Microchip Technology ARD00385 Data Sheet

DS39957D-page 232

 2009-2011 Microchip Technology Inc.

17.2.9

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 three
seconds per month.
To perform this calibration, find the number of error
clock pulses and store the value into the lower half of
the RTCCAL register. The 8-bit, signed value, loaded
into RTCCAL, is multiplied by four and will be either
added or subtracted from the RTCC timer, once every
minute.
To calibrate the RTCC module:
1.

Use another timer resource on the device to find
the error of the 32.768 kHz crystal.

2.

Convert the number of error clock pulses per
minute (see 

• If the oscillator is faster than ideal (negative

result from Step 2), the RCFGCALL register
value needs to be negative. This causes the
specified number of clock pulses to be
subtracted from the timer counter, once every
minute.

• If the oscillator is slower than ideal (positive

result from Step 2), the RCFGCALL register
value needs to be positive. This causes the
specified number of clock pulses to be added to
the timer counter, once every minute.

3.

Load the RTCCAL register with the correct
value.

Writes to the RTCCAL register should occur only when
the timer is turned off or immediately after the rising
edge of the seconds pulse.

The Alarm features and characteristics are:
• Configurable from half a second to one year
• Enabled using the ALRMEN bit (ALRMCFG<7>, 

)

• Offers one-time and repeat alarm options

17.3.1

CONFIGURING THE ALARM

The alarm feature is enabled using the ALRMEN bit.
This bit is cleared when an alarm is issued. The bit will
not be cleared if the CHIME bit = 1 or if ALRMRPT 

 0.

The interval selection of the alarm is configured
through the ALRMCFG bits (AMASK<3:0>) (see

). 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 number of times this
occurs, after the alarm is enabled, is stored in the
ALRMRPT register. 

00

ALRMMIN

ALRMSEC

01

ALRMWD

ALRMHR

10

ALRMMNTH

ALRMDAY

11

—

—

(Ideal Frequency (32,758) – Measured Frequency) * 60 = 
Error Clocks per Minute

In determining the crystal’s error value, it
is the user’s responsibility to include the
crystal’s initial error from drift due to
temperature or crystal aging.

While the alarm is enabled (ALRMEN = 1),
changing any of the registers, other than
the RTCCAL, ALRMCFG and ALRMRPT
registers and the CHIME bit, can result in a
false alarm event leading to a false alarm
interrupt. To avoid this, only change the
timer and alarm values while the alarm is
disabled (ALRMEN = 0). It is recommended
that the ALRMCFG and ALRMRPT regis-
ters and CHIME bit be changed when
RTCSYNC = 0.