MCP7940N-ESLASHST datasheet(13/42 Pages) MICROCHIP

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DS25010E-page 13

MCP7940N

• The 12/24-hour bits 0xCh.6 and 0x13h.6 are cop-

ies of the bit in 0x02h.6. The bits are read-only.

0x18h-0x1Bh are used for the timesaver function.

These registers are loaded at the time when VCC fails

and the RTCC operates on the VBAT. The VBAT bit is

also set at this time. These registers are cleared when

the VBAT bit is cleared in software.

0x1Ch-0x1Fh are used for the timesaver function.

These registers are loaded at the time when VCC is

restored and the RTCC switches to VDD. These

registers are cleared when the VBAT bit is cleared in

software.

5.2

FEATURES

5.2.1

CALIBRATION

The MCP7940N utilizes digital calibration to correct for

inaccuracies of the input clock source (either external

or crystal). Calibration is enabled by setting the value

of the Calibration register at address 08H. Calibration

is achieved by adding or subtracting a number of input

clock cycles per minute in order to achieve ppm level

adjustments in the internal timing function of the

MCP7940N.

The MSB of the Calibration register is the sign bit, with

a ‘1’ indicating subtraction and a ‘0’ indicating addition.

The remaining seven bits in the register indicate the

number of input clock cycles (multiplied by two) that

are subtracted or added per minute to the internal

timing function.

The internal timing function can be monitored using

the MFP open-drain output pin by setting bit [6]

(SQWE) and bits [2:0] (RS2, RS1, RS0) of the control

waveform is disabled when the MCP7940N is running

in VBAT mode. With the SQWE bit set to ‘1’, there are

two methods that can be used to observe the internal

timing function of the MCP7940N:

A. RS2 BIT SET TO ‘0’

With the RS2 bit set to ‘0’, the RS1 and RS0 bits

enable the following internal timing signals to be

output on the MFP pin:

The frequencies listed in the table presume an input

clock source of exactly 32.768 kHz. In terms of the

equivalent number of input clock cycles, the table

becomes:

With regards to the calibration function, the Calibration

clock signal when bits RS1 and RS0 are set to ‘11’.

The setting of the Calibration register to a non-zero

value (i.e., values other than 00H or 80H) enables the

calibration function which can be observed on the

MFP output pin. The calibration function can be

expressed in terms of the number of input clock cycles

added/subtracted from the internal timing function.

001 – Minutes match

010 – Hours match (takes into account 12/24

hour)

011 – Matches the current day, interrupt at

12.00.00 a.m. Example: 12 midnight on

100

–Date

101

– RESERVED

110

– RESERVED

111

– Seconds, Minutes, Hour, Day, Date,

Month

Note:

It is strongly recommended that the

timesaver function only be used when the

oscillator is running. This will ensure

accurate functionality.

RS2

RS1

RS0

Output Signal

1 Hz

4.096 kHz

8.192 kHz

32.768 kHz

RS2

RS1

RS0

Output Signal

32768

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