DVR21 datasheet(268/416 Pages) MICROCHIP

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PIC24FJ256GA705 FAMILY

DS30010118D-page 268

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21.4

Calibration

21.4.1

CLOCK SOURCE CALIBRATION

A crystal oscillator that is connected to the RTCC may

be calibrated to provide an accurate one second clock

in two ways. First, coarse frequency adjustment is per-

formed by adjusting the value written to the DIV<15:0>

bits. Secondly, a 5-bit value can be written to the

FDIV<4:0> control bits to perform a fine clock division.

The DIVx and FDIVx values can be concatenated and

considered as a 21-bit prescaler value. If the oscillator

source is slightly faster than ideal, the FDIV<4:0> value

can be increased to make a small decrease in the RTC

frequency. The value of DIV<15:0> should be

increased to make larger decreases in the RTC

frequency. If the oscillator source is slower than ideal,

FDIV<4:0> may be decreased for small calibration

changes and DIV<15:0> may need to be decreased to

make larger calibration changes.

Before calibration, the user must determine the error of

the crystal. This should be done using another timer

resource on the device or an external timing reference.

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.

21.5

Alarm

• Configurable from half second to one year

• Enabled using the ALRMEN bit

(RTCCON1H<15>)

• One-time alarm and repeat alarm options are

available

21.5.1

CONFIGURING THE ALARM

The alarm feature is enabled using the ALRMEN bit.

This bit is cleared when an alarm is issued. Writes to

the Alarm Value registers should only take place when

ALRMEN = 0.

As shown in Figure 21-2, the interval selection of the

alarm is configured through the AMASK<3:0> bits

(RTCCON1H<11:8>). 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

ALMRPT<7:0> bits (RTCCON1H<7:0>). When the

value of the ALMRPTx bits equals 00h and the CHIME

bit (RTCCON1H<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

ALMRPT<7:0> with FFh.

After each alarm is issued, the value of the ALMRPTx

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 ALMRPTx bits reaches 00h, it

rolls over to FFh and continues counting indefinitely

while CHIME is set.

21.5.2

ALARM INTERRUPT

At every alarm event, an interrupt is generated. This

output is completely synchronous to the RTCC clock

and can be used as a trigger clock to the other

peripherals.

Note:

Changing any of the register bits, other

than the RTCOE bit (RTCCON1L<7>), the

ALMRPT<7:0> bits (RTCCON1H<7:0>

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).

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