Datasheet

PIC24FJ64GA004 FAMILY
DS39881E-page 186  2010-2013 Microchip Technology Inc.
19.2 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 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 multi-
plied by four and will be either added or subtracted from
the RTCC timer, once every minute. Refer to the steps
below 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.
EQUATION 19-1:
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 subtracted from
the timer counter, once every minute.
4. Divide the number of error clocks per minute by
4 to get the correct CAL<7:0> bits value and
load the RCFGCAL register with the correct
value.
(Each 1-bit increment in the CALx bits value
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.
19.3 Alarm
• Configurable from half second to one year
• Enabled using the ALRMEN bit
(ALCFGRPT<15>, Register 19-3)
• One-time alarm and repeat alarm options are
available
19.3.1 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 19-2, the interval selection of the
alarm is configured through the AMASK<3:0> 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<7:0>
bits (ALCFGRPT<7:0>). When the value of the ARPTx
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 ARPTx 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 CHIME
(ALCFGRPT<14>) = 1. Instead of the alarm being dis-
abled when the value of the ARPTX bits reaches 00h, it
rolls over to FFh and continues counting indefinitely
while CHIME is set.
19.3.2 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.
(Ideal Frequency† – Measured Frequency) * 60 =
Clocks per Minute
† Ideal frequency = 32,768 Hz
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.
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.