MCP79411 datasheet(25/60 Pages) MICROCHIP

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DS20002266E-page 25

MCP79410/MCP79411/MCP79412

8.3

Digital Trimming

The MCP7941X uses digital trimming to correct for

inaccuracies of the input clock source (either externally

driven or from a crystal). These inaccuracies are due to

crystal, capacitor and temperature variations. This

enables the user to compensate for differences in tem-

perature over the operating conditions of the device,

offering higher time accuracy over an uncalibrated

RTCC. Digital trimming is always enabled in the

MCP794XX device. Digital trimming is achieved by dig-

itally modifying the number of clock cycles per minute

to achieve PPM level adjustments in the internal timing

function of the MCP7941X. The amount by which the

MCP794XX adjusts the time is determined by the value

loaded into the calibration register. A value of 0x00 in

the calibration register results in no time adjustment.

The Calibration value is maintained during a VCC

power-fail if the backup supply is enabled. Digital

trimming is also performed during this time. The same

calibration value is used until it is changed by the

system firmware.

The MSB of the Calibration register is the sign bit, with

a ‘1’ indicating a negative PPM calibration and a ‘0’

indicating a positive PPM calibration. The calibration

value can range from 0 to 127, combined with the sign

bit this gives the MCP7941X the ability to calibrate

±129 ppm of combined error. The calibration is

performed on the minute rollover.

Given that each bit provides ±2 internal clocks of

trimming, the effective PPM of each bit is ±1.017 ppm.

The calibration value is determined by measuring the

error over a period of time. If the time is running fast

then a positive calibration is loaded into the Calibration

negative calibration is loaded.

Example: Time is running fast five seconds per day.

Five seconds per day can be expressed in PPM using

Equation 8-1.

EQUATION 8-1:

In this example, the calibration value to be loaded is 57.

The same method can be used if the MCP7941X is

running slow.

CALIBRATION 0X08

R/W-0

CALSIGN

CALIBRATION

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 clear

“X” = Bit is unknown

bit 7

CALSIGN: Sign of Calibration, allows for positive and negative calibration

bit 6-0

CALIBRATION<6:0>: Calibration Value bits

Note 1:

This is an 8-bit register that is used to add or subtract clocks from the RTCC counter every minute. The

MSB is the sign bit and indicates if the count should be added or subtracted. The remaining 7 bits, with

each bit adding or subtracting two clocks, give the user the ability to add or subtract up to 254 clocks per

minute. Each bit represents ±1.017 ppm.

PPM

Seconds per day 1x10





24 60 60



----------------------------------------------------------------------

Giving

58PPM

5x10



86400



-------------------

58PPM

1.017PPM

--------------------------

Calibration register

Value

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