Datasheet

MCP7941X
DS22266A-page 14 Preliminary  2010 Microchip Technology Inc.
In the case where the MSB of the Calibration register
is set to ‘1’, the MFP output waveforms that appear
when bits RS1 and RS0 are set to ‘01’ or ‘10’ are not
as responsive to the setting of the Calibration register.
For example, when outputting the 4.096 kHz
waveform (RS1, RS0 set to ‘01’), the output waveform
is generated using only eight input clock cycles.
Consequently, attempting to subtract more than eight
input clock cycles from this output does not have a
meaningful effect on the resulting waveform. Any
effect on the output will appear as a modification in
both the frequency and duty cycle of the waveform
appearing on the MFP output pin.
B.RS2 BIT SET TO ‘1’
With the RS2 bit set to ‘1’, the following internal timing
signal is output on the MFP pin:
The frequency listed in the table presumes an input
clock source of exactly 32.768 kHz. In terms of the
equivalent number of input clock cycles, the table
becomes:
Unlike the method 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.
4.2.4 MFP
Pin 7 is a multi-function pin and supports the following
functions:
• Use of the OUT bit in the Control register for
single bit I/O
• Alarm Outputs – Available in VBAT mode
• FOUT mode – driven from a FOSC divider – Not
available in V
BAT mode
The internal control logic for the MFP is connected to
the switched internal supply bus, this allows operation
in V
BAT mode. The Alarm Output is the only mode that
operates in V
BAT mode, other modes are suspended.
4.2.5 VBAT
If the VBAT feature is not being used, the VBAT pin
should be connected to GND. A low-value series
resistor is recommended between the external battery
and the V
BAT pin.
The V
BAT point is defined as 1.5V typical. When VDD
falls below 1.5V the system will continue to operate
the RTCC and SRAM using the V
BAT supply. The
following conditions apply:
4.2.6 CRYSTAL SPECS
The MCP7941X has been designed to operate with a
standard 32 kHz crystal. Devices with a specified load
capacitance of either 12pF or 6pF can be used. The
end user should fully validate the chosen crystal across
all the expected design parameters of the system to
ensure correct operation.
The following crystals have been tested and shown to
work with the MCP7941X:
• CM200S 12pF surface mount crystals from
Citizen
• ECS-.327 12pF surface mount crystals from ECS
INC
• CFS206 12pF leaded crystals from Citizen
This is not a definitive list and all crystals should be
tested in the target application across all temperature,
voltage and other significant environmental conditions.
RS2 RS1 RS0 Output Signal
1 x x 64.0 Hz
RS2 RS1 RS0 Output Signal
1 x x 512
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 4-2:
Supply
Condition
Read/Write
Access
Powered
By
V
CC < VTRIP, VCC < VBAT No VBAT
VCC > VTRIP, VCC < VBAT Yes VCC
VCC > VTRIP, VCC > VBAT Yes VCC