F21 datasheet(251/416 Pages) MICROCHIP

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DS30010118D-page 251

PIC24FJ256GA705 FAMILY

21.0 REAL-TIME CLOCK AND

CALENDAR (RTCC) WITH

TIMESTAMP

The RTCC provides the user with a Real-Time Clock

and Calendar (RTCC) function that can be calibrated.

Key features of the RTCC module are:

• Selectable Clock Source

• Provides Hours, Minutes and Seconds Using

24-Hour Format

• Visibility of One Half Second Period

• Provides Calendar – Weekday, Date, Month

and Year

• Alarm-Configurable for Half a Second, 1 Second,

10 Seconds, 1 Minute, 10 Minutes, 1 Hour, 1 Day,

1 Week, 1 Month or 1 Year

• Alarm Repeat with Decrementing Counter

• Alarm with Indefinite Repeat Chime

• Year 2000 to 2099 Leap Year Correction

• BCD Format for Smaller Software Overhead

• Optimized for Long-Term Battery Operation

• User Calibration of the 32.768 kHz Clock

Crystal/32 kHz INTRC Frequency with Periodic

Auto-Adjust

• Fractional Second Synchronization

• Calibration to within ±2.64 Seconds Error

per Month

• Calibrates up to 260 ppm of Crystal Error

• Ability to Periodically Wake-up External Devices

without CPU Intervention (external power control)

• Power Control Output for External Circuit Control

• Calibration takes Effect Every 15 Seconds

• Timestamp Capture register for Time and Date

• Programmable Prescaler and Clock Divider

Circuit allows Operation with Any Clock Source

up to 32 MHz, Including 32.768 kHz Crystal,

50/60 Hz Powerline Clock, External Real-Time

Clock (RTC) or 31.25 kHz LPRC Clock

21.1

RTCC Source Clock

The RTCC clock divider block converts the incoming

oscillator source into accurate 1/2 and 1 second clocks

for the RTCC. The clock divider is optimized to work

with three different oscillator sources:

• 32.768 kHz crystal oscillator

• 31 kHz Low-Power RC Oscillator (LPRC)

• External 50 Hz or 60 Hz powerline frequency

An asynchronous prescaler, PS<1:0> (RTCCON2L<5:4>),

is provided that allows the RTCC to work with higher

speed clock sources, such as the system clock. Divide

ratios of 1:16, 1:64 or 1:256 may be selected, allowing

sources up to 32 MHz to clock the RTCC.

21.1.1

COARSE FREQUENCY DIVISION

The clock divider block has a 16-bit counter used to

divide the input clock frequency. The divide ratio is set

by the DIV<15:0> register bits (RTCCON2H<15:0>).

The DIV<15:0> bits should be programmed with a

value to produce a nominal 1/2 second clock divider

count period.

21.1.2

FINE FREQUENCY DIVISION

The fine frequency division is set using the FDIV<4:0>

(RTCCON2L<15:11>) bits. Increasing the FDIVx value

will lengthen the overall clock divider period.

If FDIV<4:0> = 00000, the fine frequency division circuit

is effectively disabled. Otherwise, it will optionally

remove a clock pulse from the input of the clock divider

every 1/2 second. This functionality will allow the user to

remove up to 31 pulses over a fixed period of

16 seconds, depending on the value of FDIVx.

The value for DIV<15:0> is calculated as shown in

Equation 21-1. The fractional remainder of the

DIV<15:0> calculation result can be used to calculate

the value for FDIV<4:0>.

EQUATION 21-1:

RTCC CLOCK DIVIDER

OUTPUT FREQUENCY

Note:

This data sheet summarizes the features of

this group of PIC24F devices. It is not

intended to be a comprehensive reference

source. For more information on the

Real-Time Clock and Calendar, refer to

the “dsPIC33/PIC24 Family Reference

Manual”, “RTCC with Timestamp”

(DS70005193), which is available from the

Microchip website (www.microchip.com).

The information in this data sheet

supersedes the information in the FRM.

The DIV<15:0> value is the integer part of this calculation:

The FDIV<4:0> value is the fractional part of the DIV<15:0>

calculation, multiplied by 32.

FOUT =

FIN

2 • (PS<1:0> Prescaler) • (DIV<15:0> + 1) +

FDIV<4:0>

()

DIV<15:0> =

FIN

2 • (PS<1:0> Prescaler)

– 1

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