64GA106 datasheet(219/330 Pages) MICROCHIP

219 / 330 page

 2010 Microchip Technology Inc.

DS39905E-page 219

PIC24FJ256GA110 FAMILY

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 for one minute 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 and

loaded into the RCFGCAL register.

EQUATION 19-1:

RTCC CALIBRATION

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 added from the

timer counter once every minute.

Divide the number of error clocks per minute by

4 to get the correct CAL value and load the

RCFGCAL register with the correct value.

(Each 1-bit increment in CAL 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.

U-0

R/W-x

—

MINTEN2

MINTEN1

MINTEN0

MINONE3

MINONE2

MINONE1

MINONE0

bit 15

bit 8

U-0

R/W-x

—

SECTEN2

SECTEN1

SECTEN0

SECONE3

SECONE2

SECONE1

SECONE0

bit 7

bit 0

Legend:

R = Readable bit

W = Writable bit

U = Unimplemented bit, read as ‘0’

-n = Value at POR

‘1’ = Bit is set

‘0’ = Bit is cleared

x = Bit is unknown

bit 15

Unimplemented:

Read as ‘0’

bit 14-12

MINTEN<2:0>:

Binary Coded Decimal Value of Minute’s Tens Digit bits

Contains a value from 0 to 5.

bit 11-8

MINONE<3:0>:

Binary Coded Decimal Value of Minute’s Ones Digit bits

Contains a value from 0 to 9.

bit 7

Unimplemented:

Read as ‘0’

bit 6-4

SECTEN<2:0>:

Binary Coded Decimal Value of Second’s Tens Digit bits

Contains a value from 0 to 5.

bit 3-0

SECONE<3:0>:

Binary Coded Decimal Value of Second’s Ones Digit bits

Contains a value from 0 to 9.

Error (Clocks per Minute) = (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.

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