Datasheet

 2010-2013 Microchip Technology Inc. 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 V
CC
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
register. Conversely, if the time is running slow, a
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.
REGISTER 8-3: 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
6
 
24 60 60

----------------------------------------------------------------------=
Giving
58PPM
5x10
6

86400
-------------------=
57
58PPM
1.017PPM
--------------------------=
Calibration register
Value