real-time clock (RTC) precision requirement most embedded applications

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real-time clock (RTC) precision requirement most embedded applications

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STM32F101xx STM32F103xx calibration
real-time clock (RTC) precision requirement most embedded applications, external environment temperature change, frequency variation crystal that clocks precision accurate expected. embedded STM32F101xx STM32F103xx comes with digital clock calibration circuit suitable manufacturing environments, that allows applications compensate crystal temperature variations. This application note discusses calibration basics explains calibration used improve timekeeping accuracy.
August 2007
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Contents
AN2604 Application note
Contents
calibration basics
Crystal accuracy Methodology
Calculating needed amount calibration Calculating calibration over temperature range Conclusion Revision history
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AN2604 Application note
List tables
List tables
Table Table Calibration table: compensation values seconds month days) Document revision history
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List figures
AN2604 Application note
List figures
Figure Figure Figure Typical crystal accuracy plotted against temperature calibration clock output Crystal accuracy over temperature range
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AN2604 Application note
calibration basics
calibration basics
Crystal accuracy
term "quartz-accurate" become familiar phrase used describe accuracy many time keeping functions. Quartz oscillators provide accuracy superior that other conventional oscillator designs, they perfect. Quartz crystals sensitive temperature variations. Figure shows relationship between accuracy (acc), temperature curvature typical 32.768 crystal. curve follows general formula given below: where:
-0.040 ppm/°C2
Note:
variable crystal-dependent, value indicated here crystal mounted STM3210B-EVAL board. Refer crystal manufacturer more details this parameter. clocks used most applications require high degree accuracy, there several factors involved achieving this accuracy. Typically most crystals compensated adjusting load capacitance oscillator. This method, though effective, several disadvantages: requires external components (trim capacitors) increase oscillator current major factor battery-supported applications)
Instead this crude analog method, STM32F10xxx products digital calibration feature that gives user software control over calibration procedure, make userfriendly. Figure Typical crystal accuracy plotted against temperature
Temperature (°C)
Accuracy (ppm) -100
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calibration basics
AN2604 Application note
Methodology
STM32F10xxx products driven quartz crystal-controlled oscillator with nominal frequency 32.768 kHz. crystal oscillator most accurate circuits provide fixed frequency. There causes clock error: temperature variation crystal variation
mentioned previously, most clock chips compensate crystal frequency temperature variations using cumbersome trim capacitors. STM32F10xxx design employs periodic counter corrections. digital calibration circuit removes cycles every clock cycles (see Figure 2.). number times pulses blanked depends upon value that been loaded into seven least significant bits BKP's clock calibration register. Since clock calibration register backup domain, calibration value lost even device powered provided that battery connected VBAT pin. Figure calibration clock output
CAL[6:0]bits BKP_RTCCR register HSE/128 Oscillator RTCCLK
Clock calibration
ANTI_TAMP
Div64
output frequency test
Enabled BKP_RTCCR register
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clock output ANTI_TAMP clock before calibration, value changed calibration. Each calibration step effect subtracting oscillator cycle every (220)
actual oscillator cycles. That 0.954(1000000/220) adjustment calibration step calibration register. result, oscillator clock slowed down from ppm. Table page shows many seconds month days) each represents real time.
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AN2604 Application note Table
calibration basics
Calibration table: compensation values seconds month days)
Value seconds month days) rounded nearest second Value Calibration rounded value nearest Value seconds month days) rounded nearest second
Value Calibration rounded value nearest
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calibration basics Table
AN2604 Application note Calibration table: compensation values seconds month days) (continued)
Value seconds month days) rounded nearest second Value Calibration rounded value nearest Value seconds month days) rounded nearest second
Value Calibration rounded value nearest
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AN2604 Application note Table
calibration basics
Calibration table: compensation values seconds month days) (continued)
Value seconds month days) rounded nearest second Value Calibration rounded value nearest Value seconds month days) rounded nearest second
Value Calibration rounded value nearest
described above, STM32F10xxx clock calibration circuit subtracts cycles only from crystal clocks. based fact that prescaler value default 768, faster crystal frequencies calibrated whereas slower crystal frequencies cannot compensated for. only crystal frequencies range 772, 768] calibrated. Since crystal frequency vary about 32.768 kHz, solution considered that consists setting prescaler (instead 768). crystal frequency thus compared instead 768. this way, crystal frequency range 770, 766] compensated. Throughout rest document, considered prescaler value will 766.
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Calculating needed amount calibration
AN2604 Application note
Calculating needed amount calibration
establish much calibration required given application, method specially suited manufacturing environments retained. involves clock output mode, which derives signal from clock divider chain indicated Figure page This signal used measure accuracy crystal oscillator. This method divided into following steps: Enable speed external oscillator (LSE), select clock source, then enable clock. Enable clock output with frequency divided ANTI_TAMP crystal frequency measurement. This achieved setting BKP_RTCCR. Calculate crystal frequency deviation ppm. deviation quickly calculated dividing measured deviation from 511.968 511.968 and, multiplying result million. Find nearest calibration value using Table page This table direct look-up table calibration values based upon variation values expressed ppm. Load calibration value calibration register compensate crystal deviation.
Note:
prescaler 766, write into prescaler load register. example, frequency measured during test mode 511.982 delta 0.014. dividing 511.968 multiplying million, result 27.35 ppm. this case, nearest compensation value inaccuracy will reduced from 27.35 (~71 seconds month) 0.65 (~1.7 second month).
Note:
Since calibration based removing clock cycles, does improve counting over short periods time, only improves counting over long periods. example counting 1/100 using will more accurate without calibration than with calibration. Since calibration cycle removal occur during considered time frame, resulting value change significantly. depending application better calibration.
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AN2604 Application note
Calculating calibration over temperature range
Calculating calibration over temperature range
calibration procedure described aims calculating correction specific temperature. This section provides procedure minimizing frequency variation over wider temperature range. This involves adjusting frequency curve that there equal amount error above below zero point. Figure page shows frequency error minimized over given temperature range. variables equation: following:
(see Section page
Accuracy, ppm, frequency, turnover temperature Curvature characteristic -0.04 ppm/°C2 Turnover temperature degrees Celsius Working temperature degrees Celsius example, device shows deviation room temperature, operating temperature application, equation used calculate required calibration value follows: 27ppm 0.04ppm 18ppm Since accuracy deviation ppm, nearest calibration value indicated Table page Figure Crystal accuracy over temperature range
Temperature
Accuracy (ppm)
After calibration
Before calibration
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Conclusion
AN2604 Application note
Conclusion
STM32F10xxx digital clock calibration feature allows user adjust clock accuracy during manufacturing later) minimal cost. This feature also provides method whereby "drift" (due temperature variation) corrected and/or anticipated. However, method described this application note applicable only prescaler (instead 768).
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AN2604 Application note
Revision history
Revision history
Table
Date 31-Aug-2007
Document revision history
Revision Initial release. Changes
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AN2604 Application note
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real-time clock (RTC) precision requirement most embedded applications

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