MCP7940NT-ESLASHMS datasheet(30/58 Pages) MICROCHIP

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MCP7940N

DS20005010J-page 30

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5.6.1

CALIBRATION

In order to perform calibration, the number of error

clock pulses per minute must be found and the corre-

sponding

trim

value

must

loaded

into

TRIMVAL<6:0>.

There are two methods for determining the trim value.

The first method involves measuring an output fre-

quency directly and calculating the deviation from ideal.

The second method involves observing the number of

seconds gained or lost over a period of time.

Once the OSCTRIM register has been loaded, digital

trimming will automatically occur every minute.

5.6.1.1

Calibration by Measuring Frequency

To calibrate the MCP7940N by measuring the output

frequency, perform the following steps:

Enable the crystal oscillator or external clock

input by setting the ST bit or EXTOSC bit,

respectively.

Ensure TRIMVAL<6:0> is reset to 0x00.

Select

output

frequency

setting

SQWFS<1:0>.

Set SQWEN to enable the square wave output.

Measure the resulting output frequency using a

calibrated measurement tool, such as a

frequency counter.

Calculate the number of error clocks per minute

(see Equation 5-2).

EQUATION 5-2:

CALCULATING TRIM

VALUE FROM MEASURED

FREQUENCY

• If the number of error clocks per minute is

negative, then the oscillator is faster than

ideal and the SIGN bit must be cleared.

• If the number of error clocks per minute is

positive, then the oscillator is slower than

ideal and the SIGN bit must be set.

Load the correct value into TRIMVAL<6:0>.

5.6.1.2

Calibration by Observing Time

Deviation

To calibrate the MCP7940N by observing the deviation

over time, perform the following steps:

Ensure TRIMVAL<6:0> is reset to 0x00.

Load the timekeeping registers to synchronize

the MCP7940N with a known-accurate refer-

ence time.

Enable the crystal oscillator or external clock

input by setting the ST bit or EXTOSC bit,

respectively.

Observe how many seconds are gained or lost

over a period of time (larger time periods offer

more accuracy).

Calculate the PPM deviation (see Equation 5-3).

EQUATION 5-3:

CALCULATING ERROR

PPM

• If the MCP7940N has gained time relative to

the reference clock, then the oscillator is

faster than ideal and the SIGN bit must be

cleared.

• If the MCP7940N has lost time relative to the

reference clock, then the oscillator is slower

than ideal and the SIGN bit must be set.

Calculate the trim value (see Equation 5-4).

EQUATION 5-4:

CALCULATING TRIM

VALUE FROM ERROR

PPM

Load the correct value into TRIMVAL<6:0>.

Note:

Using a lower output frequency and/or

averaging the measured frequency over a

number of clock pulses will reduce the

effects of jitter and improve accuracy.

TRIMVAL<6:0>

FIDEAL FMEAS

–

 32768

FIDEAL

------------------- 60



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

Where:

FIDEAL

Ideal frequency based on SQWFS<1:0>

FMEAS

Measured frequency

Note 1: Choosing a longer time period for observ-

ing deviation will improve accuracy.

2: Large temperature variations during the

observation period can skew results.

PPM

SecDeviation

ExpectedSec

----------------------------------- 1000000



Where:

ExpectedSec

Number of seconds in chosen period

SecDeviation

Number of seconds gained or lost

TRIMVAL<6:0>

PPM 32768 60



1000000 2



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