PIC24F16KA102 datasheet(162/254 Pages) MICROCHIP

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PIC24F16KA102 FAMILY

DS39927B-page 160

Preliminary

19.3

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

multiplied by four and will be either added or subtracted

from the RTCC timer, once every minute. Refer to the

steps below for RTCC calibration:

Using another timer resource on the device, the

user must find the error of the 32.768 kHz crystal.

Once the error is known, it must be converted to

the number of error clock pulses per minute.

a) If the oscillator is faster than ideal (negative

result form step 2), the RCFGCAL register value

must 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 than ideal (positive

result from step 2), the RCFGCAL register value

must be positive. This causes the specified

number of clock pulses to be subtracted from

the timer counter, once every minute.

Divide the number of error clocks per minute by 4 to get

the correct calibration value and load the RCFGCAL

the calibration adds or subtracts 4 pulses).

EQUATION 19-1:

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.

19.4

Alarm

• Configurable from half second to one year

• Enabled using the ALRMEN bit

(ALCFGRPT<15>)

• One-time alarm and repeat alarm options

available

19.4.1

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 displayed in Figure 19-2, 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<7:0> bits (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.

19.4.2

ALARM INTERRUPT

At every alarm event, an interrupt is generated. In

addition, 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 than

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.

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