PIC24FJ256DA210 Datasheet, PDF (294/408 Pages) Microchip Technology – 64/100-Pin, 16-Bit Flash Microcontrollers with Graphics Controller and USB On-The-Go (OTG)

PIC24FJ256DA210 FAMILY
20.2 Calibration
The real-time crystal input can be calibrated using the
periodic auto-adjust feature. When properly calibrated,
the RTCC can provide an error of less than 3 seconds
per month. This is accomplished by finding the number
of error clock pulses for one minute and storing the
value into the lower half of the RCFGCAL register. The
8-bit signed value loaded into the lower half of
RCFGCAL is multiplied by four and will either be added
or subtracted from the RTCC timer, once every minute.
Refer to the following steps for RTCC calibration:
1. Using another timer resource on the device, the
user must find the error of the 32.768 kHz
crystal.
2. Once the error is known, it must be converted to
the number of error clock pulses per minute and
loaded into the RCFGCAL register.
EQUATION 20-1: RTCC CALIBRATION
Error (clocks per minute) = (Ideal Frequency† –
Measured Frequency) x 60
†Ideal Frequency = 32,768H
3. a) If the oscillator is faster then ideal (negative
result form Step 2), the RCFGCAL register value
needs to be negative. This causes the specified
number of clock pulses to be subtracted from
the timer counter, once every minute.
b) If the oscillator is slower then ideal (positive
result from Step 2), the RCFGCAL register value
needs to be positive. This causes the specified
number of clock pulses to be added to the timer
counter, once every minute.
4. Divide the number of error clocks per minute by
4 to get the correct CAL value and load the
RCFGCAL register with the correct value.
(Each 1-bit increment in CAL adds or subtracts
4 pulses).
Writes to the lower half of the RCFGCAL register
should only occur when the timer is turned off or
immediately after the rising edge of the seconds pulse.
Note:
It is up to the user to include in the error
value the initial error of the crystal, drift
due to temperature and drift due to crystal
aging.
20.3 Alarm
• Configurable from half second to one year
• Enabled using the ALRMEN bit
(ALCFGRPT<15>, Register 20-3)
• One-time alarm and repeat alarm options
available
20.3.1 CONFIGURING THE ALARM
The alarm feature is enabled using the ALRMEN bit.
This bit is cleared when an alarm is issued. Writes to
ALRMVAL should only take place when ALRMEN = 0.
As shown in Figure 20-2, the interval selection of the
alarm is configured through the AMASK bits
(ALCFGRPT<13:10>). These bits determine which and
how many digits of the alarm must match the clock
value for the alarm to occur.
The alarm can also be configured to repeat based on a
preconfigured interval. The amount of times this
occurs, once the alarm is enabled, is stored in the
ARPT bits, ARPT<7:0> (ALCFGRPT<7:0>). When the
value of the ARPT bits equals 00h and the CHIME bit
(ALCFGRPT<14>) is cleared, the repeat function is
disabled and only a single alarm will occur. The alarm
can be repeated up to 255 times by loading
ARPT<7:0> with FFh.
After each alarm is issued, the value of the ARPT bits
is decremented by one. Once the value has reached
00h, the alarm will be issued one last time, after which
the ALRMEN bit will be cleared automatically and the
alarm will turn off.
Indefinite repetition of the alarm can occur if the CHIME
bit = 1. Instead of the alarm being disabled when the
value of the ARPT bits reaches 00h, it rolls over to FFh
and continues counting indefinitely while CHIME is set.
20.3.2 ALARM INTERRUPT
At every alarm event, an interrupt is generated. In addi-
tion, an alarm pulse output is provided that operates at
half the frequency of the alarm. This output is
completely synchronous to the RTCC clock and can be
used as a trigger clock to other peripherals.
Note:
Changing any of the registers, other then
the RCFGCAL and ALCFGRPT registers
and the CHIME bit while the alarm is
enabled (ALRMEN = 1), can result in a
false alarm event leading to a false alarm
interrupt. To avoid a false alarm event, the
timer and alarm values should only be
changed while the alarm is disabled
(ALRMEN = 0). It is recommended that the
ALCFGRPT register and CHIME bit be
changed when RTCSYNC = 0.
DS39969B-page 294
 2010 Microchip Technology Inc.