ADSP-214xx SHARC Processor Hardware Reference 18-9
Real-Time Clock—ADSP-2147x
As a second example, if there is a +50 ppm error in the 1Hz frequency,
this also translates into 86400 × 50ppm seconds (=–4.32 seconds) error at
the end of the day which in this case the time has to be subtracted. This is
corrected in the RTC by counting 00:00:00 to 00:00:03 twice, so that the
time is effectively subtracted. As soon as the RTC reaches 00:00:04, the
error reduces to –0.32 seconds over the course of the previous day and
accumulated error is minimized.
When the RTC is powered up for the first time, the calibration values are
written once to ensure proper functionality. (If they are not to be used,
write 0000). These register bits are sticky, which means that once set, they
retain their value irrespective of the status of core supply.
The addition or subtraction of time can only be in integer multiples of
seconds. Zero to seven seconds can be added or subtracted using 4 bits.
The MSB indicates addition (0) or subtraction (1). The three LSBs indi-
cate number of seconds (0 – 7 represented by their binary 3 bit
equivalents). Because the clock runs at a time period of one second (@ 1
Hz frequency) 0.25 or 0.5 second resolution is not possible.
The calibration technique introduces a guard band for alarm by defini-
tion. In case the alarm is set within the duration of the time
(Day:00:00:00 to Day:00:00:06) corrected by the calibration register,
then it occurs at the nearest corrected time. This is shown in the following
two examples.
•If the
CALIB value in the RTC_INIT register is 0101, the RTC clock
jumps from 23:59:59 to 00:00:05 due to calibration. If the alarm is
set to 00:00:01, it occurs at the RTC time 00:00:05.
•If the
CALIB value in the RTC_INIT register is 1101, then the RTC
clock counts from 00:00:00 to 00:00:04 twice and then moves to
00:00:05. If the alarm is set to 00:00:01, it occurs at the RTC time
00:00:05.
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