Datasheet

MCP7940M
DS22292A-page 14 Preliminary  2012 Microchip Technology Inc.
4.2.2 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
• FOUT mode – driven from a FOSC divider
4.2.3 CRYSTAL SPECS
The MCP7940M has been designed to operate with a
standard 32.768 kHz tuning fork crystal. The on-board
oscillator has been characterized to operate with a
crystal of maximum ESR of 70K Ohms.
Crystals with a comparable specification are also suit-
able for use with the MCP7940M.
The table below is given as design guidance and a
starting point for crystal and capacitor selection.
EQUATION 4-1:
The following must also be taken into consideration:
• Pin capacitance (to be included in Cx2 and Cx1)
• Stray Board Capacitance
The recommended board layout for the oscillator area
is shown in Figure 4-3. This actual board shows the
crystal and the load capacitors. In this example, C2 is
CX1, C3 is CX2 and the crystal is designated as Y1.
FIGURE 4-3: BOARD LAYOUT
Gerber files are available from www/microchip.com/
rtcc.
It is required that the final application should be tested
with the chosen crystal and capacitor combinations
across all operating and environmental conditions.
Please also consult with the crystal specification to
observe correct handling and reflow conditions and for
information on ideal capacitor values.
For more information please see application note
AN1365, “RTCC Best Practices” (DS01365).
Manufacturer Part Number
Crystal
Capacitance
CX1 Value CX2 Value
Micro Crystal CM7V-T1A 7pF 10pF 12pF
Citizen CM200S-32.768KDZB-UT 6pF 10pF 8 pF
Please work with your crystal vendor.
C
load
CX2 CX1 
CX2 CX1+
----------------------------- C
stray
+=