The time seconds register and time prescaler register can be written only when SR[TCE]
is clear. Always write to the prescaler register before writing to the seconds register,
because the seconds register increments on the falling edge of bit 14 of the prescaler
register.
The time prescaler register increments provided SR[TCE] is set, SR[TIF] is clear,
SR[TOF] is clear, and the 32.768 kHz (or 1 kHz) clock source is present. After enabling
the oscillator, wait the oscillator startup time before setting SR[TCE] to allow time for
the oscillator clock output to stabilize.
If the time seconds register overflows then the SR[TOF] will set and the time prescaler
register will stop incrementing. Clear SR[TOF] by initializing the time seconds register.
The time seconds register and time prescaler register read as zero whenever SR[TOF] is
set.
SR[TIF] is set on POR and software reset and is cleared by initializing the time seconds
register. The time seconds register and time prescaler register read as zero whenever
SR[TIF] is set.
45.4.3 Compensation
The compensation logic provides an accurate and wide compensation range and can
correct errors as high as 3906 ppm and as low as 0.12 ppm. The compensation factor
must be calculated externally to the RTC and supplied by software to the compensation
register. The RTC itself does not calculate the amount of compensation that is required,
although the 1 Hz clock is output to an external pin in support of external calibration
logic.
Crystal compensation can be supported by using firmware and crystal characteristics to
determine the compensation amount. Temperature compensation can be supported by
firmware that periodically measures the external temperature via ADC and updates the
compensation register based on a look-up table that specifies the change in crystal
frequency over temperature.
The compensation logic alters the number of 32.768 kHz clock cycles it takes for the
prescaler register to overflow and increment the time seconds counter. The time
compensation value is used to adjust the number of clock cycles between -127 and +128.
Cycles are added or subtracted from the prescaler register when the prescaler register
equals 0x3FFF and then increments. The compensation interval is used to adjust the
frequency at which the time compensation value is used, that is, from once a second to
once every 256 seconds.
Functional description
Kinetis KE1xF Sub-Family Reference Manual, Rev. 4, 06/2019
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NXP Semiconductors
Содержание KE1xF Series
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