Datasheet

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
register (see Section 6.0, Control register). Note that
the MFP output waveform is disabled when the
MCP7941X is running in V
BAT 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 V
CC.
• Alarm Outputs – Available when operating from
V
CC 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 Ta bl e 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.
T
output
= (2 * (256 +/- (2 * CALREG))) T
input
where:
T
output
= clock period of MFP output signal
T
input
= clock period of input signal
CALREG = decimal value of the Calibration
register setting, and the sign is
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