Datasheet

 2012-2018 Microchip Technology Inc. DS20002300C-page 30
MCP7951X/MCP7952X
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<7:0>.
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
(CRSTRIM = 0).
5.6.1.1 Calibration by Measuring Frequency
To calibrate the MCP795XX by measuring the output
frequency, perform the following steps:
1. Enable the crystal oscillator or external clock
input by setting the ST bit or EXTOSC bit,
respectively.
2. Ensure TRIMVAL<7:0> is reset to 0x00.
3. Select an output frequency by setting
SQWFS<1:0>.
4. Set SQWEN to enable the square wave output.
5. Measure the resulting output frequency using a
calibrated measurement tool, such as a
frequency counter.
6. Calculate the number of error clocks per minute
(see Equation 5-3).
EQUATION 5-3: 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 TRIMSIGN bit must be cleared.
• If the number of error clocks per minute is
positive, then the oscillator is slower than
ideal and the TRIMSIGN bit must be set.
7. Load the correct value into TRIMVAL<7:0>.
5.6.1.2 Calibration by Observing Time
Deviation
To calibrate the MCP795XX by observing the deviation
over time, perform the following steps:
1. Ensure TRIMVAL<7:0> is reset to 0x00.
2. Load the timekeeping registers to synchronize
the MCP795XX with a known-accurate
reference time.
3. Enable the crystal oscillator or external clock
input by setting the ST bit or EXTOSC bit,
respectively.
4. Observe how many seconds are gained or lost
over a period of time (larger time periods offer
more accuracy).
5. Calculate the PPM deviation (see Equation 5-4).
EQUATION 5-4: CALCULATING ERROR
PPM
• If the MCP795XX has gained time relative to
the reference clock, then the oscillator is
faster than ideal and the TRIMSIGN bit must
be cleared.
• If the MCP795XX has lost time relative to the
reference clock, then the oscillator is slower
than ideal and the TRIMSIGN bit must be set.
6. Calculate the trim value (see Equation 5-5).
EQUATION 5-5: CALCULATING TRIM
VALUE FROM ERROR
PPM
7. Load the correct value into TRIMVAL<7:0>.
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<7:0>
F
IDEAL FMEAS–
32768
F
IDEAL
-------------------60
2
---------------------------------------------------------------------------------=
Where:
F
IDEAL Ideal frequency based on SQWFS<1:0>=
F
MEAS Measured frequency=
Note 1: Choosing a longer time period for
observing deviation will improve
accuracy.
2: Large temperature variations during the
observation period can skew results.
PPM
SecDeviation
ExpectedSec
----------------------------------- 1 0 0 0 0 0 0=
Where:
ExpectedSec Number of seconds in chosen period=
SecDeviation Number of seconds gained or lost=
TRIMVAL<7:0>
PPM 32768 60
1000000 2
-------------------------------------------=