the main system for time critical tasks e Higher timekeeping accuracy However even when using an RTC the time keeping can only be as accurate as the reference used A crystal s frequency characteristic depends on the shape or cut of the crystal A manufacturer can control the crystal turnover frequency by the angle at which the crystal is cut However having to manufacture crystals with different cutting angles adds complexity and cost A tuning fork crystal is usually cut in such a way that its frequency over temperature is a parabolic curve centered around 25 C see Fig 1 This means that a tuning fork crystal oscillator will resonate close to its target frequency at room temperature but will slow down when the temperature either increases or decreases The frequency of a typical crystal at a specific temperature T is given by f fll
26. value can be calculated The microcontroller can calculate the deviation between the results of this equation and the temperature measured Alternatively the designer can create a lookup table from which the deviations are read Example Below such a lookup table is given for a crystal with B 0 035 ppm C and To 25 C for temperatures from 40 C to 85 C in steps of 5 C Tolerances in To and fo have not been taken into account and thus the equation used to calculate Af f in Table 4 reduces to equation 2 below For the lookup table only the columns Temperature T and Offset value in decimal are relevant ap Jom f 7 2 Table 4 Example lookup table only valid for PCF85063 and PCF8523 Only the columns Temperature T and Offset value in decimal need to be included Parabolic Temperature T Temperature Deviation Offset value in coefficient B offset Aff decimal ppm C7 C C ppm normal mode 0 035 40 65 147 88 34 0 035 35 60 126 00 29 0 035 30 55 105 88 24 AN11247_1 All information provided in this document is subject to legal disclaimers NXP B V 2012 All rights reserved Application note Rev 1 17 December 2012 12 of 16 NXP Semiconductors AN1 1 247 Improved timekeeping accuracy using an external temperature sensor Parabolic Temperature T Temperature Deviation Offset value in coefficient B offset Aff decimal ppm C 7 C C ppm normal
27. waking up the microcontroller and the temperature sensor obviously will increase power consumption Thus how often this is done will be a compromise between accuracy requirements and power consumption The interval should be chosen such that the temperature is not expected to change much during the chosen timeframe A value could be once per minute or once per five minutes In order to implement all steps correctly details that have to be taken into account for the practical implementation are described in sections 5 1 until 5 3 Step 1 Setting the timer The first step is to decide at which intervals the microcontroller should measure the temperature and set the timer accordingly The RTCs described here have a freely programmable 8 bit countdown timer which can be used for this except PCF85063TP which only offers the choice of a 30 s and a 1 minute interrupt The timer control registers of the other RTCs determine the source clock frequency for the countdown timer and enable or disable the timer The timer counts down from a software loaded 8 bit binary value At the end of every countdown the timer sets the Timer Flag TF The bit TF may only be cleared by software The asserted TF can be used to generate an interrupt INT Suppose that the RTC used is the PCF8523 and an interrupt to the microcontroller needs to be generated every 5 minutes Further suppose that Timer B is used This timer needs to be switched on by bit TBC in register Tm
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