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11100E–ATARM–24-Jul-13
16.5.7 RTC Accurate Clock Calibration
The crystal oscillator that drives the RTC may not be as accurate as expected mainly due to temperature variation. The
RTC is equipped with circuitry able to correct slow clock crystal drift.
RTC is equipped with circuitry able to correct slow clock crystal drift.
To compensate for possible temperature variations over time, this accurate clock calibration circuitry can be programmed
on-the-fly and also programmed during application manufacturing, in order to correct the crystal frequency accuracy at
room temperature (20-25°C). The typical clock drift range at room temperature is
on-the-fly and also programmed during application manufacturing, in order to correct the crystal frequency accuracy at
room temperature (20-25°C). The typical clock drift range at room temperature is
±
20 ppm.
In the device operating temperature range, the 32.768 kHz crystal oscillator clock inaccuracy can be up to -200 ppm.
The RTC clock calibration circuitry allows positive or negative correction in a range of 1.5 ppm to 1950 ppm. After
correction, the remaining crystal drift is as follows:
correction, the remaining crystal drift is as follows:
Below 1 ppm, for an initial crystal drift between 1.5 ppm up to 90 ppm
Below 2 ppm, for an initial crystal drift between 90 ppm up to 130 ppm
Below 5 ppm, for an initial crystal drift between 130 ppm up to 200 ppm
The calibration circuitry acts by slightly modifying the 1 Hz clock period from time to time. When the period is modified,
depending on the sign of the correction, the 1 Hz clock period increases or reduces by around 4 ms. The period interval
between 2 correction events is programmable in order to cover the possible crystal oscillator clock variations.
depending on the sign of the correction, the 1 Hz clock period increases or reduces by around 4 ms. The period interval
between 2 correction events is programmable in order to cover the possible crystal oscillator clock variations.
The inaccuracy of a crystal oscillator at typical room temperature (
±
20 ppm at 20-25 degrees Celsius) can be
compensated if a reference clock/signal is used to measure such inaccuracy. This kind of calibration operation can be set
up during the final product manufacturing by means of measurement equipment embedding such a reference clock. The
correction of value must be programmed into the RTC Mode Register (RTC_MR), and this value is kept as long as the
circuitry is powered (backup area). Removing the backup power supply cancels this calibration. This room temperature
calibration can be further processed by means of the networking capability of the target application.
up during the final product manufacturing by means of measurement equipment embedding such a reference clock. The
correction of value must be programmed into the RTC Mode Register (RTC_MR), and this value is kept as long as the
circuitry is powered (backup area). Removing the backup power supply cancels this calibration. This room temperature
calibration can be further processed by means of the networking capability of the target application.
To ease the comparison of the inherent crystal accuracy with the reference clock/signal during manufacturing, an internal
prescaled 32.768 kHz clock derivative signal can be assigned to drive RTC output. To accommodate the measure,
several clock frequencies can be selected among 1 Hz, 32 Hz, 64 Hz, 512 Hz.
prescaled 32.768 kHz clock derivative signal can be assigned to drive RTC output. To accommodate the measure,
several clock frequencies can be selected among 1 Hz, 32 Hz, 64 Hz, 512 Hz.
In any event, this adjustment does not take into account the temperature variation.
The frequency drift (up to -200 ppm) due to temperature variation can be compensated using a reference time if the
application can access such a reference. If a reference time cannot be used, a temperature sensor can be placed close
to the crystal oscillator in order to get the operating temperature of the crystal oscillator. Once obtained, the temperature
may be converted using a lookup table (describing the accuracy/temperature curve of the crystal oscillator used) and
RTC_MR configured accordingly. The calibration can be performed on-the-fly. This adjustment method is not based on a
measurement of the crystal frequency/drift and therefore can be improved by means of the networking capability of the
target application.
application can access such a reference. If a reference time cannot be used, a temperature sensor can be placed close
to the crystal oscillator in order to get the operating temperature of the crystal oscillator. Once obtained, the temperature
may be converted using a lookup table (describing the accuracy/temperature curve of the crystal oscillator used) and
RTC_MR configured accordingly. The calibration can be performed on-the-fly. This adjustment method is not based on a
measurement of the crystal frequency/drift and therefore can be improved by means of the networking capability of the
target application.
If no crystal frequency adjustment has been done during manufacturing, it is still possible to do it. In the case where a
reference time of the day can be obtained through LAN/WAN network, it is possible to calculate the drift of the application
crystal oscillator by comparing the values read on RTC Time Register (RTC_TIMR) and programming the HIGHPPM and
CORRECTION bitfields on RTC_MR according to the difference measured between the reference time and those of
RTC_TIMR.
reference time of the day can be obtained through LAN/WAN network, it is possible to calculate the drift of the application
crystal oscillator by comparing the values read on RTC Time Register (RTC_TIMR) and programming the HIGHPPM and
CORRECTION bitfields on RTC_MR according to the difference measured between the reference time and those of
RTC_TIMR.
16.5.8 Waveform Generation
Waveforms can be generated by the RTC in order to take advantage of the RTC inherent prescalers while the RTC is the
only powered circuitry (low power mode of operation, backup mode) or in any active modes. Going into backup or low
power operating modes does not affect the waveform generation outputs.
only powered circuitry (low power mode of operation, backup mode) or in any active modes. Going into backup or low
power operating modes does not affect the waveform generation outputs.
The RTC outputs (RTCOUT0 and RTCOUT1) have a source driver selected among 7 possibilities.
The first selection choice sticks the associated output at 0 (This is the reset value and it can be used at any time to
disable the waveform generation).
disable the waveform generation).
Selection choices 1 to 4 respectively select 1 Hz, 32 Hz, 64 Hz and 512 Hz.