MCP79400 datasheet(17/42 Pages) MICROCHIP

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 2011 Microchip Technology Inc.

DS25009C-page 17

MCP7940X

5.2.5

CRYSTAL SPECS

The MCP7940X 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 MCP7940X.

The table below is given as design guidance and a

starting point for crystal and capacitor selection.

EQUATION 5-1:

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 5-3. 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 5-3:

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 AN1365, “RTCC Best

Practices Application Note” (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.

Cload

CX2 CX1



CX2

CX1

-----------------------------

Cstray

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