Datasheet

DocID012578 Rev 16 35/64
M41T82, M41T83 Clock operation
Two methods are available for ascertaining how much calibration a given M41T8x may
require:
 The first involves setting the clock, letting it run for a month and comparing it to a
known accurate reference and recording deviation over a fixed period of time. This
allows the designer to give the end user the ability to calibrate the clock as the
environment requires, even if the final product is packaged in a non-user serviceable
enclosure. The designer could provide a simple utility that accesses either or both of
the calibration bytes.
 The second approach is better suited to a manufacturing environment, and uses the
512 Hz frequency test output. This is the IRQ1/FT/OUT pin on the M41T83, and the
FT/RST pin on the M41T82 (see Section 3.14 and Section 3.15 for more information on
enabling the FT output). The 512 Hz frequency test signal can be measured using a
highly accurate timing device such as a frequency counter. The measured value is then
compared to 512 Hz and the oscillator error in ppm is then determined.
Any deviation from 512 Hz indicates the degree and direction of oscillator frequency
shift at the test temperature. For example, a reading of 512.010124 Hz would indicate a
+20 ppm oscillator frequency error, requiring either a –10 (xx001010) to be loaded into
the digital calibration byte, or +6 pF (00011000) into the analog calibration byte for
correction.
Note: Setting or changing the digital calibration byte does not affect the frequency test, square
wave, or watchdog timer frequency, but changing the analog calibration byte DOES affect
all functions derived from the low current oscillator (see Figure 20).
Figure 20. Clock divider chain and calibration circuits
AI11806c
Analog Calibration
Circuitry
Remainder of
Divider Circuit
1Hz Signal
512Hz Output
Frequency Test
32KHz
Low Current
Oscillator
C
XI
C
XO
÷64
÷64
÷2
Digital Calibration Circuitry
(divide by 511/512/513)
Clock
Counters
Square Wave
Watchdog Timer
8-bit Timer