MCP79400 datasheet(30/62 Pages) MICROCHIP

30 / 62 page

MCP79400/MCP79401/MCP79402

DS20005009F-page 30

 2011-2016 Microchip Technology Inc.

5.6.1

CALIBRATION

In order to perform calibration, the number of error

clock pulses per minute must be found and the

corresponding trim value must be loaded into

TRIMVAL<6:0>.

There are two methods for determining the trim value.

The first method involves measuring an output

frequency 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 MCP7940X 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 MCP7940X 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

MCP7940X

with

known-accurate

reference 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 MCP7940X has gained time relative to the

reference clock, then the oscillator is faster

than ideal and the SIGN bit must be cleared.

• If the MCP7940X 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

observing

deviation

will

improve

accuracy.

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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