MCP79411 datasheet(26/60 Pages) MICROCHIP

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MCP79410/MCP79411/MCP79412

DS20002266E-page 26

 2010-2013 Microchip Technology Inc.

8.4

Digital Calibration Mode

The MCP7941X utilizes digital trimming to correct for

inaccuracies of the input clock source. However, as

this internal trimming is performed on the minute

rollover, a Digital Calibration mode is available. Using

this mode the oscillator frequency can be directly

observable.

The internal timing function can be monitored using

the MFP open-drain output pin by setting bit<6:0>

(SQWE) and bits<2:0> (RS2, RS1, RS0) of the control

the MFP output waveform is disabled when the

MCP7941X is running in VBAT mode. With the SQWE

bit set to ‘1’, and the RS2 bit set to ‘1’ the Digital

Calibration mode is enabled.

Unlike digital trimming previously described, the

calibration setting is continuously applied and affects

every cycle of the output waveform. This results in the

modulation of the frequency of the output waveform

based upon the setting of the Calibration register.

Using this setting, the calibration function can be

expressed as:

Since the calibration is done every cycle, the frequency

of the output MFP waveform is constant. With a crystal

frequency of exactly 32.768 kHz the output on the MFP

pin will be 64 Hz. Deviation of the crystal frequency will

shift this frequency.

8.5

Multifunction PIN (MFP)

Pin 7 is a multifunction pin and supports the following

functions:

• The value of the OUT bit determines the logic

level of the I/O. This is only available when

operating from VCC.

• Alarm Outputs – Available when operating from

VCC or backup power supply

• FOUT mode – driven from a FOSC divider – Not

available when operating from backup power

supply (CLKOUT mode)

The internal control logic for the MFP is connected to

the switched internal supply bus, this allows operation

in VBAT mode. The Alarm Output is the only mode that

operates when operating from backup power supply,

other modes are suspended, see Table 8-2.

The following diagram shows how the internal MFP

control logic is implemented. This pin in an open-drain

and can be left floating if it is not being used.

When the MFP is used for CLKOUT mode of operation

the following frequencies are available.

Toutput = (2 * (256 +/- (2 * CALREG))) Tinput

where:

Toutput = clock period of MFP output signal

Tinput = clock period of input signal

CALREG

decimal value of the Calibration

determined by the MSB of the

Calibration register.

TABLE 8-1:

CLKOUT DIVIDER

RS2

RS1

RS0

CLKOUT

Frequency

000

32.768 kHz*

001

8.192 kHz

010

4.096 kHz

011

1Hz

Note:

*Not effected by calibration

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