Datasheet

Figure 27-5. Calibration Circuitry Waveforms
Time
Monotonic 1 Hz
Counter value
32.768 kHz -50 ppm
32.768 kHz +50 ppm
Crystal frequency
remains unadjusted
Internal 1 Hz clock
is adjusted
Phase adjustment
(~4 ms)
User configurable period
(integer multiple of 1s or 20s)
Time
-50 ppm
-25 ppm
-50 ppm correction period
-25 ppm correction period
Nominal 32.768 kHz
Crystal clock
Internally divided clock (256 Hz)
Internally divided clock (128 Hz)
Clock pulse periodically suppressed
when correction period elapses
128 Hz clock edge delayed by 3.906 ms
when correction period elapses
Internally divided clock (256 Hz)
Internally divided clock (128 Hz)
Internally divided clock (64 Hz)
128 Hz clock edge delayed by 3.906 ms
when correction period elapses
Clock edge periodically added
when correction period elapses
1.000 second
1.003906 second
1.000 second
0.996094 second
NEGATIVE CORRECTION
POSITIVE CORRECTION
dashed lines = no correction
The inaccuracy of a crystal oscillator at typical room temperature (±20 ppm at 20–25 °C) 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_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.
SAM E70/S70/V70/V71 Family
Real-time Clock (RTC)
© 2019 Microchip T
echnology Inc.
Datasheet
DS60001527D-page 209