MCP795WXX/MCP795BXX
9.2
Crystal Specification and
Selection
The MCP795XXX 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 MCP795XXX.
The table below is given as design guidance and a
starting point for crystal and capacitor selection.
Crystal
Capacitance
Manufacturer
Part Number
CX1 Value
CX2 Value
Micro Crystal
Citizen
CM7V-T1A
CM200S-32.768KDZB-UT
7pF
6pF
10pF
10pF
12pF
8 pF
Please work with your crystal vendor.
Gerber files are available on request. Please contact
your Microchip Sales representative.
EQUATION 9-1:
CX2 CX1
Cload = ----------------------------- + C stray
CX2 + CX1
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.
The following must also be taken into consideration:
• Pin capacitance (to be included in Cx2 and Cx1)
• Stray Board Capacitance
For more information please see the RTCC Best
Practices AN1365 (DS01365).
The recommended board layout for the oscillator area
is shown in Figure 9-9. This actual board shows the
crystal and the load capacitors. In this example, C2 is
CX1, C1 is CX2 and the crystal is designated as Y1.
When calculating the effective load capacitance,
Equation 9-1 can be used.
FIGURE 9-9:
BOARD LAYOUT
2011 Microchip Technology Inc.
Preliminary
DS22280A-page 37
MAS [ MICRO ANALOG SYSTEMS ]
